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| author | Franklin Schmidt <fschmidt@gmail.com> |
|---|---|
| date | Sat, 02 May 2015 20:13:24 -0600 |
| parents | df95199ca4c0 |
| children | eddf7c73373b |
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local Io = require "luan:Io" local Html = require "luan:Html" local Http = require "luan:web/Http" local Shared = require "site:/Shared" function service() Io.stdout = Http.response.text_writer() %> <html> <head> <% Html.simply_html_head() %> <title>Luan Reference Manual</title> </head> <body> <div container> <% Shared.header() %> <h1>Luan Reference Manual</h1> <p> <small> Original copyright © 2015 Lua.org, PUC-Rio. Freely available under the terms of the <a href="http://www.lua.org/license.html">Lua license</a>. Modified for Luan. </small> </p> <hr/> <h2>Contents</h2> <div margin-bottom="1em"><a href="#intro">Introduction</a></div> <div margin-bottom="1em"> <a href="#basic">Basic Concepts</a> <ul> <li><a href="#types">Values and Types</a></li> <li><a href="#env">Environments</a></li> <li><a href="#error">Error Handling</a></li> <li><a href="#meta">Metatables and Metamethods</a></li> <li><a href="#gc">Garbage Collection</a></li> </ul> </div> <div margin-bottom="1em"> <a href="#lang">The Language</a> <ul> <li><a href="#lex">Lexical Conventions</a></li> <li><a href="#vars">Variables</a></li> <li> <a href="#stmts">Statements</a> <ul> <li><a href="#blocks">Blocks</a></li> <li><a href="#chunks">Chunks</a></li> <li><a href="#assignment">Assignment</a></li> <li><a href="#control">Control Structures</a></li> <li><a href="#for">For Statement</a></li> <li><a href="#fn_stmt">Function Calls as Statements</a></li> <li><a href="#local_stmt">Local Declarations</a></li> </ul> </li> <li> <a href="#expressions">Expressions</a> <ul> <li><a href="#arithmetic">Arithmetic Operators</a></li> <li><a href="#conversions">Coercions and Conversions</a></li> <li><a href="#relational">Relational Operators</a></li> <li><a href="#logical_ops">Logical Operators</a></li> <li><a href="#concatenation">Concatenation</a></li> <li><a href="#length">The Length Operator</a></li> <li><a href="#precedence">Precedence</a></li> <li><a href="#constructors">Table Constructors</a></li> <li><a href="#fn_calls">Function Calls</a></li> <li><a href="#fn_def">Function Definitions</a></li> </ul> </li> <li><a href="#visibility">>Visibility Rules</a></li> </ul> </div> <hr/> <h2 margin-top="1em"><a name="intro">Introduction</a></h2> <p>Luan is a high level programming language based on <a href="http://www.lua.org">Lua</a>. A great strength of Lua is its simplicity and Luan takes this even further, being even simpler than Lua. The goal is to provide a simple programming language for the casual programmer with as few concepts as possible so that one can quickly learn the language and then easily understand any code written in Luan.</p> <p>Luan is implemented in Java and is tightly coupled with Java. So it makes a great scripting language for Java programmers.</p> <p>Unlike Lua which is meant to be embedded, Luan is meant to be a full scripting language. This done not by adding feature to Luan, but rather by providing a complete set of libraries.</p> <h2 margin-top="1em"><a name="basic">Basic Concepts</a></h2> <p>This section describes the basic concepts of the language.</p> <h3 margin-top="1em"><a name="types">Values and Types</a></h3> <p> Luan is a <i>dynamically typed language</i>. This means that variables do not have types; only values do. There are no type definitions in the language. All values carry their own type. <p> All values in Luan are <i>first-class values</i>. This means that all values can be stored in variables, passed as arguments to other functions, and returned as results. <p> There are eight basic types in Luan: <i>nil</i>, <i>boolean</i>, <i>number</i>, <i>string</i>, <i>binary</i>, <i>function</i>, <i>userdata</i>, and <i>table</i>. <i>Nil</i> is the type of the value <b>nil</b>, whose main property is to be different from any other value; it usually represents the absence of a useful value. <i>Nil</i> is implemented as the Java value <i>null</i>. <i>Boolean</i> is the type of the values <b>false</b> and <b>true</b>. <i>Boolean</i> is implemented as the Java class <i>Boolean</i>. <i>Number</i> represents both integer numbers and real (floating-point) numbers. <i>Number</i> is implemented as the Java class <i>Number</i>. Any Java subclass of <i>Number</i> is allowed and this is invisible to the Luan user. Operations on numbers follow the same rules of the underlying Java implementation. <i>String</i> is implemented as the Java class <i>String</i>. <i>Binary</i> is implemented as the Java type <i>byte[]</i>. <p> Luan can call (and manipulate) functions written in Luan and functions written in Java (see <a href="#3.4.10">§3.4.10</a>). Both are represented by the type <i>function</i>. <p> The type <i>userdata</i> is provided to allow arbitrary Java objects to be stored in Luan variables. A userdata value is a Java object that isn't one of the standard Luan types. Userdata has no predefined operations in Luan, except assignment and identity test. Userdata is useful then Java access is enabled in Luan <p> The type <i>table</i> implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any Luan value except <b>nil</b>. Tables can be <i>heterogeneous</i>; that is, they can contain values of all types (except <b>nil</b>). Any key with value <b>nil</b> is not considered part of the table. Conversely, any key that is not part of a table has an associated value <b>nil</b>. <p> Tables are the sole data-structuring mechanism in Luan; they can be used to represent ordinary arrays, sequences, symbol tables, sets, records, graphs, trees, etc. To represent records, Luan uses the field name as an index. The language supports this representation by providing <tt>a.name</tt> as syntactic sugar for <tt>a["name"]</tt>. There are several convenient ways to create tables in Luan (see <a href="#3.4.9">§3.4.9</a>). <p> We use the term <i>sequence</i> to denote a table where the set of all positive numeric keys is equal to {1..<i>n</i>} for some non-negative integer <i>n</i>, which is called the length of the sequence (see <a href="#3.4.7">§3.4.7</a>). <p> Like indices, the values of table fields can be of any type. In particular, because functions are first-class values, table fields can contain functions. Thus tables can also carry <i>methods</i> (see <a href="#3.4.11">§3.4.11</a>). <p> The indexing of tables follows the definition of raw equality in the language. The expressions <tt>a[i]</tt> and <tt>a[j]</tt> denote the same table element if and only if <tt>i</tt> and <tt>j</tt> are raw equal (that is, equal without metamethods). In particular, floats with integral values are equal to their respective integers (e.g., <tt>1.0 == 1</tt>). <p> Luan values are <i>objects</i>: variables do not actually <i>contain</i> values, only <i>references</i> to them. Assignment, parameter passing, and function returns always manipulate references to values; these operations do not imply any kind of copy. <p> The library function <a href="#pdf-type"><tt>Luan.type</tt></a> returns a string describing the type of a given value (see <a href="#6.1">§6.1</a>). <h3 margin-top="1em"><a name="env">Environments</a></h3> <p> As will be discussed in <a href="#3.2">§3.2</a> and <a href="#3.3.3">§3.3.3</a>, any reference to a free name (that is, a name not bound to any declaration) <tt>var</tt> is syntactically translated to <tt>_ENV.var</tt>. Moreover, every chunk is compiled in the scope of an external local variable named <tt>_ENV</tt> (see <a href="#3.3.2">§3.3.2</a>), so <tt>_ENV</tt> itself is never a free name in a chunk. <p> Despite the existence of this external <tt>_ENV</tt> variable and the translation of free names, <tt>_ENV</tt> is a completely regular name. In particular, you can define new variables and parameters with that name. Each reference to a free name uses the <tt>_ENV</tt> that is visible at that point in the program, following the usual visibility rules of Luan (see <a href="#3.5">§3.5</a>). <p> Any table used as the value of <tt>_ENV</tt> is called an <i>environment</i>. <p> When Luan loads a chunk, the default value for its <tt>_ENV</tt> is an empty table. <p> Luan also provides all chunks with two other local values: <tt>require</tt> and <tt>java</tt>. These are functions used to load and access libraries and other modules. <h3 margin-top="1em"><a name="error">Error Handling</a></h3> <p> Luan code can explicitly generate an error by calling the <a href="#pdf-error"><tt>error</tt></a> function. If you need to catch errors in Luan, you can use <a href="#pdf-pcall"><tt>pcall</tt></a> or <a href="#pdf-xpcall"><tt>try</tt></a> to call a given function in <i>protected mode</i>. <p> Whenever there is an error, an <i>error object</i> (also called an <i>error message</i>) is propagated with information about the error. Luan itself only generates errors whose error object is a string, but programs may generate errors with any value as the error object. It is up to the Luan program or its host to handle such error objects. <h3 margin-top="1em"><a name="meta">Metatables and Metamethods</a></h3> <p> Every table in Luan can have a <i>metatable</i>. This <i>metatable</i> is an ordinary Luan table that defines the behavior of the original value under certain special operations. You can change several aspects of the behavior of operations over a value by setting specific fields in its metatable. For instance, when a table is the operand of an addition, Luan checks for a function in the field "<tt>__add</tt>" of the table's metatable. If it finds one, Luan calls this function to perform the addition. <p> The keys in a metatable are derived from the <i>event</i> names; the corresponding values are called <ii>metamethods</i>. In the previous example, the event is <tt>"add"</tt> and the metamethod is the function that performs the addition. <p> You can query the metatable of any table using the <a href="#pdf-getmetatable"><tt>get_metatable</tt></a> function. <p> You can replace the metatable of tables using the <a href="#pdf-setmetatable"><tt>set_metatable</tt></a> function. <p> A metatable controls how a table behaves in arithmetic operations, bitwise operations, order comparisons, concatenation, length operation, calls, and indexing. <p> A detailed list of events controlled by metatables is given next. Each operation is identified by its corresponding event name. The key for each event is a string with its name prefixed by two underscores, '<tt>__</tt>'; for instance, the key for operation "add" is the string "<tt>__add</tt>". Note that queries for metamethods are always raw; the access to a metamethod does not invoke other metamethods. You can emulate how Luan queries a metamethod for an object <tt>obj</tt> with the following code: <p><tt><pre> raw_get(get_metatable(obj) or {}, "__" .. event_name) </pre></tt></p> <p> Here are the events: <ul> <li><b>"add": </b> the <tt>+</tt> operation. If any operand for an addition is a table, Luan will try to call a metamethod. First, Luan will check the first operand (even if it is valid). If that operand does not define a metamethod for the "<tt>__add</tt>" event, then Luan will check the second operand. If Luan can find a metamethod, it calls the metamethod with the two operands as arguments, and the result of the call (adjusted to one value) is the result of the operation. Otherwise, it raises an error. </li> <li><b>"sub": </b> the <tt>-</tt> operation. Behavior similar to the "add" operation. </li> <li><b>"mul": </b> the <tt>*</tt> operation. Behavior similar to the "add" operation. </li> <li><b>"div": </b> the <tt>/</tt> operation. Behavior similar to the "add" operation. </li> <li><b>"mod": </b> the <tt>%</tt> operation. Behavior similar to the "add" operation. </li> <li><b>"pow": </b> the <tt>^</tt> (exponentiation) operation. Behavior similar to the "add" operation. </li> <li><b>"unm": </b> the <tt>-</tt> (unary minus) operation. Behavior similar to the "add" operation. </li> <li><b>"concat": </b> the <tt>..</tt> (concatenation) operation. Behavior similar to the "add" operation. </li> <li><b>"len": </b> the <tt>#</tt> (length) operation. If there is a metamethod, Luan calls it with the object as argument, and the result of the call (always adjusted to one value) is the result of the operation. If there is no metamethod but the object is a table, then Luan uses the table length operation (see <a href="#3.4.7">§3.4.7</a>). Otherwise, Luan raises an error. </li> <li><b>"eq": </b> the <tt>==</tt> (equal) operation. Behavior similar to the "add" operation, except that Luan will try a metamethod only when the values being compared are both tables and they are not primitively equal. The result of the call is always converted to a boolean. </li> <li><b>"lt": </b> the <tt><</tt> (less than) operation. Behavior similar to the "add" operation. The result of the call is always converted to a boolean. </li> <li><b>"le": </b> the <tt><=</tt> (less equal) operation. Unlike other operations, The less-equal operation can use two different events. First, Luan looks for the "<tt>__le</tt>" metamethod in both operands, like in the "lt" operation. If it cannot find such a metamethod, then it will try the "<tt>__lt</tt>" event, assuming that <tt>a <= b</tt> is equivalent to <tt>not (b < a)</tt>. As with the other comparison operators, the result is always a boolean. </li> <li><b>"index": </b> The indexing access <tt>table[key]</tt>. This event happens when <tt>key</tt> is not present in <tt>table</tt>. The metamethod is looked up in <tt>table</tt>. <p> Despite the name, the metamethod for this event can be either a function or a table. If it is a function, it is called with <tt>table</tt> and <tt>key</tt> as arguments. If it is a table, the final result is the result of indexing this table with <tt>key</tt>. (This indexing is regular, not raw, and therefore can trigger another metamethod.) </li> <li><b>"new_index": </b> The indexing assignment <tt>table[key] = value</tt>. Like the index event, this event happens when when <tt>key</tt> is not present in <tt>table</tt>. The metamethod is looked up in <tt>table</tt>. <p> Like with indexing, the metamethod for this event can be either a function or a table. If it is a function, it is called with <tt>table</tt>, <tt>key</tt>, and <tt>value</tt> as arguments. If it is a table, Luan does an indexing assignment to this table with the same key and value. (This assignment is regular, not raw, and therefore can trigger another metamethod.) <p> Whenever there is a "new_index" metamethod, Luan does not perform the primitive assignment. (If necessary, the metamethod itself can call <a href="#pdf-rawset"><tt>raw_set</tt></a> to do the assignment.) </li> <li><b>"call": </b> The call operation <tt>func(args)</tt>. This event happens when Luan tries to call a table. The metamethod is looked up in <tt>func</tt>. If present, the metamethod is called with <tt>func</tt> as its first argument, followed by the arguments of the original call (<tt>args</tt>). </li> </ul> <h3 margin-top="1em"><a name="gc">Garbage Collection</a></h3> <p> Luan uses Java's garbage collection. <h2 margin-top="1em"><a name="lang">The Language</a></h2> <p> This section describes the lexis, the syntax, and the semantics of Luan. In other words, this section describes which tokens are valid, how they can be combined, and what their combinations mean. <p> Language constructs will be explained using the usual extended BNF notation, in which {<i>a</i>} means 0 or more <i>a</i>'s, and [<i>a</i>] means an optional <i>a</i>. Non-terminals are shown like non-terminal, keywords are shown like <b>kword</b>, and other terminal symbols are shown like ‘<b>=</b>’. The complete syntax of Luan can be found in <a href="#9">§9</a> at the end of this manual. <h3 margin-top="1em"><a name="lex">Lexical Conventions</a></h3> <p> Luan ignores spaces and comments between lexical elements (tokens), except as delimiters between names and keywords. Luan generally considers the end of a line to be the end of a statement. This catches errors and encourages readability. The exception to this is in paranthesis ( <i>(...)</i>, <i>[...]</i>, and <i>{...}</i> ) where the end of line is treated as white space. <p> <i>Names</i> (also called <i>identifiers</i>) in Luan can be any string of letters, digits, and underscores, not beginning with a digit. Identifiers are used to name variables, table fields, and labels. <p> The following <i>keywords</i> are reserved and cannot be used as names: <p><tt><pre> and break do else elseif end false for function goto if in local nil not or repeat return then true until while </pre></tt></p> <p> Luan is a case-sensitive language: <tt>and</tt> is a reserved word, but <tt>And</tt> and <tt>AND</tt> are two different, valid names. <p> The following strings denote other tokens: <p><tt><pre> + - * / % ^ # & ~ | << >> // == ~= <= >= < > = ( ) { } [ ] :: ; : , . .. ... </pre></tt></p> <p> <i>Literal strings</i> can be delimited by matching single or double quotes, and can contain the following C-like escape sequences: '<tt>\a</tt>' (bell), '<tt>\b</tt>' (backspace), '<tt>\f</tt>' (form feed), '<tt>\n</tt>' (newline), '<tt>\r</tt>' (carriage return), '<tt>\t</tt>' (horizontal tab), '<tt>\v</tt>' (vertical tab), '<tt>\\</tt>' (backslash), '<tt>\"</tt>' (quotation mark [double quote]), and '<tt>\'</tt>' (apostrophe [single quote]). A backslash followed by a real newline results in a newline in the string. The escape sequence '<tt>\z</tt>' skips the following span of white-space characters, including line breaks; it is particularly useful to break and indent a long literal string into multiple lines without adding the newlines and spaces into the string contents. <p> Luan can specify any character in a literal string by its numerical value. This can be done with the escape sequence <tt>\x<i>XX</i></tt>, where <i>XX</i> is a sequence of exactly two hexadecimal digits, or with the escape sequence <tt>\<i>ddd</i></tt>, where <i>ddd</i> is a sequence of up to three decimal digits. (Note that if a decimal escape sequence is to be followed by a digit, it must be expressed using exactly three digits.) <p> Literal strings can also be defined using a long format enclosed by <i>long brackets</i>. We define an <i>opening long bracket of level <i>n</i></i> as an opening square bracket followed by <i>n</i> equal signs followed by another opening square bracket. So, an opening long bracket of level 0 is written as <tt>[[</tt>, an opening long bracket of level 1 is written as <tt>[=[</tt>, and so on. A <i>closing long bracket</i> is defined similarly; for instance, a closing long bracket of level 4 is written as <tt>]====]</tt>. A <i>long literal</i> starts with an opening long bracket of any level and ends at the first closing long bracket of the same level. It can contain any text except a closing bracket of the same level. Literals in this bracketed form can run for several lines, do not interpret any escape sequences, and ignore long brackets of any other level. Any kind of end-of-line sequence (carriage return, newline, carriage return followed by newline, or newline followed by carriage return) is converted to a simple newline. <p> Any character in a literal string not explicitly affected by the previous rules represents itself. However, Luan opens files for parsing in text mode, and the system file functions may have problems with some control characters. So, it is safer to represent non-text data as a quoted literal with explicit escape sequences for non-text characters. <p> For convenience, when the opening long bracket is immediately followed by a newline, the newline is not included in the string. As an example the five literal strings below denote the same string: <p><tt><pre> a = 'alo\n123"' a = "alo\n123\"" a = '\97lo\10\04923"' a = [[alo 123"]] a = [==[ alo 123"]==] </pre></tt></p> <p> A <i>numerical constant</i> (or <i>numeral</i>) can be written with an optional fractional part and an optional decimal exponent, marked by a letter '<tt>e</tt>' or '<tt>E</tt>'. Luan also accepts hexadecimal constants, which start with <tt>0x</tt> or <tt>0X</tt>. Hexadecimal constants also accept an optional fractional part plus an optional binary exponent, marked by a letter '<tt>p</tt>' or '<tt>P</tt>'. A numeric constant with a fractional dot or an exponent denotes a float; otherwise it denotes an integer. Examples of valid integer constants are <p><tt><pre> 3 345 0xff 0xBEBADA </pre></tt></p> <p> Examples of valid float constants are <p><tt><pre> 3.0 3.1416 314.16e-2 0.31416E1 34e1 0x0.1E 0xA23p-4 0X1.921FB54442D18P+1 </pre></tt></p> <p> A <i>comment</i> starts with a double hyphen (<tt>--</tt>) anywhere outside a string. If the text immediately after <tt>--</tt> is not an opening long bracket, the comment is a <i>short comment</i>, which runs until the end of the line. Otherwise, it is a <i>long comment</i>, which runs until the corresponding closing long bracket. Long comments are frequently used to disable code temporarily. <h3 margin-top="1em"><a name="vars">Variables</a></h3> <p> Variables are places that store values. There are three kinds of variables in Luan: global variables, local variables, and table fields. <p> A single name can denote a global variable or a local variable (or a function's formal parameter, which is a particular kind of local variable): <p><tt><pre> var ::= Name </pre></tt></p> <p> Name denotes identifiers, as defined in <a href="#3.1">§3.1</a>. <p> Any variable name is assumed to be global unless explicitly declared as a local (see <a href="#3.3.7">§3.3.7</a>). Local variables are <i>lexically scoped</i>: local variables can be freely accessed by functions defined inside their scope (see <a href="#3.5">§3.5</a>). <p> Before the first assignment to a variable, its value is <b>nil</b>. <p> Square brackets are used to index a table: <p><tt><pre> var ::= prefixexp ‘<b>[</b>’ exp ‘<b>]</b>’ </pre></tt></p> <p> The meaning of accesses to table fields can be changed via metatables. An access to an indexed variable <tt>t[i]</tt> is equivalent to a call <tt>gettable_event(t,i)</tt>. (See <a href="#2.4">§2.4</a> for a complete description of the <tt>gettable_event</tt> function. This function is not defined or callable in Luan. We use it here only for explanatory purposes.) <p> The syntax <tt>var.Name</tt> is just syntactic sugar for <tt>var["Name"]</tt>: <p><tt><pre> var ::= prefixexp ‘<b>.</b>’ Name </pre></tt></p> <p> An access to a global variable <tt>x</tt> is equivalent to <tt>_ENV.x</tt>. Due to the way that chunks are compiled, <tt>_ENV</tt> is never a global name (see <a href="#2.2">§2.2</a>). <h3 margin-top="1em"><a name="stmts">Statements</a></h3> <p> Luan supports an almost conventional set of statements, similar to those in Pascal or C. This set includes assignments, control structures, function calls, and variable declarations. <h4 margin-top="1em"><a name="blocks">Blocks</a></h4> <p> A block is a list of statements, which are executed sequentially: <p><tt><pre> block ::= {stat} </pre></tt></p> <p> Luan has <i>empty statements</i> that allow you to separate statements with semicolons, start a block with a semicolon or write two semicolons in sequence: <p><tt><pre> stat ::= ‘<b>;</b>’ </pre></tt></p> <p> A block can be explicitly delimited to produce a single statement: <p><tt><pre> stat ::= <b>do</b> block <b>end</b> </pre></tt></p> <p> Explicit blocks are useful to control the scope of variable declarations. Explicit blocks are also sometimes used to add a <b>return</b> statement in the middle of another block (see <a href="#3.3.4">§3.3.4</a>). <h4 margin-top="1em"><a name="chunks">Chunks</a></h4> <p> The unit of compilation of Luan is called a <i>chunk</i>. Syntactically, a chunk is simply a block: <p><tt><pre> chunk ::= block </pre></tt></p> <p> Luan handles a chunk as the body of an anonymous function with a variable number of arguments (see <a href="#3.4.11">§3.4.11</a>). As such, chunks can define local variables, receive arguments, and return values. <p> A chunk can be stored in a file or in a string inside the host program. To execute a chunk, Luan first <i>loads</i> it, compiling the chunk's code, and then Luan executes the compiled code. <h4 margin-top="1em"><a name="assignment">Assignment</a></h4> <p> Luan allows multiple assignments. Therefore, the syntax for assignment defines a list of variables on the left side and a list of expressions on the right side. The elements in both lists are separated by commas: <p><tt><pre> stat ::= varlist ‘<b>=</b>’ explist varlist ::= var {‘<b>,</b>’ var} explist ::= exp {‘<b>,</b>’ exp} </pre></tt></p> <p> Expressions are discussed in <a href="#3.4">§3.4</a>. <p> Before the assignment, the list of values is <i>adjusted</i> to the length of the list of variables. If there are more values than needed, the excess values are thrown away. If there are fewer values than needed, the list is extended with as many <b>nil</b>'s as needed. If the list of expressions ends with a function call, then all values returned by that call enter the list of values, before the adjustment (except when the call is enclosed in parentheses; see <a href="#3.4">§3.4</a>). <p> The assignment statement first evaluates all its expressions and only then the assignments are performed. Thus the code <p><tt><pre> i = 3 i, a[i] = i+1, 20 </pre></tt></p> <p> sets <tt>a[3]</tt> to 20, without affecting <tt>a[4]</tt> because the <tt>i</tt> in <tt>a[i]</tt> is evaluated (to 3) before it is assigned 4. Similarly, the line <p><tt><pre> x, y = y, x </pre></tt></p> <p> exchanges the values of <tt>x</tt> and <tt>y</tt>, and <p><tt><pre> x, y, z = y, z, x </pre></tt></p> <p> cyclically permutes the values of <tt>x</tt>, <tt>y</tt>, and <tt>z</tt>. <p> The meaning of assignments to global variables and table fields can be changed via metatables. An assignment to an indexed variable <tt>t[i] = val</tt> is equivalent to <tt>settable_event(t,i,val)</tt>. (See <a href="#2.4">§2.4</a> for a complete description of the <tt>settable_event</tt> function. This function is not defined or callable in Luan. We use it here only for explanatory purposes.) <p> An assignment to a global name <tt>x = val</tt> is equivalent to the assignment <tt>_ENV.x = val</tt> (see <a href="#2.2">§2.2</a>). <h4 margin-top="1em"><a name="control">Control Structures</a></h4> <p> The control structures <b>if</b>, <b>while</b>, and <b>repeat</b> have the usual meaning and familiar syntax: <p><tt><pre> stat ::= <b>while</b> exp <b>do</b> block <b>end</b> stat ::= <b>repeat</b> block <b>until</b> exp stat ::= <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b> </pre></tt></p> <p> Luan also has a <b>for</b> statement (see <a href="#3.3.5">§3.3.5</a>). <p> The condition expression of a control structure must be a boolean. Any other value type will produce an error. This helps catch errors and makes code more readable. <p> In the <b>repeat</b>–<b>until</b> loop, the inner block does not end at the <b>until</b> keyword, but only after the condition. So, the condition can refer to local variables declared inside the loop block. <p> The <b>break</b> statement terminates the execution of a <b>while</b>, <b>repeat</b>, or <b>for</b> loop, skipping to the next statement after the loop: <p><tt><pre> stat ::= <b>break</b> </pre></tt></p> <p> A <b>break</b> ends the innermost enclosing loop. <p> The <b>return</b> statement is used to return values from a function or a chunk (which is an anonymous function). Functions can return more than one value, so the syntax for the <b>return</b> statement is <p><tt><pre> stat ::= <b>return</b> [explist] [‘<b>;</b>’] </pre></tt></p> <h4 margin-top="1em"><a name="for">For Statement</a></h4> <p> The <b>for</b> statement works over functions, called <i>iterators</i>. On each iteration, the iterator function is called to produce a new value, stopping when this new value is <b>nil</b>. The <b>for</b> loop has the following syntax: <p><tt><pre> stat ::= <b>for</b> namelist <b>in</b> exp <b>do</b> block <b>end</b> namelist ::= Name {‘<b>,</b>’ Name} </pre></tt></p> <p> A <b>for</b> statement like <p><tt><pre> for <i>var_1</i>, ···, <i>var_n</i> in <i>exp</i> do <i>block</i> end </pre></tt></p> <p> is equivalent to the code: <p><tt><pre> do local <i>f</i> = <i>exp</i> while true do local <i>var_1</i>, ···, <i>var_n</i> = <i>f</i>() if <i>var_1</i> == nil then break end <i>block</i> end end </pre></tt></p> <p> Note the following: <ul> <li> <tt><i>exp</i></tt> is evaluated only once. Its result is an <i>iterator</i> function. </li> <li> <tt><i>f</i></tt> is an invisible variable. The name is here for explanatory purposes only. </li> <li> You can use <b>break</b> to exit a <b>for</b> loop. </li> <li> The loop variables <tt><i>var_i</i></tt> are local to the loop; you cannot use their values after the <b>for</b> ends. If you need these values, then assign them to other variables before breaking or exiting the loop. </li> </ul> <h4 margin-top="1em"><a name="fn_stmt">Function Calls as Statements</a></h4> <p> To allow possible side-effects, function calls can be executed as statements: <p><tt><pre> stat ::= functioncall </pre></tt></p> <p> In this case, all returned values are thrown away. Function calls are explained in <a href="#3.4.10">§3.4.10</a>. <h4 margin-top="1em"><a name="local_stmt">Local Declarations</a></h4> <p> Local variables can be declared anywhere inside a block. The declaration can include an initial assignment: <p><tt><pre> stat ::= <b>local</b> namelist [‘<b>=</b>’ explist] </pre></tt></p> <p> If present, an initial assignment has the same semantics of a multiple assignment (see <a href="#3.3.3">§3.3.3</a>). Otherwise, all variables are initialized with <b>nil</b>. <p> A chunk is also a block (see <a href="#3.3.2">§3.3.2</a>), and so local variables can be declared in a chunk outside any explicit block. <p> The visibility rules for local variables are explained in <a href="#3.5">§3.5</a>. <h3 margin-top="1em"><a name="expressions">Expressions</a></h3> <p> The basic expressions in Luan are the following: <p><tt><pre> exp ::= prefixexp exp ::= <b>nil</b> | <b>false</b> | <b>true</b> exp ::= Numeral exp ::= LiteralString exp ::= functiondef exp ::= tableconstructor exp ::= ‘<b>...</b>’ exp ::= exp binop exp exp ::= unop exp prefixexp ::= var | functioncall | ‘<b>(</b>’ exp ‘<b>)</b>’ </pre></tt></p> <p> Numerals and literal strings are explained in <a href="#3.1">§3.1</a>; variables are explained in <a href="#3.2">§3.2</a>; function definitions are explained in <a href="#3.4.11">§3.4.11</a>; function calls are explained in <a href="#3.4.10">§3.4.10</a>; table constructors are explained in <a href="#3.4.9">§3.4.9</a>. Vararg expressions, denoted by three dots ('<tt>...</tt>'), can only be used when directly inside a vararg function; they are explained in <a href="#3.4.11">§3.4.11</a>. <p> Binary operators comprise arithmetic operators (see <a href="#3.4.1">§3.4.1</a>), relational operators (see <a href="#3.4.4">§3.4.4</a>), logical operators (see <a href="#3.4.5">§3.4.5</a>), and the concatenation operator (see <a href="#3.4.6">§3.4.6</a>). Unary operators comprise the unary minus (see <a href="#3.4.1">§3.4.1</a>), the unary logical <b>not</b> (see <a href="#3.4.5">§3.4.5</a>), and the unary <em>length operator</em> (see <a href="#3.4.7">§3.4.7</a>). <p> Both function calls and vararg expressions can result in multiple values. If a function call is used as a statement (see <a href="#3.3.6">§3.3.6</a>), then its return list is adjusted to zero elements, thus discarding all returned values. If an expression is used as the last (or the only) element of a list of expressions, then no adjustment is made (unless the expression is enclosed in parentheses). In all other contexts, Luan adjusts the result list to one element, either discarding all values except the first one or adding a single <b>nil</b> if there are no values. <p> Here are some examples: <p><tt><pre> f() -- adjusted to 0 results g(f(), x) -- f() is adjusted to 1 result g(x, f()) -- g gets x plus all results from f() a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil) a,b = ... -- a gets the first vararg parameter, b gets -- the second (both a and b can get nil if there -- is no corresponding vararg parameter) a,b,c = x, f() -- f() is adjusted to 2 results a,b,c = f() -- f() is adjusted to 3 results return f() -- returns all results from f() return ... -- returns all received vararg parameters return x,y,f() -- returns x, y, and all results from f() {f()} -- creates a list with all results from f() {...} -- creates a list with all vararg parameters {f(), nil} -- f() is adjusted to 1 result </pre></tt></p> <p> Any expression enclosed in parentheses always results in only one value. Thus, <tt>(f(x,y,z))</tt> is always a single value, even if <tt>f</tt> returns several values. (The value of <tt>(f(x,y,z))</tt> is the first value returned by <tt>f</tt> or <b>nil</b> if <tt>f</tt> does not return any values.) <h4 margin-top="1em"><a name="arithmetic">Arithmetic Operators</a></h4> <p> Luan supports the following arithmetic operators: <ul> <li><b><tt>+</tt>: </b>addition</li> <li><b><tt>-</tt>: </b>subtraction</li> <li><b><tt>*</tt>: </b>multiplication</li> <li><b><tt>/</tt>: </b>division</li> <li><b><tt>%</tt>: </b>modulo</li> <li><b><tt>^</tt>: </b>exponentiation</li> <li><b><tt>-</tt>: </b>unary minus</li> </ul> <p> Addition, subtraction, multiplication, division, and unary minus are the same as these operators in Java. Exponentiation uses Java's <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#pow(double,%20double)">Math.pow</a> function. <p> Modulo is defined as the remainder of a division that rounds the quotient towards minus infinite (floor division). (The Java modulo operator is not used.) <h4 margin-top="1em"><a name="conversions">Coercions and Conversions</a></h4> <p> Luan provides some automatic conversions between some types and representations at run time. String concatenation converts all of its arguments to strings. Luan also converts strings to numbers whenever a number is expected. <h4 margin-top="1em"><a name="relational">Relational Operators</a></h4> <p> Luan supports the following relational operators: <ul> <li><b><tt>==</tt>: </b>equality</li> <li><b><tt>~=</tt>: </b>inequality</li> <li><b><tt><</tt>: </b>less than</li> <li><b><tt>></tt>: </b>greater than</li> <li><b><tt><=</tt>: </b>less or equal</li> <li><b><tt>>=</tt>: </b>greater or equal</li> </ul><p> These operators always result in <b>false</b> or <b>true</b>. <p> Equality (<tt>==</tt>) first compares the type of its operands. If the types are different, then the result is <b>false</b>. Otherwise, the values of the operands are compared. Strings, numbers, and binary values are compared in the obvious way (by value). <p> Tables are compared by reference: two objects are considered equal only if they are the same object. Every time you create a new table, it is different from any previously existing table. Closures are also compared by reference. <p> You can change the way that Luan compares tables by using the "eq" metamethod (see <a href="#2.4">§2.4</a>). <p> Userdata are Java objects. They are compared for equality with the Java <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#equals(java.lang.Object)"><tt>equals</tt></a> method. <p> Equality comparisons do not convert strings to numbers or vice versa. Thus, <tt>"0"==0</tt> evaluates to <b>false</b>, and <tt>t[0]</tt> and <tt>t["0"]</tt> denote different entries in a table. <p> The operator <tt>~=</tt> is exactly the negation of equality (<tt>==</tt>). <p> The order operators work as follows. If both arguments are numbers, then they are compared following the usual rule for binary operations. Otherwise, if both arguments are strings, then their values are compared according to the current locale. Otherwise, Luan tries to call the "lt" or the "le" metamethod (see <a href="#2.4">§2.4</a>). A comparison <tt>a > b</tt> is translated to <tt>b < a</tt> and <tt>a >= b</tt> is translated to <tt>b <= a</tt>. <h4 margin-top="1em"><a name="logical_ops">Logical Operators</a></h4> <p> The logical operators in Luan are <b>and</b>, <b>or</b>, and <b>not</b>. The <b>and</b> and <b>or</b> operators consider both <b>false</b> and <b>nil</b> as false and anything else as true. Like the control structures (see <a href="#3.3.4">§3.3.4</a>), the <b>not</b> operator requires a boolean value. <p> The negation operator <b>not</b> always returns <b>false</b> or <b>true</b>. The conjunction operator <b>and</b> returns its first argument if this value is <b>false</b> or <b>nil</b>; otherwise, <b>and</b> returns its second argument. The disjunction operator <b>or</b> returns its first argument if this value is different from <b>nil</b> and <b>false</b>; otherwise, <b>or</b> returns its second argument. Both <b>and</b> and <b>or</b> use short-circuit evaluation; that is, the second operand is evaluated only if necessary. Here are some examples: <p><tt><pre> 10 or 20 --> 10 10 or error() --> 10 nil or "a" --> "a" nil and 10 --> nil false and error() --> false false and nil --> false false or nil --> nil 10 and 20 --> 20 </pre></tt></p> <p> (In this manual, <tt>--></tt> indicates the result of the preceding expression.) <h4 margin-top="1em"><a name="concatenation">Concatenation</a></h4> <p> The string concatenation operator in Luan is denoted by two dots ('<tt>..</tt>'). All operands are converted to strings. <h4 margin-top="1em"><a name="length">The Length Operator</a></h4> <p> The length operator is denoted by the unary prefix operator <tt>#</tt>. The length of a string is its number of characters. The length of a binary is its number of bytes. <p> A program can modify the behavior of the length operator for any table through the <tt>__len</tt> metamethod (see <a href="#2.4">§2.4</a>). <p> Unless a <tt>__len</tt> metamethod is given, the length of a table <tt>t</tt> is defined as the number of elements in <em>sequence</em>, that is, the size of the set of its positive numeric keys is equal to <em>{1..n}</em> for some non-negative integer <em>n</em>. In that case, <em>n</em> is its length. Note that a table like <p><tt><pre> {10, 20, nil, 40} </pre></tt></p> <p> has a length of <tt>2</tt>, because that is the last key in sequence. <h4 margin-top="1em"><a name="precedence">Precedence</a></h4> <p> Operator precedence in Luan follows the table below, from lower to higher priority: <p><tt><pre> or and < > <= >= ~= == .. + - * / % unary operators (not # -) ^ </pre></tt></p> <p> As usual, you can use parentheses to change the precedences of an expression. The concatenation ('<tt>..</tt>') and exponentiation ('<tt>^</tt>') operators are right associative. All other binary operators are left associative. <h4 margin-top="1em"><a name="constructors">Table Constructors</a></h4> <p> Table constructors are expressions that create tables. Every time a constructor is evaluated, a new table is created. A constructor can be used to create an empty table or to create a table and initialize some of its fields. The general syntax for constructors is <p><tt><pre> tableconstructor ::= ‘<b>{</b>’ [fieldlist] ‘<b>}</b>’ fieldlist ::= field {fieldsep field} [fieldsep] field ::= ‘<b>[</b>’ exp ‘<b>]</b>’ ‘<b>=</b>’ exp | Name ‘<b>=</b>’ exp | exp fieldsep ::= ‘<b>,</b>’ | ‘<b>;</b>’ </pre></tt></p> <p> Each field of the form <tt>[exp1] = exp2</tt> adds to the new table an entry with key <tt>exp1</tt> and value <tt>exp2</tt>. A field of the form <tt>name = exp</tt> is equivalent to <tt>["name"] = exp</tt>. Finally, fields of the form <tt>exp</tt> are equivalent to <tt>[i] = exp</tt>, where <tt>i</tt> are consecutive integers starting with 1. Fields in the other formats do not affect this counting. For example, <p><tt><pre> a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 } </pre></tt></p> <p> is equivalent to <p><tt><pre> do local t = {} t[f(1)] = g t[1] = "x" -- 1st exp t[2] = "y" -- 2nd exp t.x = 1 -- t["x"] = 1 t[3] = f(x) -- 3rd exp t[30] = 23 t[4] = 45 -- 4th exp a = t end </pre></tt></p> <p> The order of the assignments in a constructor is undefined. (This order would be relevant only when there are repeated keys.) <p> If the last field in the list has the form <tt>exp</tt> and the expression is a function call or a vararg expression, then all values returned by this expression enter the list consecutively (see <a href="#3.4.10">§3.4.10</a>). <p> The field list can have an optional trailing separator, as a convenience for machine-generated code. <h4 margin-top="1em"><a name="fn_calls">Function Calls</a></h4> <p> A function call in Luan has the following syntax: <p><tt><pre> functioncall ::= prefixexp args </pre></tt></p> <p> In a function call, first prefixexp and args are evaluated. If the value of prefixexp has type <i>function</i>, then this function is called with the given arguments. Otherwise, the prefixexp "call" metamethod is called, having as first parameter the value of prefixexp, followed by the original call arguments (see <a href="#2.4">§2.4</a>). <p> Arguments have the following syntax: <p><tt><pre> args ::= ‘<b>(</b>’ [explist] ‘<b>)</b>’ args ::= tableconstructor args ::= LiteralString </pre></tt></p> <p> All argument expressions are evaluated before the call. A call of the form <tt>f{<i>fields</i>}</tt> is syntactic sugar for <tt>f({<i>fields</i>})</tt>; that is, the argument list is a single new table. A call of the form <tt>f'<i>string</i>'</tt> (or <tt>f"<i>string</i>"</tt> or <tt>f[[<i>string</i>]]</tt>) is syntactic sugar for <tt>f('<i>string</i>')</tt>; that is, the argument list is a single literal string. <p> A call of the form <tt>return <i>functioncall</i></tt> is called a <i>tail call</i>. Luan implements <i>proper tail calls</i> (or <i>proper tail recursion</i>): in a tail call, the called function reuses the stack entry of the calling function. Therefore, there is no limit on the number of nested tail calls that a program can execute. However, a tail call erases any debug information about the calling function. Note that a tail call only happens with a particular syntax, where the <b>return</b> has one single function call as argument; this syntax makes the calling function return exactly the returns of the called function. So, none of the following examples are tail calls: <p><tt><pre> return (f(x)) -- results adjusted to 1 return 2 * f(x) return x, f(x) -- additional results f(x); return -- results discarded return x or f(x) -- results adjusted to 1 </pre></tt></p> <h4 margin-top="1em"><a name="fn_def">Function Definitions</a></h4> <p> The syntax for function definition is <p><tt><pre> functiondef ::= <b>function</b> funcbody funcbody ::= ‘<b>(</b>’ [parlist] ‘<b>)</b>’ block <b>end</b> </pre></tt></p> <p> The following syntactic sugar simplifies function definitions: <p><tt><pre> stat ::= <b>function</b> funcname funcbody stat ::= <b>local</b> <b>function</b> Name funcbody funcname ::= Name {‘<b>.</b>’ Name} [‘<b>:</b>’ Name] </pre></tt></p> <p> The statement <p><tt><pre> function f () <i>body</i> end </pre></tt></p> <p> translates to <p><tt><pre> f = function () <i>body</i> end </pre></tt></p> <p> The statement <p><tt><pre> function t.a.b.c.f () <i>body</i> end </pre></tt></p> <p> translates to <p><tt><pre> t.a.b.c.f = function () <i>body</i> end </pre></tt></p> <p> The statement <p><tt><pre> local function f () <i>body</i> end </pre></tt></p> <p> translates to <p><tt><pre> local f; f = function () <em>body</em> end </pre></tt></p> <p> not to <p><tt><pre> local f = function () <em>body</em> end </pre></tt></p> <p> (This only makes a difference when the body of the function contains references to <tt>f</tt>.) <p> A function definition is an executable expression, whose value has type <i>function</i>. When Luan precompiles a chunk, all its function bodies are precompiled too. Then, whenever Luan executes the function definition, the function is <i>instantiated</i> (or <i>closed</i>). This function instance (or <i>closure</i>) is the final value of the expression. <p> Parameters act as local variables that are initialized with the argument values: <p><tt><pre> parlist ::= namelist [‘<b>,</b>’ ‘<b>...</b>’] | ‘<b>...</b>’ </pre></tt></p> <p> When a function is called, the list of arguments is adjusted to the length of the list of parameters, unless the function is a <i>vararg function</i>, which is indicated by three dots ('<tt>...</tt>') at the end of its parameter list. A vararg function does not adjust its argument list; instead, it collects all extra arguments and supplies them to the function through a <i>vararg expression</i>, which is also written as three dots. The value of this expression is a list of all actual extra arguments, similar to a function with multiple results. If a vararg expression is used inside another expression or in the middle of a list of expressions, then its return list is adjusted to one element. If the expression is used as the last element of a list of expressions, then no adjustment is made (unless that last expression is enclosed in parentheses). <p> As an example, consider the following definitions: <p><tt><pre> function f(a, b) end function g(a, b, ...) end function r() return 1,2,3 end </pre></tt></p> <p> Then, we have the following mapping from arguments to parameters and to the vararg expression: <p><tt><pre> CALL PARAMETERS f(3) a=3, b=nil f(3, 4) a=3, b=4 f(3, 4, 5) a=3, b=4 f(r(), 10) a=1, b=10 f(r()) a=1, b=2 g(3) a=3, b=nil, ... --> (nothing) g(3, 4) a=3, b=4, ... --> (nothing) g(3, 4, 5, 8) a=3, b=4, ... --> 5 8 g(5, r()) a=5, b=1, ... --> 2 3 </pre></tt></p> <p> Results are returned using the <b>return</b> statement (see <a href="#3.3.4">§3.3.4</a>). If control reaches the end of a function without encountering a <b>return</b> statement, then the function returns with no results. <h3 margin-top="1em"><a name="visibility">Visibility Rules</a></h3> <p> Luan is a lexically scoped language. The scope of a local variable begins at the first statement after its declaration and lasts until the last non-void statement of the innermost block that includes the declaration. Consider the following example: <p><tt><pre> x = 10 -- global variable do -- new block local x = x -- new 'x', with value 10 print(x) --> 10 x = x+1 do -- another block local x = x+1 -- another 'x' print(x) --> 12 end print(x) --> 11 end print(x) --> 10 (the global one) </pre></tt></p> <p> Notice that, in a declaration like <tt>local x = x</tt>, the new <tt>x</tt> being declared is not in scope yet, and so the second <tt>x</tt> refers to the outside variable. <p> Because of the lexical scoping rules, local variables can be freely accessed by functions defined inside their scope. A local variable used by an inner function is called an <i>upvalue</i>, or <i>external local variable</i>, inside the inner function. <p> Notice that each execution of a <b>local</b> statement defines new local variables. Consider the following example: <p><tt><pre> a = {} local x = 20 for i=1,10 do local y = 0 a[i] = function () y=y+1; return x+y end end </pre></tt></p> <p> The loop creates ten closures (that is, ten instances of the anonymous function). Each of these closures uses a different <tt>y</tt> variable, while all of them share the same <tt>x</tt>. <h1>6 – <a name="6">Standard Libraries</a></h1> <p> The standard Lua libraries provide useful functions that are implemented directly through the C API. Some of these functions provide essential services to the language (e.g., <a href="#pdf-type"><code>type</code></a> and <a href="#pdf-getmetatable"><code>getmetatable</code></a>); others provide access to "outside" services (e.g., I/O); and others could be implemented in Lua itself, but are quite useful or have critical performance requirements that deserve an implementation in C (e.g., <a href="#pdf-table.sort"><code>table.sort</code></a>). <p> All libraries are implemented through the official C API and are provided as separate C modules. Currently, Lua has the following standard libraries: <ul> <li>basic library (<a href="#6.1">§6.1</a>);</li> <li>coroutine library (<a href="#6.2">§6.2</a>);</li> <li>package library (<a href="#6.3">§6.3</a>);</li> <li>string manipulation (<a href="#6.4">§6.4</a>);</li> <li>basic UTF-8 support (<a href="#6.5">§6.5</a>);</li> <li>table manipulation (<a href="#6.6">§6.6</a>);</li> <li>mathematical functions (<a href="#6.7">§6.7</a>) (sin, log, etc.);</li> <li>input and output (<a href="#6.8">§6.8</a>);</li> <li>operating system facilities (<a href="#6.9">§6.9</a>);</li> <li>debug facilities (<a href="#6.10">§6.10</a>).</li> </ul><p> Except for the basic and the package libraries, each library provides all its functions as fields of a global table or as methods of its objects. <p> To have access to these libraries, the C host program should call the <a href="#luaL_openlibs"><code>luaL_openlibs</code></a> function, which opens all standard libraries. Alternatively, the host program can open them individually by using <a href="#luaL_requiref"><code>luaL_requiref</code></a> to call <a name="pdf-luaopen_base"><code>luaopen_base</code></a> (for the basic library), <a name="pdf-luaopen_package"><code>luaopen_package</code></a> (for the package library), <a name="pdf-luaopen_coroutine"><code>luaopen_coroutine</code></a> (for the coroutine library), <a name="pdf-luaopen_string"><code>luaopen_string</code></a> (for the string library), <a name="pdf-luaopen_utf8"><code>luaopen_utf8</code></a> (for the UTF8 library), <a name="pdf-luaopen_table"><code>luaopen_table</code></a> (for the table library), <a name="pdf-luaopen_math"><code>luaopen_math</code></a> (for the mathematical library), <a name="pdf-luaopen_io"><code>luaopen_io</code></a> (for the I/O library), <a name="pdf-luaopen_os"><code>luaopen_os</code></a> (for the operating system library), and <a name="pdf-luaopen_debug"><code>luaopen_debug</code></a> (for the debug library). These functions are declared in <a name="pdf-lualib.h"><code>lualib.h</code></a>. <h2>6.1 – <a name="6.1">Basic Functions</a></h2> <p> The basic library provides core functions to Lua. If you do not include this library in your application, you should check carefully whether you need to provide implementations for some of its facilities. <p> <hr><h3><a name="pdf-assert"><code>assert (v [, message])</code></a></h3> <p> Calls <a href="#pdf-error"><code>error</code></a> if the value of its argument <code>v</code> is false (i.e., <b>nil</b> or <b>false</b>); otherwise, returns all its arguments. In case of error, <code>message</code> is the error object; when absent, it defaults to "<code>assertion failed!</code>" <p> <hr><h3><a name="pdf-collectgarbage"><code>collectgarbage ([opt [, arg]])</code></a></h3> <p> This function is a generic interface to the garbage collector. It performs different functions according to its first argument, <code>opt</code>: <ul> <li><b>"<code>collect</code>": </b> performs a full garbage-collection cycle. This is the default option. </li> <li><b>"<code>stop</code>": </b> stops automatic execution of the garbage collector. The collector will run only when explicitly invoked, until a call to restart it. </li> <li><b>"<code>restart</code>": </b> restarts automatic execution of the garbage collector. </li> <li><b>"<code>count</code>": </b> returns the total memory in use by Lua in Kbytes. The value has a fractional part, so that it multiplied by 1024 gives the exact number of bytes in use by Lua (except for overflows). </li> <li><b>"<code>step</code>": </b> performs a garbage-collection step. The step "size" is controlled by <code>arg</code>. With a zero value, the collector will perform one basic (indivisible) step. For non-zero values, the collector will perform as if that amount of memory (in KBytes) had been allocated by Lua. Returns <b>true</b> if the step finished a collection cycle. </li> <li><b>"<code>setpause</code>": </b> sets <code>arg</code> as the new value for the <em>pause</em> of the collector (see <a href="#2.5">§2.5</a>). Returns the previous value for <em>pause</em>. </li> <li><b>"<code>setstepmul</code>": </b> sets <code>arg</code> as the new value for the <em>step multiplier</em> of the collector (see <a href="#2.5">§2.5</a>). Returns the previous value for <em>step</em>. </li> <li><b>"<code>isrunning</code>": </b> returns a boolean that tells whether the collector is running (i.e., not stopped). </li> </ul> <p> <hr><h3><a name="pdf-dofile"><code>dofile ([filename])</code></a></h3> Opens the named file and executes its contents as a Lua chunk. When called without arguments, <code>dofile</code> executes the contents of the standard input (<code>stdin</code>). Returns all values returned by the chunk. In case of errors, <code>dofile</code> propagates the error to its caller (that is, <code>dofile</code> does not run in protected mode). <p> <hr><h3><a name="pdf-error"><code>error (message [, level])</code></a></h3> Terminates the last protected function called and returns <code>message</code> as the error object. Function <code>error</code> never returns. <p> Usually, <code>error</code> adds some information about the error position at the beginning of the message, if the message is a string. The <code>level</code> argument specifies how to get the error position. With level 1 (the default), the error position is where the <code>error</code> function was called. Level 2 points the error to where the function that called <code>error</code> was called; and so on. Passing a level 0 avoids the addition of error position information to the message. <p> <hr><h3><a name="pdf-_G"><code>_G</code></a></h3> A global variable (not a function) that holds the global environment (see <a href="#2.2">§2.2</a>). Lua itself does not use this variable; changing its value does not affect any environment, nor vice versa. <p> <hr><h3><a name="pdf-getmetatable"><code>getmetatable (object)</code></a></h3> <p> If <code>object</code> does not have a metatable, returns <b>nil</b>. Otherwise, if the object's metatable has a <code>"__metatable"</code> field, returns the associated value. Otherwise, returns the metatable of the given object. <p> <hr><h3><a name="pdf-ipairs"><code>ipairs (t)</code></a></h3> <p> Returns three values (an iterator function, the table <code>t</code>, and 0) so that the construction <pre> for i,v in ipairs(t) do <em>body</em> end </pre><p> will iterate over the key–value pairs (<code>1,t[1]</code>), (<code>2,t[2]</code>), ..., up to the first nil value. <p> <hr><h3><a name="pdf-load"><code>load (chunk [, chunkname [, mode [, env]]])</code></a></h3> <p> Loads a chunk. <p> If <code>chunk</code> is a string, the chunk is this string. If <code>chunk</code> is a function, <code>load</code> calls it repeatedly to get the chunk pieces. Each call to <code>chunk</code> must return a string that concatenates with previous results. A return of an empty string, <b>nil</b>, or no value signals the end of the chunk. <p> If there are no syntactic errors, returns the compiled chunk as a function; otherwise, returns <b>nil</b> plus the error message. <p> If the resulting function has upvalues, the first upvalue is set to the value of <code>env</code>, if that parameter is given, or to the value of the global environment. Other upvalues are initialized with <b>nil</b>. (When you load a main chunk, the resulting function will always have exactly one upvalue, the <code>_ENV</code> variable (see <a href="#2.2">§2.2</a>). However, when you load a binary chunk created from a function (see <a href="#pdf-string.dump"><code>string.dump</code></a>), the resulting function can have an arbitrary number of upvalues.) All upvalues are fresh, that is, they are not shared with any other function. <p> <code>chunkname</code> is used as the name of the chunk for error messages and debug information (see <a href="#4.9">§4.9</a>). When absent, it defaults to <code>chunk</code>, if <code>chunk</code> is a string, or to "<code>=(load)</code>" otherwise. <p> The string <code>mode</code> controls whether the chunk can be text or binary (that is, a precompiled chunk). It may be the string "<code>b</code>" (only binary chunks), "<code>t</code>" (only text chunks), or "<code>bt</code>" (both binary and text). The default is "<code>bt</code>". <p> Lua does not check the consistency of binary chunks. Maliciously crafted binary chunks can crash the interpreter. <p> <hr><h3><a name="pdf-loadfile"><code>loadfile ([filename [, mode [, env]]])</code></a></h3> <p> Similar to <a href="#pdf-load"><code>load</code></a>, but gets the chunk from file <code>filename</code> or from the standard input, if no file name is given. <p> <hr><h3><a name="pdf-next"><code>next (table [, index])</code></a></h3> <p> Allows a program to traverse all fields of a table. Its first argument is a table and its second argument is an index in this table. <code>next</code> returns the next index of the table and its associated value. When called with <b>nil</b> as its second argument, <code>next</code> returns an initial index and its associated value. When called with the last index, or with <b>nil</b> in an empty table, <code>next</code> returns <b>nil</b>. If the second argument is absent, then it is interpreted as <b>nil</b>. In particular, you can use <code>next(t)</code> to check whether a table is empty. <p> The order in which the indices are enumerated is not specified, <em>even for numeric indices</em>. (To traverse a table in numeric order, use a numerical <b>for</b>.) <p> The behavior of <code>next</code> is undefined if, during the traversal, you assign any value to a non-existent field in the table. You may however modify existing fields. In particular, you may clear existing fields. <p> <hr><h3><a name="pdf-pairs"><code>pairs (t)</code></a></h3> <p> If <code>t</code> has a metamethod <code>__pairs</code>, calls it with <code>t</code> as argument and returns the first three results from the call. <p> Otherwise, returns three values: the <a href="#pdf-next"><code>next</code></a> function, the table <code>t</code>, and <b>nil</b>, so that the construction <pre> for k,v in pairs(t) do <em>body</em> end </pre><p> will iterate over all key–value pairs of table <code>t</code>. <p> See function <a href="#pdf-next"><code>next</code></a> for the caveats of modifying the table during its traversal. <p> <hr><h3><a name="pdf-pcall"><code>pcall (f [, arg1, ···])</code></a></h3> <p> Calls function <code>f</code> with the given arguments in <em>protected mode</em>. This means that any error inside <code>f</code> is not propagated; instead, <code>pcall</code> catches the error and returns a status code. Its first result is the status code (a boolean), which is true if the call succeeds without errors. In such case, <code>pcall</code> also returns all results from the call, after this first result. In case of any error, <code>pcall</code> returns <b>false</b> plus the error message. <p> <hr><h3><a name="pdf-print"><code>print (···)</code></a></h3> Receives any number of arguments and prints their values to <code>stdout</code>, using the <a href="#pdf-tostring"><code>tostring</code></a> function to convert each argument to a string. <code>print</code> is not intended for formatted output, but only as a quick way to show a value, for instance for debugging. For complete control over the output, use <a href="#pdf-string.format"><code>string.format</code></a> and <a href="#pdf-io.write"><code>io.write</code></a>. <p> <hr><h3><a name="pdf-rawequal"><code>rawequal (v1, v2)</code></a></h3> Checks whether <code>v1</code> is equal to <code>v2</code>, without invoking any metamethod. Returns a boolean. <p> <hr><h3><a name="pdf-rawget"><code>rawget (table, index)</code></a></h3> Gets the real value of <code>table[index]</code>, without invoking any metamethod. <code>table</code> must be a table; <code>index</code> may be any value. <p> <hr><h3><a name="pdf-rawlen"><code>rawlen (v)</code></a></h3> Returns the length of the object <code>v</code>, which must be a table or a string, without invoking any metamethod. Returns an integer. <p> <hr><h3><a name="pdf-rawset"><code>rawset (table, index, value)</code></a></h3> Sets the real value of <code>table[index]</code> to <code>value</code>, without invoking any metamethod. <code>table</code> must be a table, <code>index</code> any value different from <b>nil</b> and NaN, and <code>value</code> any Lua value. <p> This function returns <code>table</code>. <p> <hr><h3><a name="pdf-select"><code>select (index, ···)</code></a></h3> <p> If <code>index</code> is a number, returns all arguments after argument number <code>index</code>; a negative number indexes from the end (-1 is the last argument). Otherwise, <code>index</code> must be the string <code>"#"</code>, and <code>select</code> returns the total number of extra arguments it received. <p> <hr><h3><a name="pdf-setmetatable"><code>setmetatable (table, metatable)</code></a></h3> <p> Sets the metatable for the given table. (You cannot change the metatable of other types from Lua, only from C.) If <code>metatable</code> is <b>nil</b>, removes the metatable of the given table. If the original metatable has a <code>"__metatable"</code> field, raises an error. <p> This function returns <code>table</code>. <p> <hr><h3><a name="pdf-tonumber"><code>tonumber (e [, base])</code></a></h3> <p> When called with no <code>base</code>, <code>tonumber</code> tries to convert its argument to a number. If the argument is already a number or a string convertible to a number, then <code>tonumber</code> returns this number; otherwise, it returns <b>nil</b>. <p> The conversion of strings can result in integers or floats, according to the lexical conventions of Lua (see <a href="#3.1">§3.1</a>). (The string may have leading and trailing spaces and a sign.) <p> When called with <code>base</code>, then <code>e</code> must be a string to be interpreted as an integer numeral in that base. The base may be any integer between 2 and 36, inclusive. In bases above 10, the letter '<code>A</code>' (in either upper or lower case) represents 10, '<code>B</code>' represents 11, and so forth, with '<code>Z</code>' representing 35. If the string <code>e</code> is not a valid numeral in the given base, the function returns <b>nil</b>. <p> <hr><h3><a name="pdf-tostring"><code>tostring (v)</code></a></h3> Receives a value of any type and converts it to a string in a human-readable format. Floats always produce strings with some floating-point indication (either a decimal dot or an exponent). (For complete control of how numbers are converted, use <a href="#pdf-string.format"><code>string.format</code></a>.) <p> If the metatable of <code>v</code> has a <code>"__to_string"</code> field, then <code>tostring</code> calls the corresponding value with <code>v</code> as argument, and uses the result of the call as its result. <p> <hr><h3><a name="pdf-type"><code>type (v)</code></a></h3> Returns the type of its only argument, coded as a string. The possible results of this function are "<code>nil</code>" (a string, not the value <b>nil</b>), "<code>number</code>", "<code>string</code>", "<code>boolean</code>", "<code>table</code>", "<code>function</code>", "<code>thread</code>", and "<code>userdata</code>". <p> <hr><h3><a name="pdf-_VERSION"><code>_VERSION</code></a></h3> A global variable (not a function) that holds a string containing the current interpreter version. The current value of this variable is "<code>Lua 5.3</code>". <p> <hr><h3><a name="pdf-xpcall"><code>xpcall (f, msgh [, arg1, ···])</code></a></h3> <p> This function is similar to <a href="#pdf-pcall"><code>pcall</code></a>, except that it sets a new message handler <code>msgh</code>. <h2>6.2 – <a name="6.2">Coroutine Manipulation</a></h2> <p> The operations related to coroutines comprise a sub-library of the basic library and come inside the table <a name="pdf-coroutine"><code>coroutine</code></a>. See <a href="#2.6">§2.6</a> for a general description of coroutines. <p> <hr><h3><a name="pdf-coroutine.create"><code>coroutine.create (f)</code></a></h3> <p> Creates a new coroutine, with body <code>f</code>. <code>f</code> must be a Lua function. Returns this new coroutine, an object with type <code>"thread"</code>. <p> <hr><h3><a name="pdf-coroutine.isyieldable"><code>coroutine.isyieldable ()</code></a></h3> <p> Returns true when the running coroutine can yield. <p> A running coroutine is yieldable if it is not the main thread and it is not inside a non-yieldable C function. <p> <hr><h3><a name="pdf-coroutine.resume"><code>coroutine.resume (co [, val1, ···])</code></a></h3> <p> Starts or continues the execution of coroutine <code>co</code>. The first time you resume a coroutine, it starts running its body. The values <code>val1</code>, ... are passed as the arguments to the body function. If the coroutine has yielded, <code>resume</code> restarts it; the values <code>val1</code>, ... are passed as the results from the yield. <p> If the coroutine runs without any errors, <code>resume</code> returns <b>true</b> plus any values passed to <code>yield</code> (when the coroutine yields) or any values returned by the body function (when the coroutine terminates). If there is any error, <code>resume</code> returns <b>false</b> plus the error message. <p> <hr><h3><a name="pdf-coroutine.running"><code>coroutine.running ()</code></a></h3> <p> Returns the running coroutine plus a boolean, true when the running coroutine is the main one. <p> <hr><h3><a name="pdf-coroutine.status"><code>coroutine.status (co)</code></a></h3> <p> Returns the status of coroutine <code>co</code>, as a string: <code>"running"</code>, if the coroutine is running (that is, it called <code>status</code>); <code>"suspended"</code>, if the coroutine is suspended in a call to <code>yield</code>, or if it has not started running yet; <code>"normal"</code> if the coroutine is active but not running (that is, it has resumed another coroutine); and <code>"dead"</code> if the coroutine has finished its body function, or if it has stopped with an error. <p> <hr><h3><a name="pdf-coroutine.wrap"><code>coroutine.wrap (f)</code></a></h3> <p> Creates a new coroutine, with body <code>f</code>. <code>f</code> must be a Lua function. Returns a function that resumes the coroutine each time it is called. Any arguments passed to the function behave as the extra arguments to <code>resume</code>. Returns the same values returned by <code>resume</code>, except the first boolean. In case of error, propagates the error. <p> <hr><h3><a name="pdf-coroutine.yield"><code>coroutine.yield (···)</code></a></h3> <p> Suspends the execution of the calling coroutine. Any arguments to <code>yield</code> are passed as extra results to <code>resume</code>. <h2>6.3 – <a name="6.3">Modules</a></h2> <p> The package library provides basic facilities for loading modules in Lua. It exports one function directly in the global environment: <a href="#pdf-require"><code>require</code></a>. Everything else is exported in a table <a name="pdf-package"><code>package</code></a>. <p> <hr><h3><a name="pdf-require"><code>require (modname)</code></a></h3> <p> Loads the given module. The function starts by looking into the <a href="#pdf-package.loaded"><code>package.loaded</code></a> table to determine whether <code>modname</code> is already loaded. If it is, then <code>require</code> returns the value stored at <code>package.loaded[modname]</code>. Otherwise, it tries to find a <em>loader</em> for the module. <p> To find a loader, <code>require</code> is guided by the <a href="#pdf-package.searchers"><code>package.searchers</code></a> sequence. By changing this sequence, we can change how <code>require</code> looks for a module. The following explanation is based on the default configuration for <a href="#pdf-package.searchers"><code>package.searchers</code></a>. <p> First <code>require</code> queries <code>package.preload[modname]</code>. If it has a value, this value (which must be a function) is the loader. Otherwise <code>require</code> searches for a Lua loader using the path stored in <a href="#pdf-package.path"><code>package.path</code></a>. If that also fails, it searches for a C loader using the path stored in <a href="#pdf-package.cpath"><code>package.cpath</code></a>. If that also fails, it tries an <em>all-in-one</em> loader (see <a href="#pdf-package.searchers"><code>package.searchers</code></a>). <p> Once a loader is found, <code>require</code> calls the loader with two arguments: <code>modname</code> and an extra value dependent on how it got the loader. (If the loader came from a file, this extra value is the file name.) If the loader returns any non-nil value, <code>require</code> assigns the returned value to <code>package.loaded[modname]</code>. If the loader does not return a non-nil value and has not assigned any value to <code>package.loaded[modname]</code>, then <code>require</code> assigns <b>true</b> to this entry. In any case, <code>require</code> returns the final value of <code>package.loaded[modname]</code>. <p> If there is any error loading or running the module, or if it cannot find any loader for the module, then <code>require</code> raises an error. <p> <hr><h3><a name="pdf-package.config"><code>package.config</code></a></h3> <p> A string describing some compile-time configurations for packages. This string is a sequence of lines: <ul> <li>The first line is the directory separator string. Default is '<code>\</code>' for Windows and '<code>/</code>' for all other systems.</li> <li>The second line is the character that separates templates in a path. Default is '<code>;</code>'.</li> <li>The third line is the string that marks the substitution points in a template. Default is '<code>?</code>'.</li> <li>The fourth line is a string that, in a path in Windows, is replaced by the executable's directory. Default is '<code>!</code>'.</li> <li>The fifth line is a mark to ignore all text after it when building the <code>luaopen_</code> function name. Default is '<code>-</code>'.</li> </ul> <p> <hr><h3><a name="pdf-package.cpath"><code>package.cpath</code></a></h3> <p> The path used by <a href="#pdf-require"><code>require</code></a> to search for a C loader. <p> Lua initializes the C path <a href="#pdf-package.cpath"><code>package.cpath</code></a> in the same way it initializes the Lua path <a href="#pdf-package.path"><code>package.path</code></a>, using the environment variable <a name="pdf-LUA_CPATH_5_3"><code>LUA_CPATH_5_3</code></a> or the environment variable <a name="pdf-LUA_CPATH"><code>LUA_CPATH</code></a> or a default path defined in <code>luaconf.h</code>. <p> <hr><h3><a name="pdf-package.loaded"><code>package.loaded</code></a></h3> <p> A table used by <a href="#pdf-require"><code>require</code></a> to control which modules are already loaded. When you require a module <code>modname</code> and <code>package.loaded[modname]</code> is not false, <a href="#pdf-require"><code>require</code></a> simply returns the value stored there. <p> This variable is only a reference to the real table; assignments to this variable do not change the table used by <a href="#pdf-require"><code>require</code></a>. <p> <hr><h3><a name="pdf-package.loadlib"><code>package.loadlib (libname, funcname)</code></a></h3> <p> Dynamically links the host program with the C library <code>libname</code>. <p> If <code>funcname</code> is "<code>*</code>", then it only links with the library, making the symbols exported by the library available to other dynamically linked libraries. Otherwise, it looks for a function <code>funcname</code> inside the library and returns this function as a C function. So, <code>funcname</code> must follow the <a href="#lua_CFunction"><code>lua_CFunction</code></a> prototype (see <a href="#lua_CFunction"><code>lua_CFunction</code></a>). <p> This is a low-level function. It completely bypasses the package and module system. Unlike <a href="#pdf-require"><code>require</code></a>, it does not perform any path searching and does not automatically adds extensions. <code>libname</code> must be the complete file name of the C library, including if necessary a path and an extension. <code>funcname</code> must be the exact name exported by the C library (which may depend on the C compiler and linker used). <p> This function is not supported by Standard C. As such, it is only available on some platforms (Windows, Linux, Mac OS X, Solaris, BSD, plus other Unix systems that support the <code>dlfcn</code> standard). <p> <hr><h3><a name="pdf-package.path"><code>package.path</code></a></h3> <p> The path used by <a href="#pdf-require"><code>require</code></a> to search for a Lua loader. <p> At start-up, Lua initializes this variable with the value of the environment variable <a name="pdf-LUA_PATH_5_3"><code>LUA_PATH_5_3</code></a> or the environment variable <a name="pdf-LUA_PATH"><code>LUA_PATH</code></a> or with a default path defined in <code>luaconf.h</code>, if those environment variables are not defined. Any "<code>;;</code>" in the value of the environment variable is replaced by the default path. <p> <hr><h3><a name="pdf-package.preload"><code>package.preload</code></a></h3> <p> A table to store loaders for specific modules (see <a href="#pdf-require"><code>require</code></a>). <p> This variable is only a reference to the real table; assignments to this variable do not change the table used by <a href="#pdf-require"><code>require</code></a>. <p> <hr><h3><a name="pdf-package.searchers"><code>package.searchers</code></a></h3> <p> A table used by <a href="#pdf-require"><code>require</code></a> to control how to load modules. <p> Each entry in this table is a <em>searcher function</em>. When looking for a module, <a href="#pdf-require"><code>require</code></a> calls each of these searchers in ascending order, with the module name (the argument given to <a href="#pdf-require"><code>require</code></a>) as its sole parameter. The function can return another function (the module <em>loader</em>) plus an extra value that will be passed to that loader, or a string explaining why it did not find that module (or <b>nil</b> if it has nothing to say). <p> Lua initializes this table with four searcher functions. <p> The first searcher simply looks for a loader in the <a href="#pdf-package.preload"><code>package.preload</code></a> table. <p> The second searcher looks for a loader as a Lua library, using the path stored at <a href="#pdf-package.path"><code>package.path</code></a>. The search is done as described in function <a href="#pdf-package.searchpath"><code>package.searchpath</code></a>. <p> The third searcher looks for a loader as a C library, using the path given by the variable <a href="#pdf-package.cpath"><code>package.cpath</code></a>. Again, the search is done as described in function <a href="#pdf-package.searchpath"><code>package.searchpath</code></a>. For instance, if the C path is the string <pre> "./?.so;./?.dll;/usr/local/?/init.so" </pre><p> the searcher for module <code>foo</code> will try to open the files <code>./foo.so</code>, <code>./foo.dll</code>, and <code>/usr/local/foo/init.so</code>, in that order. Once it finds a C library, this searcher first uses a dynamic link facility to link the application with the library. Then it tries to find a C function inside the library to be used as the loader. The name of this C function is the string "<code>luaopen_</code>" concatenated with a copy of the module name where each dot is replaced by an underscore. Moreover, if the module name has a hyphen, its suffix after (and including) the first hyphen is removed. For instance, if the module name is <code>a.b.c-v2.1</code>, the function name will be <code>luaopen_a_b_c</code>. <p> The fourth searcher tries an <em>all-in-one loader</em>. It searches the C path for a library for the root name of the given module. For instance, when requiring <code>a.b.c</code>, it will search for a C library for <code>a</code>. If found, it looks into it for an open function for the submodule; in our example, that would be <code>luaopen_a_b_c</code>. With this facility, a package can pack several C submodules into one single library, with each submodule keeping its original open function. <p> All searchers except the first one (preload) return as the extra value the file name where the module was found, as returned by <a href="#pdf-package.searchpath"><code>package.searchpath</code></a>. The first searcher returns no extra value. <p> <hr><h3><a name="pdf-package.searchpath"><code>package.searchpath (name, path [, sep [, rep]])</code></a></h3> <p> Searches for the given <code>name</code> in the given <code>path</code>. <p> A path is a string containing a sequence of <em>templates</em> separated by semicolons. For each template, the function replaces each interrogation mark (if any) in the template with a copy of <code>name</code> wherein all occurrences of <code>sep</code> (a dot, by default) were replaced by <code>rep</code> (the system's directory separator, by default), and then tries to open the resulting file name. <p> For instance, if the path is the string <pre> "./?.lua;./?.lc;/usr/local/?/init.lua" </pre><p> the search for the name <code>foo.a</code> will try to open the files <code>./foo/a.lua</code>, <code>./foo/a.lc</code>, and <code>/usr/local/foo/a/init.lua</code>, in that order. <p> Returns the resulting name of the first file that it can open in read mode (after closing the file), or <b>nil</b> plus an error message if none succeeds. (This error message lists all file names it tried to open.) <h2>6.4 – <a name="6.4">String Manipulation</a></h2> <p> This library provides generic functions for string manipulation, such as finding and extracting substrings, and pattern matching. When indexing a string in Lua, the first character is at position 1 (not at 0, as in C). Indices are allowed to be negative and are interpreted as indexing backwards, from the end of the string. Thus, the last character is at position -1, and so on. <p> The string library provides all its functions inside the table <a name="pdf-string"><code>string</code></a>. It also sets a metatable for strings where the <code>__index</code> field points to the <code>string</code> table. Therefore, you can use the string functions in object-oriented style. For instance, <code>string.byte(s,i)</code> can be written as <code>s:byte(i)</code>. <p> The string library assumes one-byte character encodings. <p> <hr><h3><a name="pdf-string.byte"><code>string.byte (s [, i [, j]])</code></a></h3> Returns the internal numerical codes of the characters <code>s[i]</code>, <code>s[i+1]</code>, ..., <code>s[j]</code>. The default value for <code>i</code> is 1; the default value for <code>j</code> is <code>i</code>. These indices are corrected following the same rules of function <a href="#pdf-string.sub"><code>string.sub</code></a>. <p> Numerical codes are not necessarily portable across platforms. <p> <hr><h3><a name="pdf-string.char"><code>string.char (···)</code></a></h3> Receives zero or more integers. Returns a string with length equal to the number of arguments, in which each character has the internal numerical code equal to its corresponding argument. <p> Numerical codes are not necessarily portable across platforms. <p> <hr><h3><a name="pdf-string.dump"><code>string.dump (function [, strip])</code></a></h3> <p> Returns a string containing a binary representation (a <em>binary chunk</em>) of the given function, so that a later <a href="#pdf-load"><code>load</code></a> on this string returns a copy of the function (but with new upvalues). If <code>strip</code> is a true value, the binary representation is created without debug information about the function (local variable names, lines, etc.). <p> Functions with upvalues have only their number of upvalues saved. When (re)loaded, those upvalues receive fresh instances containing <b>nil</b>. (You can use the debug library to serialize and reload the upvalues of a function in a way adequate to your needs.) <p> <hr><h3><a name="pdf-string.find"><code>string.find (s, pattern [, init [, plain]])</code></a></h3> <p> Looks for the first match of <code>pattern</code> (see <a href="#6.4.1">§6.4.1</a>) in the string <code>s</code>. If it finds a match, then <code>find</code> returns the indices of <code>s</code> where this occurrence starts and ends; otherwise, it returns <b>nil</b>. A third, optional numerical argument <code>init</code> specifies where to start the search; its default value is 1 and can be negative. A value of <b>true</b> as a fourth, optional argument <code>plain</code> turns off the pattern matching facilities, so the function does a plain "find substring" operation, with no characters in <code>pattern</code> being considered magic. Note that if <code>plain</code> is given, then <code>init</code> must be given as well. <p> If the pattern has captures, then in a successful match the captured values are also returned, after the two indices. <p> <hr><h3><a name="pdf-string.format"><code>string.format (formatstring, ···)</code></a></h3> <p> Returns a formatted version of its variable number of arguments following the description given in its first argument (which must be a string). The format string follows the same rules as the ISO C function <code>sprintf</code>. The only differences are that the options/modifiers <code>*</code>, <code>h</code>, <code>L</code>, <code>l</code>, <code>n</code>, and <code>p</code> are not supported and that there is an extra option, <code>q</code>. The <code>q</code> option formats a string between double quotes, using escape sequences when necessary to ensure that it can safely be read back by the Lua interpreter. For instance, the call <pre> string.format('%q', 'a string with "quotes" and \n new line') </pre><p> may produce the string: <pre> "a string with \"quotes\" and \ new line" </pre> <p> Options <code>A</code> and <code>a</code> (when available), <code>E</code>, <code>e</code>, <code>f</code>, <code>G</code>, and <code>g</code> all expect a number as argument. Options <code>c</code>, <code>d</code>, <code>i</code>, <code>o</code>, <code>u</code>, <code>X</code>, and <code>x</code> expect an integer. Option <code>q</code> expects a string; option <code>s</code> expects a string without embedded zeros. If the argument to option <code>s</code> is not a string, it is converted to one following the same rules of <a href="#pdf-tostring"><code>tostring</code></a>. <p> <hr><h3><a name="pdf-string.gmatch"><code>string.gmatch (s, pattern)</code></a></h3> Returns an iterator function that, each time it is called, returns the next captures from <code>pattern</code> (see <a href="#6.4.1">§6.4.1</a>) over the string <code>s</code>. If <code>pattern</code> specifies no captures, then the whole match is produced in each call. <p> As an example, the following loop will iterate over all the words from string <code>s</code>, printing one per line: <pre> s = "hello world from Lua" for w in string.gmatch(s, "%a+") do print(w) end </pre><p> The next example collects all pairs <code>key=value</code> from the given string into a table: <pre> t = {} s = "from=world, to=Lua" for k, v in string.gmatch(s, "(%w+)=(%w+)") do t[k] = v end </pre> <p> For this function, a caret '<code>^</code>' at the start of a pattern does not work as an anchor, as this would prevent the iteration. <p> <hr><h3><a name="pdf-string.gsub"><code>string.gsub (s, pattern, repl [, n])</code></a></h3> Returns a copy of <code>s</code> in which all (or the first <code>n</code>, if given) occurrences of the <code>pattern</code> (see <a href="#6.4.1">§6.4.1</a>) have been replaced by a replacement string specified by <code>repl</code>, which can be a string, a table, or a function. <code>gsub</code> also returns, as its second value, the total number of matches that occurred. The name <code>gsub</code> comes from <em>Global SUBstitution</em>. <p> If <code>repl</code> is a string, then its value is used for replacement. The character <code>%</code> works as an escape character: any sequence in <code>repl</code> of the form <code>%<em>d</em></code>, with <em>d</em> between 1 and 9, stands for the value of the <em>d</em>-th captured substring. The sequence <code>%0</code> stands for the whole match. The sequence <code>%%</code> stands for a single <code>%</code>. <p> If <code>repl</code> is a table, then the table is queried for every match, using the first capture as the key. <p> If <code>repl</code> is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order. <p> In any case, if the pattern specifies no captures, then it behaves as if the whole pattern was inside a capture. <p> If the value returned by the table query or by the function call is a string or a number, then it is used as the replacement string; otherwise, if it is <b>false</b> or <b>nil</b>, then there is no replacement (that is, the original match is kept in the string). <p> Here are some examples: <pre> x = string.gsub("hello world", "(%w+)", "%1 %1") --> x="hello hello world world" x = string.gsub("hello world", "%w+", "%0 %0", 1) --> x="hello hello world" x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1") --> x="world hello Lua from" x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv) --> x="home = /home/roberto, user = roberto" x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s) return load(s)() end) --> x="4+5 = 9" local t = {name="lua", version="5.3"} x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t) --> x="lua-5.3.tar.gz" </pre> <p> <hr><h3><a name="pdf-string.len"><code>string.len (s)</code></a></h3> Receives a string and returns its length. The empty string <code>""</code> has length 0. Embedded zeros are counted, so <code>"a\000bc\000"</code> has length 5. <p> <hr><h3><a name="pdf-string.lower"><code>string.lower (s)</code></a></h3> Receives a string and returns a copy of this string with all uppercase letters changed to lowercase. All other characters are left unchanged. The definition of what an uppercase letter is depends on the current locale. <p> <hr><h3><a name="pdf-string.match"><code>string.match (s, pattern [, init])</code></a></h3> Looks for the first <em>match</em> of <code>pattern</code> (see <a href="#6.4.1">§6.4.1</a>) in the string <code>s</code>. If it finds one, then <code>match</code> returns the captures from the pattern; otherwise it returns <b>nil</b>. If <code>pattern</code> specifies no captures, then the whole match is returned. A third, optional numerical argument <code>init</code> specifies where to start the search; its default value is 1 and can be negative. <p> <hr><h3><a name="pdf-string.pack"><code>string.pack (fmt, v1, v2, ···)</code></a></h3> <p> Returns a binary string containing the values <code>v1</code>, <code>v2</code>, etc. packed (that is, serialized in binary form) according to the format string <code>fmt</code> (see <a href="#6.4.2">§6.4.2</a>). <p> <hr><h3><a name="pdf-string.packsize"><code>string.packsize (fmt)</code></a></h3> <p> Returns the size of a string resulting from <a href="#pdf-string.pack"><code>string.pack</code></a> with the given format. The format string cannot have the variable-length options '<code>s</code>' or '<code>z</code>' (see <a href="#6.4.2">§6.4.2</a>). <p> <hr><h3><a name="pdf-string.rep"><code>string.rep (s, n [, sep])</code></a></h3> Returns a string that is the concatenation of <code>n</code> copies of the string <code>s</code> separated by the string <code>sep</code>. The default value for <code>sep</code> is the empty string (that is, no separator). Returns the empty string if <code>n</code> is not positive. <p> <hr><h3><a name="pdf-string.reverse"><code>string.reverse (s)</code></a></h3> Returns a string that is the string <code>s</code> reversed. <p> <hr><h3><a name="pdf-string.sub"><code>string.sub (s, i [, j])</code></a></h3> Returns the substring of <code>s</code> that starts at <code>i</code> and continues until <code>j</code>; <code>i</code> and <code>j</code> can be negative. If <code>j</code> is absent, then it is assumed to be equal to -1 (which is the same as the string length). In particular, the call <code>string.sub(s,1,j)</code> returns a prefix of <code>s</code> with length <code>j</code>, and <code>string.sub(s, -i)</code> returns a suffix of <code>s</code> with length <code>i</code>. <p> If, after the translation of negative indices, <code>i</code> is less than 1, it is corrected to 1. If <code>j</code> is greater than the string length, it is corrected to that length. If, after these corrections, <code>i</code> is greater than <code>j</code>, the function returns the empty string. <p> <hr><h3><a name="pdf-string.unpack"><code>string.unpack (fmt, s [, pos])</code></a></h3> <p> Returns the values packed in string <code>s</code> (see <a href="#pdf-string.pack"><code>string.pack</code></a>) according to the format string <code>fmt</code> (see <a href="#6.4.2">§6.4.2</a>). An optional <code>pos</code> marks where to start reading in <code>s</code> (default is 1). After the read values, this function also returns the index of the first unread byte in <code>s</code>. <p> <hr><h3><a name="pdf-string.upper"><code>string.upper (s)</code></a></h3> Receives a string and returns a copy of this string with all lowercase letters changed to uppercase. All other characters are left unchanged. The definition of what a lowercase letter is depends on the current locale. <h3>6.4.1 – <a name="6.4.1">Patterns</a></h3> <p> Patterns in Lua are described by regular strings, which are interpreted as patterns by the pattern-matching functions <a href="#pdf-string.find"><code>string.find</code></a>, <a href="#pdf-string.gmatch"><code>string.gmatch</code></a>, <a href="#pdf-string.gsub"><code>string.gsub</code></a>, and <a href="#pdf-string.match"><code>string.match</code></a>. This section describes the syntax and the meaning (that is, what they match) of these strings. <h4>Character Class:</h4><p> A <em>character class</em> is used to represent a set of characters. The following combinations are allowed in describing a character class: <ul> <li><b><em>x</em>: </b> (where <em>x</em> is not one of the <em>magic characters</em> <code>^$()%.[]*+-?</code>) represents the character <em>x</em> itself. </li> <li><b><code>.</code>: </b> (a dot) represents all characters.</li> <li><b><code>%a</code>: </b> represents all letters.</li> <li><b><code>%c</code>: </b> represents all control characters.</li> <li><b><code>%d</code>: </b> represents all digits.</li> <li><b><code>%g</code>: </b> represents all printable characters except space.</li> <li><b><code>%l</code>: </b> represents all lowercase letters.</li> <li><b><code>%p</code>: </b> represents all punctuation characters.</li> <li><b><code>%s</code>: </b> represents all space characters.</li> <li><b><code>%u</code>: </b> represents all uppercase letters.</li> <li><b><code>%w</code>: </b> represents all alphanumeric characters.</li> <li><b><code>%x</code>: </b> represents all hexadecimal digits.</li> <li><b><code>%<em>x</em></code>: </b> (where <em>x</em> is any non-alphanumeric character) represents the character <em>x</em>. This is the standard way to escape the magic characters. Any non-alphanumeric character (including all punctuations, even the non-magical) can be preceded by a '<code>%</code>' when used to represent itself in a pattern. </li> <li><b><code>[<em>set</em>]</code>: </b> represents the class which is the union of all characters in <em>set</em>. A range of characters can be specified by separating the end characters of the range, in ascending order, with a '<code>-</code>'. All classes <code>%</code><em>x</em> described above can also be used as components in <em>set</em>. All other characters in <em>set</em> represent themselves. For example, <code>[%w_]</code> (or <code>[_%w]</code>) represents all alphanumeric characters plus the underscore, <code>[0-7]</code> represents the octal digits, and <code>[0-7%l%-]</code> represents the octal digits plus the lowercase letters plus the '<code>-</code>' character. <p> The interaction between ranges and classes is not defined. Therefore, patterns like <code>[%a-z]</code> or <code>[a-%%]</code> have no meaning. </li> <li><b><code>[^<em>set</em>]</code>: </b> represents the complement of <em>set</em>, where <em>set</em> is interpreted as above. </li> </ul><p> For all classes represented by single letters (<code>%a</code>, <code>%c</code>, etc.), the corresponding uppercase letter represents the complement of the class. For instance, <code>%S</code> represents all non-space characters. <p> The definitions of letter, space, and other character groups depend on the current locale. In particular, the class <code>[a-z]</code> may not be equivalent to <code>%l</code>. <h4>Pattern Item:</h4><p> A <em>pattern item</em> can be <ul> <li> a single character class, which matches any single character in the class; </li> <li> a single character class followed by '<code>*</code>', which matches zero or more repetitions of characters in the class. These repetition items will always match the longest possible sequence; </li> <li> a single character class followed by '<code>+</code>', which matches one or more repetitions of characters in the class. These repetition items will always match the longest possible sequence; </li> <li> a single character class followed by '<code>-</code>', which also matches zero or more repetitions of characters in the class. Unlike '<code>*</code>', these repetition items will always match the shortest possible sequence; </li> <li> a single character class followed by '<code>?</code>', which matches zero or one occurrence of a character in the class. It always matches one occurrence if possible; </li> <li> <code>%<em>n</em></code>, for <em>n</em> between 1 and 9; such item matches a substring equal to the <em>n</em>-th captured string (see below); </li> <li> <code>%b<em>xy</em></code>, where <em>x</em> and <em>y</em> are two distinct characters; such item matches strings that start with <em>x</em>, end with <em>y</em>, and where the <em>x</em> and <em>y</em> are <em>balanced</em>. This means that, if one reads the string from left to right, counting <em>+1</em> for an <em>x</em> and <em>-1</em> for a <em>y</em>, the ending <em>y</em> is the first <em>y</em> where the count reaches 0. For instance, the item <code>%b()</code> matches expressions with balanced parentheses. </li> <li> <code>%f[<em>set</em>]</code>, a <em>frontier pattern</em>; such item matches an empty string at any position such that the next character belongs to <em>set</em> and the previous character does not belong to <em>set</em>. The set <em>set</em> is interpreted as previously described. The beginning and the end of the subject are handled as if they were the character '<code>\0</code>'. </li> </ul> <h4>Pattern:</h4><p> A <em>pattern</em> is a sequence of pattern items. A caret '<code>^</code>' at the beginning of a pattern anchors the match at the beginning of the subject string. A '<code>$</code>' at the end of a pattern anchors the match at the end of the subject string. At other positions, '<code>^</code>' and '<code>$</code>' have no special meaning and represent themselves. <h4>Captures:</h4><p> A pattern can contain sub-patterns enclosed in parentheses; they describe <em>captures</em>. When a match succeeds, the substrings of the subject string that match captures are stored (<em>captured</em>) for future use. Captures are numbered according to their left parentheses. For instance, in the pattern <code>"(a*(.)%w(%s*))"</code>, the part of the string matching <code>"a*(.)%w(%s*)"</code> is stored as the first capture (and therefore has number 1); the character matching "<code>.</code>" is captured with number 2, and the part matching "<code>%s*</code>" has number 3. <p> As a special case, the empty capture <code>()</code> captures the current string position (a number). For instance, if we apply the pattern <code>"()aa()"</code> on the string <code>"flaaap"</code>, there will be two captures: 3 and 5. <h3>6.4.2 – <a name="6.4.2">Format Strings for Pack and Unpack</a></h3> <p> The first argument to <a href="#pdf-string.pack"><code>string.pack</code></a>, <a href="#pdf-string.packsize"><code>string.packsize</code></a>, and <a href="#pdf-string.unpack"><code>string.unpack</code></a> is a format string, which describes the layout of the structure being created or read. <p> A format string is a sequence of conversion options. The conversion options are as follows: <ul> <li><b><code><</code>: </b>sets little endian</li> <li><b><code>></code>: </b>sets big endian</li> <li><b><code>=</code>: </b>sets native endian</li> <li><b><code>![<em>n</em>]</code>: </b>sets maximum alignment to <code>n</code> (default is native alignment)</li> <li><b><code>b</code>: </b>a signed byte (<code>char</code>)</li> <li><b><code>B</code>: </b>an unsigned byte (<code>char</code>)</li> <li><b><code>h</code>: </b>a signed <code>short</code> (native size)</li> <li><b><code>H</code>: </b>an unsigned <code>short</code> (native size)</li> <li><b><code>l</code>: </b>a signed <code>long</code> (native size)</li> <li><b><code>L</code>: </b>an unsigned <code>long</code> (native size)</li> <li><b><code>j</code>: </b>a <code>lua_Integer</code></li> <li><b><code>J</code>: </b>a <code>lua_Unsigned</code></li> <li><b><code>T</code>: </b>a <code>size_t</code> (native size)</li> <li><b><code>i[<em>n</em>]</code>: </b>a signed <code>int</code> with <code>n</code> bytes (default is native size)</li> <li><b><code>I[<em>n</em>]</code>: </b>an unsigned <code>int</code> with <code>n</code> bytes (default is native size)</li> <li><b><code>f</code>: </b>a <code>float</code> (native size)</li> <li><b><code>d</code>: </b>a <code>double</code> (native size)</li> <li><b><code>n</code>: </b>a <code>lua_Number</code></li> <li><b><code>c<em>n</em></code>: </b>a fixed-sized string with <code>n</code> bytes</li> <li><b><code>z</code>: </b>a zero-terminated string</li> <li><b><code>s[<em>n</em>]</code>: </b>a string preceded by its length coded as an unsigned integer with <code>n</code> bytes (default is a <code>size_t</code>)</li> <li><b><code>x</code>: </b>one byte of padding</li> <li><b><code>X<em>op</em></code>: </b>an empty item that aligns according to option <code>op</code> (which is otherwise ignored)</li> <li><b>'<code> </code>': </b>(empty space) ignored</li> </ul><p> (A "<code>[<em>n</em>]</code>" means an optional integral numeral.) Except for padding, spaces, and configurations (options "<code>xX <=>!</code>"), each option corresponds to an argument (in <a href="#pdf-string.pack"><code>string.pack</code></a>) or a result (in <a href="#pdf-string.unpack"><code>string.unpack</code></a>). <p> For options "<code>!<em>n</em></code>", "<code>s<em>n</em></code>", "<code>i<em>n</em></code>", and "<code>I<em>n</em></code>", <code>n</code> can be any integer between 1 and 16. All integral options check overflows; <a href="#pdf-string.pack"><code>string.pack</code></a> checks whether the given value fits in the given size; <a href="#pdf-string.unpack"><code>string.unpack</code></a> checks whether the read value fits in a Lua integer. <p> Any format string starts as if prefixed by "<code>!1=</code>", that is, with maximum alignment of 1 (no alignment) and native endianness. <p> Alignment works as follows: For each option, the format gets extra padding until the data starts at an offset that is a multiple of the minimum between the option size and the maximum alignment; this minimum must be a power of 2. Options "<code>c</code>" and "<code>z</code>" are not aligned; option "<code>s</code>" follows the alignment of its starting integer. <p> All padding is filled with zeros by <a href="#pdf-string.pack"><code>string.pack</code></a> (and ignored by <a href="#pdf-string.unpack"><code>string.unpack</code></a>). <h2>6.5 – <a name="6.5">UTF-8 Support</a></h2> <p> This library provides basic support for UTF-8 encoding. It provides all its functions inside the table <a name="pdf-utf8"><code>utf8</code></a>. This library does not provide any support for Unicode other than the handling of the encoding. Any operation that needs the meaning of a character, such as character classification, is outside its scope. <p> Unless stated otherwise, all functions that expect a byte position as a parameter assume that the given position is either the start of a byte sequence or one plus the length of the subject string. As in the string library, negative indices count from the end of the string. <p> <hr><h3><a name="pdf-utf8.char"><code>utf8.char (···)</code></a></h3> Receives zero or more integers, converts each one to its corresponding UTF-8 byte sequence and returns a string with the concatenation of all these sequences. <p> <hr><h3><a name="pdf-utf8.charpattern"><code>utf8.charpattern</code></a></h3> The pattern (a string, not a function) "<code>[\0-\x7F\xC2-\xF4][\x80-\xBF]*</code>" (see <a href="#6.4.1">§6.4.1</a>), which matches exactly one UTF-8 byte sequence, assuming that the subject is a valid UTF-8 string. <p> <hr><h3><a name="pdf-utf8.codes"><code>utf8.codes (s)</code></a></h3> <p> Returns values so that the construction <pre> for p, c in utf8.codes(s) do <em>body</em> end </pre><p> will iterate over all characters in string <code>s</code>, with <code>p</code> being the position (in bytes) and <code>c</code> the code point of each character. It raises an error if it meets any invalid byte sequence. <p> <hr><h3><a name="pdf-utf8.codepoint"><code>utf8.codepoint (s [, i [, j]])</code></a></h3> Returns the codepoints (as integers) from all characters in <code>s</code> that start between byte position <code>i</code> and <code>j</code> (both included). The default for <code>i</code> is 1 and for <code>j</code> is <code>i</code>. It raises an error if it meets any invalid byte sequence. <p> <hr><h3><a name="pdf-utf8.len"><code>utf8.len (s [, i [, j]])</code></a></h3> Returns the number of UTF-8 characters in string <code>s</code> that start between positions <code>i</code> and <code>j</code> (both inclusive). The default for <code>i</code> is 1 and for <code>j</code> is -1. If it finds any invalid byte sequence, returns a false value plus the position of the first invalid byte. <p> <hr><h3><a name="pdf-utf8.offset"><code>utf8.offset (s, n [, i])</code></a></h3> Returns the position (in bytes) where the encoding of the <code>n</code>-th character of <code>s</code> (counting from position <code>i</code>) starts. A negative <code>n</code> gets characters before position <code>i</code>. The default for <code>i</code> is 1 when <code>n</code> is non-negative and <code>#s + 1</code> otherwise, so that <code>utf8.offset(s, -n)</code> gets the offset of the <code>n</code>-th character from the end of the string. If the specified character is neither in the subject nor right after its end, the function returns <b>nil</b>. <p> As a special case, when <code>n</code> is 0 the function returns the start of the encoding of the character that contains the <code>i</code>-th byte of <code>s</code>. <p> This function assumes that <code>s</code> is a valid UTF-8 string. <h2>6.6 – <a name="6.6">Table Manipulation</a></h2> <p> This library provides generic functions for table manipulation. It provides all its functions inside the table <a name="pdf-table"><code>table</code></a>. <p> Remember that, whenever an operation needs the length of a table, the table must be a proper sequence or have a <code>__len</code> metamethod (see <a href="#3.4.7">§3.4.7</a>). All functions ignore non-numeric keys in the tables given as arguments. <p> <hr><h3><a name="pdf-table.concat"><code>table.concat (list [, sep [, i [, j]]])</code></a></h3> <p> Given a list where all elements are strings or numbers, returns the string <code>list[i]..sep..list[i+1] ··· sep..list[j]</code>. The default value for <code>sep</code> is the empty string, the default for <code>i</code> is 1, and the default for <code>j</code> is <code>#list</code>. If <code>i</code> is greater than <code>j</code>, returns the empty string. <p> <hr><h3><a name="pdf-table.insert"><code>table.insert (list, [pos,] value)</code></a></h3> <p> Inserts element <code>value</code> at position <code>pos</code> in <code>list</code>, shifting up the elements <code>list[pos], list[pos+1], ···, list[#list]</code>. The default value for <code>pos</code> is <code>#list+1</code>, so that a call <code>table.insert(t,x)</code> inserts <code>x</code> at the end of list <code>t</code>. <p> <hr><h3><a name="pdf-table.move"><code>table.move (a1, f, e, t [,a2])</code></a></h3> <p> Moves elements from table <code>a1</code> to table <code>a2</code>. This function performs the equivalent to the following multiple assignment: <code>a2[t],··· = a1[f],···,a1[e]</code>. The default for <code>a2</code> is <code>a1</code>. The destination range can overlap with the source range. Index <code>f</code> must be positive. <p> <hr><h3><a name="pdf-table.pack"><code>table.pack (···)</code></a></h3> <p> Returns a new table with all parameters stored into keys 1, 2, etc. and with a field "<code>n</code>" with the total number of parameters. Note that the resulting table may not be a sequence. <p> <hr><h3><a name="pdf-table.remove"><code>table.remove (list [, pos])</code></a></h3> <p> Removes from <code>list</code> the element at position <code>pos</code>, returning the value of the removed element. When <code>pos</code> is an integer between 1 and <code>#list</code>, it shifts down the elements <code>list[pos+1], list[pos+2], ···, list[#list]</code> and erases element <code>list[#list]</code>; The index <code>pos</code> can also be 0 when <code>#list</code> is 0, or <code>#list + 1</code>; in those cases, the function erases the element <code>list[pos]</code>. <p> The default value for <code>pos</code> is <code>#list</code>, so that a call <code>table.remove(l)</code> removes the last element of list <code>l</code>. <p> <hr><h3><a name="pdf-table.sort"><code>table.sort (list [, comp])</code></a></h3> <p> Sorts list elements in a given order, <em>in-place</em>, from <code>list[1]</code> to <code>list[#list]</code>. If <code>comp</code> is given, then it must be a function that receives two list elements and returns true when the first element must come before the second in the final order (so that <code>not comp(list[i+1],list[i])</code> will be true after the sort). If <code>comp</code> is not given, then the standard Lua operator <code><</code> is used instead. <p> The sort algorithm is not stable; that is, elements considered equal by the given order may have their relative positions changed by the sort. <p> <hr><h3><a name="pdf-table.unpack"><code>table.unpack (list [, i [, j]])</code></a></h3> <p> Returns the elements from the given list. This function is equivalent to <pre> return list[i], list[i+1], ···, list[j] </pre><p> By default, <code>i</code> is 1 and <code>j</code> is <code>#list</code>. <h2>6.7 – <a name="6.7">Mathematical Functions</a></h2> <p> This library provides basic mathematical functions. It provides all its functions and constants inside the table <a name="pdf-math"><code>math</code></a>. Functions with the annotation "<code>integer/float</code>" give integer results for integer arguments and float results for float (or mixed) arguments. Rounding functions (<a href="#pdf-math.ceil"><code>math.ceil</code></a>, <a href="#pdf-math.floor"><code>math.floor</code></a>, and <a href="#pdf-math.modf"><code>math.modf</code></a>) return an integer when the result fits in the range of an integer, or a float otherwise. <p> <hr><h3><a name="pdf-math.abs"><code>math.abs (x)</code></a></h3> <p> Returns the absolute value of <code>x</code>. (integer/float) <p> <hr><h3><a name="pdf-math.acos"><code>math.acos (x)</code></a></h3> <p> Returns the arc cosine of <code>x</code> (in radians). <p> <hr><h3><a name="pdf-math.asin"><code>math.asin (x)</code></a></h3> <p> Returns the arc sine of <code>x</code> (in radians). <p> <hr><h3><a name="pdf-math.atan"><code>math.atan (y [, x])</code></a></h3> <p> Returns the arc tangent of <code>y/x</code> (in radians), but uses the signs of both parameters to find the quadrant of the result. (It also handles correctly the case of <code>x</code> being zero.) <p> The default value for <code>x</code> is 1, so that the call <code>math.atan(y)</code> returns the arc tangent of <code>y</code>. <p> <hr><h3><a name="pdf-math.ceil"><code>math.ceil (x)</code></a></h3> <p> Returns the smallest integral value larger than or equal to <code>x</code>. <p> <hr><h3><a name="pdf-math.cos"><code>math.cos (x)</code></a></h3> <p> Returns the cosine of <code>x</code> (assumed to be in radians). <p> <hr><h3><a name="pdf-math.deg"><code>math.deg (x)</code></a></h3> <p> Converts the angle <code>x</code> from radians to degrees. <p> <hr><h3><a name="pdf-math.exp"><code>math.exp (x)</code></a></h3> <p> Returns the value <em>e<sup>x</sup></em> (where <code>e</code> is the base of natural logarithms). <p> <hr><h3><a name="pdf-math.floor"><code>math.floor (x)</code></a></h3> <p> Returns the largest integral value smaller than or equal to <code>x</code>. <p> <hr><h3><a name="pdf-math.fmod"><code>math.fmod (x, y)</code></a></h3> <p> Returns the remainder of the division of <code>x</code> by <code>y</code> that rounds the quotient towards zero. (integer/float) <p> <hr><h3><a name="pdf-math.huge"><code>math.huge</code></a></h3> <p> The float value <code>HUGE_VAL</code>, a value larger than any other numerical value. <p> <hr><h3><a name="pdf-math.log"><code>math.log (x [, base])</code></a></h3> <p> Returns the logarithm of <code>x</code> in the given base. The default for <code>base</code> is <em>e</em> (so that the function returns the natural logarithm of <code>x</code>). <p> <hr><h3><a name="pdf-math.max"><code>math.max (x, ···)</code></a></h3> <p> Returns the argument with the maximum value, according to the Lua operator <code><</code>. (integer/float) <p> <hr><h3><a name="pdf-math.maxinteger"><code>math.maxinteger</code></a></h3> An integer with the maximum value for an integer. <p> <hr><h3><a name="pdf-math.min"><code>math.min (x, ···)</code></a></h3> <p> Returns the argument with the minimum value, according to the Lua operator <code><</code>. (integer/float) <p> <hr><h3><a name="pdf-math.mininteger"><code>math.mininteger</code></a></h3> An integer with the minimum value for an integer. <p> <hr><h3><a name="pdf-math.modf"><code>math.modf (x)</code></a></h3> <p> Returns the integral part of <code>x</code> and the fractional part of <code>x</code>. Its second result is always a float. <p> <hr><h3><a name="pdf-math.pi"><code>math.pi</code></a></h3> <p> The value of <em>π</em>. <p> <hr><h3><a name="pdf-math.rad"><code>math.rad (x)</code></a></h3> <p> Converts the angle <code>x</code> from degrees to radians. <p> <hr><h3><a name="pdf-math.random"><code>math.random ([m [, n]])</code></a></h3> <p> When called without arguments, returns a pseudo-random float with uniform distribution in the range <em>[0,1)</em>. When called with two integers <code>m</code> and <code>n</code>, <code>math.random</code> returns a pseudo-random integer with uniform distribution in the range <em>[m, n]</em>. (The value <em>m-n</em> cannot be negative and must fit in a Lua integer.) The call <code>math.random(n)</code> is equivalent to <code>math.random(1,n)</code>. <p> This function is an interface to the underling pseudo-random generator function provided by C. No guarantees can be given for its statistical properties. <p> <hr><h3><a name="pdf-math.randomseed"><code>math.randomseed (x)</code></a></h3> <p> Sets <code>x</code> as the "seed" for the pseudo-random generator: equal seeds produce equal sequences of numbers. <p> <hr><h3><a name="pdf-math.sin"><code>math.sin (x)</code></a></h3> <p> Returns the sine of <code>x</code> (assumed to be in radians). <p> <hr><h3><a name="pdf-math.sqrt"><code>math.sqrt (x)</code></a></h3> <p> Returns the square root of <code>x</code>. (You can also use the expression <code>x^0.5</code> to compute this value.) <p> <hr><h3><a name="pdf-math.tan"><code>math.tan (x)</code></a></h3> <p> Returns the tangent of <code>x</code> (assumed to be in radians). <p> <hr><h3><a name="pdf-math.tointeger"><code>math.tointeger (x)</code></a></h3> <p> If the value <code>x</code> is convertible to an integer, returns that integer. Otherwise, returns <b>nil</b>. <p> <hr><h3><a name="pdf-math.type"><code>math.type (x)</code></a></h3> <p> Returns "<code>integer</code>" if <code>x</code> is an integer, "<code>float</code>" if it is a float, or <b>nil</b> if <code>x</code> is not a number. <p> <hr><h3><a name="pdf-math.ult"><code>math.ult (m, n)</code></a></h3> <p> Returns a boolean, true if integer <code>m</code> is below integer <code>n</code> when they are compared as unsigned integers. <h2>6.8 – <a name="6.8">Input and Output Facilities</a></h2> <p> The I/O library provides two different styles for file manipulation. The first one uses implicit file handles; that is, there are operations to set a default input file and a default output file, and all input/output operations are over these default files. The second style uses explicit file handles. <p> When using implicit file handles, all operations are supplied by table <a name="pdf-io"><code>io</code></a>. When using explicit file handles, the operation <a href="#pdf-io.open"><code>io.open</code></a> returns a file handle and then all operations are supplied as methods of the file handle. <p> The table <code>io</code> also provides three predefined file handles with their usual meanings from C: <a name="pdf-io.stdin"><code>io.stdin</code></a>, <a name="pdf-io.stdout"><code>io.stdout</code></a>, and <a name="pdf-io.stderr"><code>io.stderr</code></a>. The I/O library never closes these files. <p> Unless otherwise stated, all I/O functions return <b>nil</b> on failure (plus an error message as a second result and a system-dependent error code as a third result) and some value different from <b>nil</b> on success. On non-POSIX systems, the computation of the error message and error code in case of errors may be not thread safe, because they rely on the global C variable <code>errno</code>. <p> <hr><h3><a name="pdf-io.close"><code>io.close ([file])</code></a></h3> <p> Equivalent to <code>file:close()</code>. Without a <code>file</code>, closes the default output file. <p> <hr><h3><a name="pdf-io.flush"><code>io.flush ()</code></a></h3> <p> Equivalent to <code>io.output():flush()</code>. <p> <hr><h3><a name="pdf-io.input"><code>io.input ([file])</code></a></h3> <p> When called with a file name, it opens the named file (in text mode), and sets its handle as the default input file. When called with a file handle, it simply sets this file handle as the default input file. When called without parameters, it returns the current default input file. <p> In case of errors this function raises the error, instead of returning an error code. <p> <hr><h3><a name="pdf-io.lines"><code>io.lines ([filename ···])</code></a></h3> <p> Opens the given file name in read mode and returns an iterator function that works like <code>file:lines(···)</code> over the opened file. When the iterator function detects the end of file, it returns no values (to finish the loop) and automatically closes the file. <p> The call <code>io.lines()</code> (with no file name) is equivalent to <code>io.input():lines("*l")</code>; that is, it iterates over the lines of the default input file. In this case it does not close the file when the loop ends. <p> In case of errors this function raises the error, instead of returning an error code. <p> <hr><h3><a name="pdf-io.open"><code>io.open (filename [, mode])</code></a></h3> <p> This function opens a file, in the mode specified in the string <code>mode</code>. It returns a new file handle, or, in case of errors, <b>nil</b> plus an error message. <p> The <code>mode</code> string can be any of the following: <ul> <li><b>"<code>r</code>": </b> read mode (the default);</li> <li><b>"<code>w</code>": </b> write mode;</li> <li><b>"<code>a</code>": </b> append mode;</li> <li><b>"<code>r+</code>": </b> update mode, all previous data is preserved;</li> <li><b>"<code>w+</code>": </b> update mode, all previous data is erased;</li> <li><b>"<code>a+</code>": </b> append update mode, previous data is preserved, writing is only allowed at the end of file.</li> </ul><p> The <code>mode</code> string can also have a '<code>b</code>' at the end, which is needed in some systems to open the file in binary mode. <p> <hr><h3><a name="pdf-io.output"><code>io.output ([file])</code></a></h3> <p> Similar to <a href="#pdf-io.input"><code>io.input</code></a>, but operates over the default output file. <p> <hr><h3><a name="pdf-io.popen"><code>io.popen (prog [, mode])</code></a></h3> <p> This function is system dependent and is not available on all platforms. <p> Starts program <code>prog</code> in a separated process and returns a file handle that you can use to read data from this program (if <code>mode</code> is <code>"r"</code>, the default) or to write data to this program (if <code>mode</code> is <code>"w"</code>). <p> <hr><h3><a name="pdf-io.read"><code>io.read (···)</code></a></h3> <p> Equivalent to <code>io.input():read(···)</code>. <p> <hr><h3><a name="pdf-io.tmpfile"><code>io.tmpfile ()</code></a></h3> <p> Returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends. <p> <hr><h3><a name="pdf-io.type"><code>io.type (obj)</code></a></h3> <p> Checks whether <code>obj</code> is a valid file handle. Returns the string <code>"file"</code> if <code>obj</code> is an open file handle, <code>"closed file"</code> if <code>obj</code> is a closed file handle, or <b>nil</b> if <code>obj</code> is not a file handle. <p> <hr><h3><a name="pdf-io.write"><code>io.write (···)</code></a></h3> <p> Equivalent to <code>io.output():write(···)</code>. <p> <hr><h3><a name="pdf-file:close"><code>file:close ()</code></a></h3> <p> Closes <code>file</code>. Note that files are automatically closed when their handles are garbage collected, but that takes an unpredictable amount of time to happen. <p> When closing a file handle created with <a href="#pdf-io.popen"><code>io.popen</code></a>, <a href="#pdf-file:close"><code>file:close</code></a> returns the same values returned by <a href="#pdf-os.execute"><code>os.execute</code></a>. <p> <hr><h3><a name="pdf-file:flush"><code>file:flush ()</code></a></h3> <p> Saves any written data to <code>file</code>. <p> <hr><h3><a name="pdf-file:lines"><code>file:lines (···)</code></a></h3> <p> Returns an iterator function that, each time it is called, reads the file according to the given formats. When no format is given, uses "<code>l</code>" as a default. As an example, the construction <pre> for c in file:lines(1) do <em>body</em> end </pre><p> will iterate over all characters of the file, starting at the current position. Unlike <a href="#pdf-io.lines"><code>io.lines</code></a>, this function does not close the file when the loop ends. <p> In case of errors this function raises the error, instead of returning an error code. <p> <hr><h3><a name="pdf-file:read"><code>file:read (···)</code></a></h3> <p> Reads the file <code>file</code>, according to the given formats, which specify what to read. For each format, the function returns a string or a number with the characters read, or <b>nil</b> if it cannot read data with the specified format. (In this latter case, the function does not read subsequent formats.) When called without formats, it uses a default format that reads the next line (see below). <p> The available formats are <ul> <li><b>"<code>n</code>": </b> reads a numeral and returns it as a float or an integer, following the lexical conventions of Lua. (The numeral may have leading spaces and a sign.) This format always reads the longest input sequence that is a valid prefix for a number; if that prefix does not form a valid number (e.g., an empty string, "<code>0x</code>", or "<code>3.4e-</code>"), it is discarded and the function returns <b>nil</b>. </li> <li><b>"<code>i</code>": </b> reads an integral number and returns it as an integer. </li> <li><b>"<code>a</code>": </b> reads the whole file, starting at the current position. On end of file, it returns the empty string. </li> <li><b>"<code>l</code>": </b> reads the next line skipping the end of line, returning <b>nil</b> on end of file. This is the default format. </li> <li><b>"<code>L</code>": </b> reads the next line keeping the end-of-line character (if present), returning <b>nil</b> on end of file. </li> <li><b><em>number</em>: </b> reads a string with up to this number of bytes, returning <b>nil</b> on end of file. If <code>number</code> is zero, it reads nothing and returns an empty string, or <b>nil</b> on end of file. </li> </ul><p> The formats "<code>l</code>" and "<code>L</code>" should be used only for text files. <p> <hr><h3><a name="pdf-file:seek"><code>file:seek ([whence [, offset]])</code></a></h3> <p> Sets and gets the file position, measured from the beginning of the file, to the position given by <code>offset</code> plus a base specified by the string <code>whence</code>, as follows: <ul> <li><b>"<code>set</code>": </b> base is position 0 (beginning of the file);</li> <li><b>"<code>cur</code>": </b> base is current position;</li> <li><b>"<code>end</code>": </b> base is end of file;</li> </ul><p> In case of success, <code>seek</code> returns the final file position, measured in bytes from the beginning of the file. If <code>seek</code> fails, it returns <b>nil</b>, plus a string describing the error. <p> The default value for <code>whence</code> is <code>"cur"</code>, and for <code>offset</code> is 0. Therefore, the call <code>file:seek()</code> returns the current file position, without changing it; the call <code>file:seek("set")</code> sets the position to the beginning of the file (and returns 0); and the call <code>file:seek("end")</code> sets the position to the end of the file, and returns its size. <p> <hr><h3><a name="pdf-file:setvbuf"><code>file:setvbuf (mode [, size])</code></a></h3> <p> Sets the buffering mode for an output file. There are three available modes: <ul> <li><b>"<code>no</code>": </b> no buffering; the result of any output operation appears immediately. </li> <li><b>"<code>full</code>": </b> full buffering; output operation is performed only when the buffer is full or when you explicitly <code>flush</code> the file (see <a href="#pdf-io.flush"><code>io.flush</code></a>). </li> <li><b>"<code>line</code>": </b> line buffering; output is buffered until a newline is output or there is any input from some special files (such as a terminal device). </li> </ul><p> For the last two cases, <code>size</code> specifies the size of the buffer, in bytes. The default is an appropriate size. <p> <hr><h3><a name="pdf-file:write"><code>file:write (···)</code></a></h3> <p> Writes the value of each of its arguments to <code>file</code>. The arguments must be strings or numbers. <p> In case of success, this function returns <code>file</code>. Otherwise it returns <b>nil</b> plus a string describing the error. <h2>6.9 – <a name="6.9">Operating System Facilities</a></h2> <p> This library is implemented through table <a name="pdf-os"><code>os</code></a>. <p> <hr><h3><a name="pdf-os.clock"><code>os.clock ()</code></a></h3> <p> Returns an approximation of the amount in seconds of CPU time used by the program. <p> <hr><h3><a name="pdf-os.date"><code>os.date ([format [, time]])</code></a></h3> <p> Returns a string or a table containing date and time, formatted according to the given string <code>format</code>. <p> If the <code>time</code> argument is present, this is the time to be formatted (see the <a href="#pdf-os.time"><code>os.time</code></a> function for a description of this value). Otherwise, <code>date</code> formats the current time. <p> If <code>format</code> starts with '<code>!</code>', then the date is formatted in Coordinated Universal Time. After this optional character, if <code>format</code> is the string "<code>*t</code>", then <code>date</code> returns a table with the following fields: <code>year</code> (four digits), <code>month</code> (1–12), <code>day</code> (1–31), <code>hour</code> (0–23), <code>min</code> (0–59), <code>sec</code> (0–61), <code>wday</code> (weekday, Sunday is 1), <code>yday</code> (day of the year), and <code>isdst</code> (daylight saving flag, a boolean). This last field may be absent if the information is not available. <p> If <code>format</code> is not "<code>*t</code>", then <code>date</code> returns the date as a string, formatted according to the same rules as the ISO C function <code>strftime</code>. <p> When called without arguments, <code>date</code> returns a reasonable date and time representation that depends on the host system and on the current locale (that is, <code>os.date()</code> is equivalent to <code>os.date("%c")</code>). <p> On non-POSIX systems, this function may be not thread safe because of its reliance on C function <code>gmtime</code> and C function <code>localtime</code>. <p> <hr><h3><a name="pdf-os.difftime"><code>os.difftime (t2, t1)</code></a></h3> <p> Returns the difference, in seconds, from time <code>t1</code> to time <code>t2</code> (where the times are values returned by <a href="#pdf-os.time"><code>os.time</code></a>). In POSIX, Windows, and some other systems, this value is exactly <code>t2</code><em>-</em><code>t1</code>. <p> <hr><h3><a name="pdf-os.execute"><code>os.execute ([command])</code></a></h3> <p> This function is equivalent to the ISO C function <code>system</code>. It passes <code>command</code> to be executed by an operating system shell. Its first result is <b>true</b> if the command terminated successfully, or <b>nil</b> otherwise. After this first result the function returns a string plus a number, as follows: <ul> <li><b>"<code>exit</code>": </b> the command terminated normally; the following number is the exit status of the command. </li> <li><b>"<code>signal</code>": </b> the command was terminated by a signal; the following number is the signal that terminated the command. </li> </ul> <p> When called without a <code>command</code>, <code>os.execute</code> returns a boolean that is true if a shell is available. <p> <hr><h3><a name="pdf-os.exit"><code>os.exit ([code [, close]])</code></a></h3> <p> Calls the ISO C function <code>exit</code> to terminate the host program. If <code>code</code> is <b>true</b>, the returned status is <code>EXIT_SUCCESS</code>; if <code>code</code> is <b>false</b>, the returned status is <code>EXIT_FAILURE</code>; if <code>code</code> is a number, the returned status is this number. The default value for <code>code</code> is <b>true</b>. <p> If the optional second argument <code>close</code> is true, closes the Lua state before exiting. <p> <hr><h3><a name="pdf-os.getenv"><code>os.getenv (varname)</code></a></h3> <p> Returns the value of the process environment variable <code>varname</code>, or <b>nil</b> if the variable is not defined. <p> <hr><h3><a name="pdf-os.remove"><code>os.remove (filename)</code></a></h3> <p> Deletes the file (or empty directory, on POSIX systems) with the given name. If this function fails, it returns <b>nil</b>, plus a string describing the error and the error code. <p> <hr><h3><a name="pdf-os.rename"><code>os.rename (oldname, newname)</code></a></h3> <p> Renames file or directory named <code>oldname</code> to <code>newname</code>. If this function fails, it returns <b>nil</b>, plus a string describing the error and the error code. <p> <hr><h3><a name="pdf-os.setlocale"><code>os.setlocale (locale [, category])</code></a></h3> <p> Sets the current locale of the program. <code>locale</code> is a system-dependent string specifying a locale; <code>category</code> is an optional string describing which category to change: <code>"all"</code>, <code>"collate"</code>, <code>"ctype"</code>, <code>"monetary"</code>, <code>"numeric"</code>, or <code>"time"</code>; the default category is <code>"all"</code>. The function returns the name of the new locale, or <b>nil</b> if the request cannot be honored. <p> If <code>locale</code> is the empty string, the current locale is set to an implementation-defined native locale. If <code>locale</code> is the string "<code>C</code>", the current locale is set to the standard C locale. <p> When called with <b>nil</b> as the first argument, this function only returns the name of the current locale for the given category. <p> This function may be not thread safe because of its reliance on C function <code>setlocale</code>. <p> <hr><h3><a name="pdf-os.time"><code>os.time ([table])</code></a></h3> <p> Returns the current time when called without arguments, or a time representing the date and time specified by the given table. This table must have fields <code>year</code>, <code>month</code>, and <code>day</code>, and may have fields <code>hour</code> (default is 12), <code>min</code> (default is 0), <code>sec</code> (default is 0), and <code>isdst</code> (default is <b>nil</b>). For a description of these fields, see the <a href="#pdf-os.date"><code>os.date</code></a> function. <p> The returned value is a number, whose meaning depends on your system. In POSIX, Windows, and some other systems, this number counts the number of seconds since some given start time (the "epoch"). In other systems, the meaning is not specified, and the number returned by <code>time</code> can be used only as an argument to <a href="#pdf-os.date"><code>os.date</code></a> and <a href="#pdf-os.difftime"><code>os.difftime</code></a>. <p> <hr><h3><a name="pdf-os.tmpname"><code>os.tmpname ()</code></a></h3> <p> Returns a string with a file name that can be used for a temporary file. The file must be explicitly opened before its use and explicitly removed when no longer needed. <p> On POSIX systems, this function also creates a file with that name, to avoid security risks. (Someone else might create the file with wrong permissions in the time between getting the name and creating the file.) You still have to open the file to use it and to remove it (even if you do not use it). <p> When possible, you may prefer to use <a href="#pdf-io.tmpfile"><code>io.tmpfile</code></a>, which automatically removes the file when the program ends. <h2>6.10 – <a name="6.10">The Debug Library</a></h2> <p> This library provides the functionality of the debug interface (<a href="#4.9">§4.9</a>) to Lua programs. You should exert care when using this library. Several of its functions violate basic assumptions about Lua code (e.g., that variables local to a function cannot be accessed from outside; that userdata metatables cannot be changed by Lua code; that Lua programs do not crash) and therefore can compromise otherwise secure code. Moreover, some functions in this library may be slow. <p> All functions in this library are provided inside the <a name="pdf-debug"><code>debug</code></a> table. All functions that operate over a thread have an optional first argument which is the thread to operate over. The default is always the current thread. <p> <hr><h3><a name="pdf-debug.debug"><code>debug.debug ()</code></a></h3> <p> Enters an interactive mode with the user, running each string that the user enters. Using simple commands and other debug facilities, the user can inspect global and local variables, change their values, evaluate expressions, and so on. A line containing only the word <code>cont</code> finishes this function, so that the caller continues its execution. <p> Note that commands for <code>debug.debug</code> are not lexically nested within any function and so have no direct access to local variables. <p> <hr><h3><a name="pdf-debug.gethook"><code>debug.gethook ([thread])</code></a></h3> <p> Returns the current hook settings of the thread, as three values: the current hook function, the current hook mask, and the current hook count (as set by the <a href="#pdf-debug.sethook"><code>debug.sethook</code></a> function). <p> <hr><h3><a name="pdf-debug.getinfo"><code>debug.getinfo ([thread,] f [, what])</code></a></h3> <p> Returns a table with information about a function. You can give the function directly or you can give a number as the value of <code>f</code>, which means the function running at level <code>f</code> of the call stack of the given thread: level 0 is the current function (<code>getinfo</code> itself); level 1 is the function that called <code>getinfo</code> (except for tail calls, which do not count on the stack); and so on. If <code>f</code> is a number larger than the number of active functions, then <code>getinfo</code> returns <b>nil</b>. <p> The returned table can contain all the fields returned by <a href="#lua_getinfo"><code>lua_getinfo</code></a>, with the string <code>what</code> describing which fields to fill in. The default for <code>what</code> is to get all information available, except the table of valid lines. If present, the option '<code>f</code>' adds a field named <code>func</code> with the function itself. If present, the option '<code>L</code>' adds a field named <code>activelines</code> with the table of valid lines. <p> For instance, the expression <code>debug.getinfo(1,"n").name</code> returns a table with a name for the current function, if a reasonable name can be found, and the expression <code>debug.getinfo(print)</code> returns a table with all available information about the <a href="#pdf-print"><code>print</code></a> function. <p> <hr><h3><a name="pdf-debug.getlocal"><code>debug.getlocal ([thread,] f, local)</code></a></h3> <p> This function returns the name and the value of the local variable with index <code>local</code> of the function at level <code>f</code> of the stack. This function accesses not only explicit local variables, but also parameters, temporaries, etc. <p> The first parameter or local variable has index 1, and so on, following the order that they are declared in the code, counting only the variables that are active in the current scope of the function. Negative indices refer to vararg parameters; -1 is the first vararg parameter. The function returns <b>nil</b> if there is no variable with the given index, and raises an error when called with a level out of range. (You can call <a href="#pdf-debug.getinfo"><code>debug.getinfo</code></a> to check whether the level is valid.) <p> Variable names starting with '<code>(</code>' (open parenthesis) represent variables with no known names (internal variables such as loop control variables, and variables from chunks saved without debug information). <p> The parameter <code>f</code> may also be a function. In that case, <code>getlocal</code> returns only the name of function parameters. <p> <hr><h3><a name="pdf-debug.getmetatable"><code>debug.getmetatable (value)</code></a></h3> <p> Returns the metatable of the given <code>value</code> or <b>nil</b> if it does not have a metatable. <p> <hr><h3><a name="pdf-debug.getregistry"><code>debug.getregistry ()</code></a></h3> <p> Returns the registry table (see <a href="#4.5">§4.5</a>). <p> <hr><h3><a name="pdf-debug.getupvalue"><code>debug.getupvalue (f, up)</code></a></h3> <p> This function returns the name and the value of the upvalue with index <code>up</code> of the function <code>f</code>. The function returns <b>nil</b> if there is no upvalue with the given index. <p> Variable names starting with '<code>(</code>' (open parenthesis) represent variables with no known names (variables from chunks saved without debug information). <p> <hr><h3><a name="pdf-debug.getuservalue"><code>debug.getuservalue (u)</code></a></h3> <p> Returns the Lua value associated to <code>u</code>. If <code>u</code> is not a userdata, returns <b>nil</b>. <p> <hr><h3><a name="pdf-debug.sethook"><code>debug.sethook ([thread,] hook, mask [, count])</code></a></h3> <p> Sets the given function as a hook. The string <code>mask</code> and the number <code>count</code> describe when the hook will be called. The string mask may have any combination of the following characters, with the given meaning: <ul> <li><b>'<code>c</code>': </b> the hook is called every time Lua calls a function;</li> <li><b>'<code>r</code>': </b> the hook is called every time Lua returns from a function;</li> <li><b>'<code>l</code>': </b> the hook is called every time Lua enters a new line of code.</li> </ul><p> Moreover, with a <code>count</code> different from zero, the hook is called also after every <code>count</code> instructions. <p> When called without arguments, <a href="#pdf-debug.sethook"><code>debug.sethook</code></a> turns off the hook. <p> When the hook is called, its first parameter is a string describing the event that has triggered its call: <code>"call"</code> (or <code>"tail call"</code>), <code>"return"</code>, <code>"line"</code>, and <code>"count"</code>. For line events, the hook also gets the new line number as its second parameter. Inside a hook, you can call <code>getinfo</code> with level 2 to get more information about the running function (level 0 is the <code>getinfo</code> function, and level 1 is the hook function). <p> <hr><h3><a name="pdf-debug.setlocal"><code>debug.setlocal ([thread,] level, local, value)</code></a></h3> <p> This function assigns the value <code>value</code> to the local variable with index <code>local</code> of the function at level <code>level</code> of the stack. The function returns <b>nil</b> if there is no local variable with the given index, and raises an error when called with a <code>level</code> out of range. (You can call <code>getinfo</code> to check whether the level is valid.) Otherwise, it returns the name of the local variable. <p> See <a href="#pdf-debug.getlocal"><code>debug.getlocal</code></a> for more information about variable indices and names. <p> <hr><h3><a name="pdf-debug.setmetatable"><code>debug.setmetatable (value, table)</code></a></h3> <p> Sets the metatable for the given <code>value</code> to the given <code>table</code> (which can be <b>nil</b>). Returns <code>value</code>. <p> <hr><h3><a name="pdf-debug.setupvalue"><code>debug.setupvalue (f, up, value)</code></a></h3> <p> This function assigns the value <code>value</code> to the upvalue with index <code>up</code> of the function <code>f</code>. The function returns <b>nil</b> if there is no upvalue with the given index. Otherwise, it returns the name of the upvalue. <p> <hr><h3><a name="pdf-debug.setuservalue"><code>debug.setuservalue (udata, value)</code></a></h3> <p> Sets the given <code>value</code> as the Lua value associated to the given <code>udata</code>. <code>udata</code> must be a full userdata. <p> Returns <code>udata</code>. <p> <hr><h3><a name="pdf-debug.traceback"><code>debug.traceback ([thread,] [message [, level]])</code></a></h3> <p> If <code>message</code> is present but is neither a string nor <b>nil</b>, this function returns <code>message</code> without further processing. Otherwise, it returns a string with a traceback of the call stack. The optional <code>message</code> string is appended at the beginning of the traceback. An optional <code>level</code> number tells at which level to start the traceback (default is 1, the function calling <code>traceback</code>). <p> <hr><h3><a name="pdf-debug.upvalueid"><code>debug.upvalueid (f, n)</code></a></h3> <p> Returns a unique identifier (as a light userdata) for the upvalue numbered <code>n</code> from the given function. <p> These unique identifiers allow a program to check whether different closures share upvalues. Lua closures that share an upvalue (that is, that access a same external local variable) will return identical ids for those upvalue indices. <p> <hr><h3><a name="pdf-debug.upvaluejoin"><code>debug.upvaluejoin (f1, n1, f2, n2)</code></a></h3> <p> Make the <code>n1</code>-th upvalue of the Lua closure <code>f1</code> refer to the <code>n2</code>-th upvalue of the Lua closure <code>f2</code>. <h1>7 – <a name="7">Lua Standalone</a></h1> <p> Although Lua has been designed as an extension language, to be embedded in a host C program, it is also frequently used as a standalone language. An interpreter for Lua as a standalone language, called simply <code>lua</code>, is provided with the standard distribution. The standalone interpreter includes all standard libraries, including the debug library. Its usage is: <pre> lua [options] [script [args]] </pre><p> The options are: <ul> <li><b><code>-e <em>stat</em></code>: </b> executes string <em>stat</em>;</li> <li><b><code>-l <em>mod</em></code>: </b> "requires" <em>mod</em>;</li> <li><b><code>-i</code>: </b> enters interactive mode after running <em>script</em>;</li> <li><b><code>-v</code>: </b> prints version information;</li> <li><b><code>-E</code>: </b> ignores environment variables;</li> <li><b><code>--</code>: </b> stops handling options;</li> <li><b><code>-</code>: </b> executes <code>stdin</code> as a file and stops handling options.</li> </ul><p> After handling its options, <code>lua</code> runs the given <em>script</em>. When called without arguments, <code>lua</code> behaves as <code>lua -v -i</code> when the standard input (<code>stdin</code>) is a terminal, and as <code>lua -</code> otherwise. <p> When called without option <code>-E</code>, the interpreter checks for an environment variable <a name="pdf-LUA_INIT_5_3"><code>LUA_INIT_5_3</code></a> (or <a name="pdf-LUA_INIT"><code>LUA_INIT</code></a> if the versioned name is not defined) before running any argument. If the variable content has the format <code>@<em>filename</em></code>, then <code>lua</code> executes the file. Otherwise, <code>lua</code> executes the string itself. <p> When called with option <code>-E</code>, besides ignoring <code>LUA_INIT</code>, Lua also ignores the values of <code>LUA_PATH</code> and <code>LUA_CPATH</code>, setting the values of <a href="#pdf-package.path"><code>package.path</code></a> and <a href="#pdf-package.cpath"><code>package.cpath</code></a> with the default paths defined in <code>luaconf.h</code>. <p> All options are handled in order, except <code>-i</code> and <code>-E</code>. For instance, an invocation like <pre> $ lua -e'a=1' -e 'print(a)' script.lua </pre><p> will first set <code>a</code> to 1, then print the value of <code>a</code>, and finally run the file <code>script.lua</code> with no arguments. (Here <code>$</code> is the shell prompt. Your prompt may be different.) <p> Before running any code, <code>lua</code> collects all command-line arguments in a global table called <code>arg</code>. The script name goes to index 0, the first argument after the script name goes to index 1, and so on. Any arguments before the script name (that is, the interpreter name plus its options) go to negative indices. For instance, in the call <pre> $ lua -la b.lua t1 t2 </pre><p> the table is like this: <pre> arg = { [-2] = "lua", [-1] = "-la", [0] = "b.lua", [1] = "t1", [2] = "t2" } </pre><p> If there is no script in the call, the interpreter name goes to index 0, followed by the other arguments. For instance, the call <pre> $ lua -e "print(arg[1])" </pre><p> will print "<code>-e</code>". If there is a script, the script is called with parameters <code>arg[1]</code>, ···, <code>arg[#arg]</code>. (Like all chunks in Lua, the script is compiled as a vararg function.) <p> In interactive mode, Lua repeatedly prompts and waits for a line. After reading a line, Lua first try to interpret the line as an expression. If it succeeds, it prints its value. Otherwise, it interprets the line as a statement. If you write an incomplete statement, the interpreter waits for its completion by issuing a different prompt. <p> In case of unprotected errors in the script, the interpreter reports the error to the standard error stream. If the error object is not a string but has a metamethod <code>__to_string</code>, the interpreter calls this metamethod to produce the final message. Otherwise, the interpreter converts the error object to a string and adds a stack traceback to it. <p> When finishing normally, the interpreter closes its main Lua state (see <a href="#lua_close"><code>lua_close</code></a>). The script can avoid this step by calling <a href="#pdf-os.exit"><code>os.exit</code></a> to terminate. <p> To allow the use of Lua as a script interpreter in Unix systems, the standalone interpreter skips the first line of a chunk if it starts with <code>#</code>. Therefore, Lua scripts can be made into executable programs by using <code>chmod +x</code> and the <code>#!</code> form, as in <pre> #!/usr/local/bin/lua </pre><p> (Of course, the location of the Lua interpreter may be different in your machine. If <code>lua</code> is in your <code>PATH</code>, then <pre> #!/usr/bin/env lua </pre><p> is a more portable solution.) <h1>8 – <a name="8">Incompatibilities with the Previous Version</a></h1> <p> Here we list the incompatibilities that you may find when moving a program from Lua 5.2 to Lua 5.3. You can avoid some incompatibilities by compiling Lua with appropriate options (see file <code>luaconf.h</code>). However, all these compatibility options will be removed in the future. <p> Lua versions can always change the C API in ways that do not imply source-code changes in a program, such as the numeric values for constants or the implementation of functions as macros. Therefore, you should not assume that binaries are compatible between different Lua versions. Always recompile clients of the Lua API when using a new version. <p> Similarly, Lua versions can always change the internal representation of precompiled chunks; precompiled chunks are not compatible between different Lua versions. <p> The standard paths in the official distribution may change between versions. <h2>8.1 – <a name="8.1">Changes in the Language</a></h2> <ul> <li> The main difference between Lua 5.2 and Lua 5.3 is the introduction of an integer subtype for numbers. Although this change should not affect "normal" computations, some computations (mainly those that involve some kind of overflow) can give different results. <p> You can fix these differences by forcing a number to be a float (in Lua 5.2 all numbers were float), in particular writing constants with an ending <code>.0</code> or using <code>x = x + 0.0</code> to convert a variable. (This recommendation is only for a quick fix for an occasional incompatibility; it is not a general guideline for good programming. For good programming, use floats where you need floats and integers where you need integers.) </li> <li> The conversion of a float to a string now adds a <code>.0</code> suffix to the result if it looks like an integer. (For instance, the float 2.0 will be printed as <code>2.0</code>, not as <code>2</code>.) You should always use an explicit format when you need a specific format for numbers. <p> (Formally this is not an incompatibility, because Lua does not specify how numbers are formatted as strings, but some programs assumed a specific format.) </li> <li> The generational mode for the garbage collector was removed. (It was an experimental feature in Lua 5.2.) </li> </ul> <h2>8.2 – <a name="8.2">Changes in the Libraries</a></h2> <ul> <li> The <code>bit32</code> library has been deprecated. It is easy to require a compatible external library or, better yet, to replace its functions with appropriate bitwise operations. (Keep in mind that <code>bit32</code> operates on 32-bit integers, while the bitwise operators in standard Lua operate on 64-bit integers.) </li> <li> The Table library now respects metamethods for setting and getting elements. </li> <li> The <a href="#pdf-ipairs"><code>ipairs</code></a> iterator now respects metamethods and its <code>__ipairs</code> metamethod has been deprecated. </li> <li> Option names in <a href="#pdf-io.read"><code>io.read</code></a> do not have a starting '<code>*</code>' anymore. For compatibility, Lua will continue to ignore this character. </li> <li> The following functions were deprecated in the mathematical library: <code>atan2</code>, <code>cosh</code>, <code>sinh</code>, <code>tanh</code>, <code>pow</code>, <code>frexp</code>, and <code>ldexp</code>. You can replace <code>math.pow(x,y)</code> with <code>x^y</code>; you can replace <code>math.atan2</code> with <code>math.atan</code>, which now accepts one or two parameters; you can replace <code>math.ldexp(x,exp)</code> with <code>x * 2.0^exp</code>. For the other operations, you can either use an external library or implement them in Lua. </li> <li> The searcher for C loaders used by <a href="#pdf-require"><code>require</code></a> changed the way it handles versioned names. Now, the version should come after the module name (as is usual in most other tools). For compatibility, that searcher still tries the old format if it cannot find an open function according to the new style. (Lua 5.2 already worked that way, but it did not document the change.) </li> </ul> <h2>8.3 – <a name="8.3">Changes in the API</a></h2> <ul> <li> Continuation functions now receive as parameters what they needed to get through <code>lua_getctx</code>, so <code>lua_getctx</code> has been removed. Adapt your code accordingly. </li> <li> Function <a href="#lua_dump"><code>lua_dump</code></a> has an extra parameter, <code>strip</code>. Use 0 as the value of this parameter to get the old behavior. </li> <li> Functions to inject/project unsigned integers (<code>lua_pushunsigned</code>, <code>lua_tounsigned</code>, <code>lua_tounsignedx</code>, <code>luaL_checkunsigned</code>, <code>luaL_optunsigned</code>) were deprecated. Use their signed equivalents with a type cast. </li> <li> Macros to project non-default integer types (<code>luaL_checkint</code>, <code>luaL_optint</code>, <code>luaL_checklong</code>, <code>luaL_optlong</code>) were deprecated. Use their equivalent over <a href="#lua_Integer"><code>lua_Integer</code></a> with a type cast (or, when possible, use <a href="#lua_Integer"><code>lua_Integer</code></a> in your code). </li> </ul> <h1>9 – <a name="9">The Complete Syntax of Lua</a></h1> <p> Here is the complete syntax of Lua in extended BNF. As usual in extended BNF, {A} means 0 or more As, and [A] means an optional A. (For operator precedences, see <a href="#3.4.8">§3.4.8</a>; for a description of the terminals Name, Numeral, and LiteralString, see <a href="#3.1">§3.1</a>.) <pre> chunk ::= block block ::= {stat} [retstat] stat ::= ‘<b>;</b>’ | varlist ‘<b>=</b>’ explist | functioncall | label | <b>break</b> | <b>goto</b> Name | <b>do</b> block <b>end</b> | <b>while</b> exp <b>do</b> block <b>end</b> | <b>repeat</b> block <b>until</b> exp | <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b> | <b>for</b> Name ‘<b>=</b>’ exp ‘<b>,</b>’ exp [‘<b>,</b>’ exp] <b>do</b> block <b>end</b> | <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b> | <b>function</b> funcname funcbody | <b>local</b> <b>function</b> Name funcbody | <b>local</b> namelist [‘<b>=</b>’ explist] retstat ::= <b>return</b> [explist] [‘<b>;</b>’] label ::= ‘<b>::</b>’ Name ‘<b>::</b>’ funcname ::= Name {‘<b>.</b>’ Name} [‘<b>:</b>’ Name] varlist ::= var {‘<b>,</b>’ var} var ::= Name | prefixexp ‘<b>[</b>’ exp ‘<b>]</b>’ | prefixexp ‘<b>.</b>’ Name namelist ::= Name {‘<b>,</b>’ Name} explist ::= exp {‘<b>,</b>’ exp} exp ::= <b>nil</b> | <b>false</b> | <b>true</b> | Numeral | LiteralString | ‘<b>...</b>’ | functiondef | prefixexp | tableconstructor | exp binop exp | unop exp prefixexp ::= var | functioncall | ‘<b>(</b>’ exp ‘<b>)</b>’ functioncall ::= prefixexp args | prefixexp ‘<b>:</b>’ Name args args ::= ‘<b>(</b>’ [explist] ‘<b>)</b>’ | tableconstructor | LiteralString functiondef ::= <b>function</b> funcbody funcbody ::= ‘<b>(</b>’ [parlist] ‘<b>)</b>’ block <b>end</b> parlist ::= namelist [‘<b>,</b>’ ‘<b>...</b>’] | ‘<b>...</b>’ tableconstructor ::= ‘<b>{</b>’ [fieldlist] ‘<b>}</b>’ fieldlist ::= field {fieldsep field} [fieldsep] field ::= ‘<b>[</b>’ exp ‘<b>]</b>’ ‘<b>=</b>’ exp | Name ‘<b>=</b>’ exp | exp fieldsep ::= ‘<b>,</b>’ | ‘<b>;</b>’ binop ::= ‘<b>+</b>’ | ‘<b>-</b>’ | ‘<b>*</b>’ | ‘<b>/</b>’ | ‘<b>//</b>’ | ‘<b>^</b>’ | ‘<b>%</b>’ | ‘<b>&</b>’ | ‘<b>~</b>’ | ‘<b>|</b>’ | ‘<b>>></b>’ | ‘<b><<</b>’ | ‘<b>..</b>’ | ‘<b><</b>’ | ‘<b><=</b>’ | ‘<b>></b>’ | ‘<b>>=</b>’ | ‘<b>==</b>’ | ‘<b>~=</b>’ | <b>and</b> | <b>or</b> unop ::= ‘<b>-</b>’ | <b>not</b> | ‘<b>#</b>’ | ‘<b>~</b>’ </pre> <p> <HR> <SMALL CLASS="footer"> Last update: Fri Jan 16 00:58:20 BRST 2015 </SMALL> <!-- Last change: minor edit --> </div> <% Html.simply_html_body_bottom() %> </body> </html> <% end