Luan Reference Manual

The reference manual is the official definition of the Luan language.

Original Copyright © 2011–2013 Lua.org, PUC-Rio. Freely available under the terms of the Lua license. Modified for Luan in 2014.

Contents

1 – Introduction

Luan is a high level programming language based on Lua. 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 programmers can quickly learn the language and then easily understand any code written in Luan.

Luan is implemented in Java and is tightly coupled with Java. So it makes a great scripting language for Java programmers. By importing the Java package, one can directly call Java from Luan.

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.

2 – Basic Concepts

This section describes the basic concepts of the language.

2.1 – Values and Types

Luan is a dynamically typed language. 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.

All values in Luan are first-class values. This means that all values can be stored in variables, passed as arguments to other functions, and returned as results.

There are eight basic types in Lua: nil, boolean, number, string, binary, function, userdata, and table. Nil is the type of the value nil, whose main property is to be different from any other value; it usually represents the absence of a useful value. Nil is implemented as the Java value null. Boolean is the type of the values false and true. Boolean is implemented as the Java class Boolean. Number represents real (double-precision floating-point) numbers. Number is implemented as the Java class Number. Any Java subclass of Number is allowed and this is invisible to the Luan user. Operations on numbers follow the same rules of the underlying Java implementation. String is implemented as the Java class String. Binary is implemented as the Java type byte[].

Luan can call (and manipulate) functions written in Luan and functions written in Java (see §3.4.9).

The type userdata is provided to allow arbitrary Java objects to be stored in Lua variables. A userdata value is a Java object that isn't of the standard Luan types.

Lua has a type thread that Luan lacks because Luan does not have the Lua concept of coroutines.

The type table implements associative arrays, that is, arrays that can be indexed not only with numbers, but with any Lua value except nil. Tables can be heterogeneous; that is, they can contain values of all types (except nil). Any key with value nil is not considered part of the table. Conversely, any key that is not part of a table has an associated value nil.

Tables are the sole data structuring mechanism in Lua; they can be used to represent ordinary arrays, sequences, symbol tables, sets, records, graphs, trees, etc. To represent records, Lua uses the field name as an index. The language supports this representation by providing a.name as syntactic sugar for a["name"]. There are several convenient ways to create tables in Lua (see §3.4.8).

We use the term sequence to denote a table where the set of all positive numeric keys is equal to {1..n} for some integer n, which is called the length of the sequence (see §3.4.6).

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 methods (see §3.4.10).

The indexing of tables follows the definition of raw equality in the language. The expressions a[i] and a[j] denote the same table element if and only if i and j are raw equal (that is, equal without metamethods).

Tables, functions, and userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.

The library function type returns a string describing the type of a given value (see §6.1).

2.2 – Environments and the Global Environment

As will be discussed in §3.2 and §3.3.3, any reference to a global name var is syntactically translated to _ENV.var. Moreover, every chunk is compiled in the scope of an external local variable called _ENV (see §3.3.2), so _ENV itself is never a global name in a chunk.

Despite the existence of this external _ENV variable and the translation of global names, _ENV is a completely regular name. In particular, you can define new variables and parameters with that name. Each reference to a global name uses the _ENV that is visible at that point in the program, following the usual visibility rules of Lua (see §3.5).

Any table used as the value of _ENV is called an environment.

Lua keeps a distinguished environment called the global environment. This value is kept in the Luan state implemented in Java. In Luan, the variable _G is initialized with this same value.

When Lua compiles a chunk, it initializes the value of its _ENV to an empty table. The values in the global environment become local variables of the chunk. All standard libraries are loaded in the global environment and so they become available as local variables. You can use load (or load_file) to load a chunk with a specific environment instead of starting empty.

If you change the values in the global environment, all chunks loaded after the change will get the new environment. Previously loaded chunks are not affected, however, as each has its own references to the values in its local variables. Moreover, the variable _G (which is stored in the original global environment) is never updated by Lua.

2.3 – Error Handling

Luan error handling is quite different from Lua.

Luan code can explicitly generate an error by calling the error function. Unlike Lua, Luan has try-catch blocks for catching errors. This means that there is no need for Lua's pcall and xpcall functions.

Whenever there is an error, an error object (also called an error message) is propagated with information about the error. Lua itself only generates errors where the error object is a string, but programs may generate errors with any value for the error object.

2.4 – Metatables and Metamethods

Every table in Luan can have a metatable. This metatable is an ordinary Luan table that defines the behavior of the original table under certain special operations. You can change several aspects of the behavior of operations over a table 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 "__add" of the table's metatable. If it finds one, Luan calls this function to perform the addition.

Inside Luan's implementation, there is a global metatable that applies to all objects. This metatable is not exposed to Luan users but can be used to change the behavior of objects other than tables.

The keys in a metatable are derived from the event names; the corresponding values are called metamethods. In the previous example, the event is "add" and the metamethod is the function that performs the addition.

You can query the metatable of any value using the get_metatable function.

You can replace the metatable of tables using the set_metatable function.

Tables have individual metatables (although multiple tables can share their metatables). By default, a table has no metatable.

A metatable controls how a table behaves in arithmetic operations, order comparisons, concatenation, length operation, and indexing. When Luan performs one of these operations over a table, it checks whether this table has a metatable with the corresponding event. If so, the value associated with that key (the metamethod) controls how Luan will perform the operation.

Metatables control the operations listed next. Each operation is identified by its corresponding name. The key for each operation is a string with its name prefixed by two underscores, '__'; for instance, the key for operation "add" is the string "__add".

The semantics of these operations is better explained by a Luan function describing how the interpreter executes the operation. The code shown here in Lua is only illustrative; the real behavior is hard coded in the interpreter and it is much more efficient than this simulation. All functions used in these descriptions (raw_get, to_number, etc.) are described in §6.1. In particular, to retrieve the metamethod of a given object, we use the expression 

     metatable(obj)[event]

This should be read as 

     raw_get(get_metatable(obj) or {}, event)

This means that the access to a metamethod does not invoke other metamethods, and access to tables with no metatables does not fail (it simply results in nil).

2.5 – Garbage Collection

Luan uses Java's garbage collection, so there is very little to say on this subject. So this section is just a place holder to replace the long explanation of Lua's garbage collection which isn't needed by Luan.

Lua has weak tables which is a good concept but is not yet implemented in Luan. It will be added when there is a need.

2.6 – Coroutines

Unlike Lua, Luan does not support coroutines. Yes coroutines are cool, but they are not simple, so in the name of simplicity, Luan does without them.

3 – The Language

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.

Language constructs will be explained using the usual extended BNF notation, in which {a} means 0 or more a's, and [a] means an optional a. Non-terminals are shown like non-terminal, keywords are shown like kword, and other terminal symbols are shown like ‘=’. The complete syntax of Luan can be found in §9 at the end of this manual.

3.1 – Lexical Conventions

Luan ignores spaces and comments between lexical elements (tokens), except as delimiters between names and keywords. But unlike Lua, Luan generally treats the newline character as a statement separator. This is how most languages work. If a newline is preceded by a backslash, then it is treated like a space. Also, inside of parenthesis (...), brackets [...], and braces {...}, a newline is treated like a space. This allows the Luan parser to catch mistakes more easily.

In interactive mode, Luan allows an expression on a line which is then evaluated and printed. This means that entering 1+1 on an interactive line will produce 2.

Names (also called identifiers) in Lua can be any string of letters, digits, and underscores, not beginning with a digit. Identifiers are used to name variables, table fields, and labels.

The following keywords are reserved and cannot be used as names: 

     and       break     do        else      elseif    end
     false     for       function  goto      if        in
     local     nil       not       or        repeat    return
     then      true      until     while
The following keywords are also reserved in Luan but not in Lua: 
     catch     import    try

Lua is a case-sensitive language: and is a reserved word, but And and AND are two different, valid names. As a convention, names starting with an underscore followed by uppercase letters (such as _VERSION) are reserved for variables used by Lua.

The following strings denote other tokens: 

     +     -     *     /     %     ^     #
     ==    ~=    <=    >=    <     >     =
     (     )     {     }     [     ]     
     ;     :     ,     .     ..    ...

Literal strings can be delimited by matching single or double quotes, and can contain the following C-like escape sequences: '\a' (bell), '\b' (backspace), '\f' (form feed), '\n' (newline), '\r' (carriage return), '\t' (horizontal tab), '\v' (vertical tab), '\\' (backslash), '\"' (quotation mark [double quote]), and '\'' (apostrophe [single quote]). A backslash followed by a real newline results in a newline in the string. The escape sequence '\z' 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.

A byte in a literal string can also be specified by its numerical value. This can be done with the escape sequence \xXX, where XX is a sequence of exactly two hexadecimal digits, or with the escape sequence \ddd, where ddd is a sequence of up to three decimal digits. (Note that if a decimal escape is to be followed by a digit, it must be expressed using exactly three digits.) Strings in Lua can contain any 8-bit value, including embedded zeros, which can be specified as '\0'.

Literal strings can also be defined using a long format enclosed by long brackets. We define an opening long bracket of level n as an opening square bracket followed by n equal signs followed by another opening square bracket. So, an opening long bracket of level 0 is written as [[, an opening long bracket of level 1 is written as [=[, and so on. A closing long bracket is defined similarly; for instance, a closing long bracket of level 4 is written as ]====]. A long literal 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 proper 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.

For convenience, when the opening long bracket is immediately followed by a newline, the newline is not included in the string. As an example, in a system using ASCII (in which 'a' is coded as 97, newline is coded as 10, and '1' is coded as 49), the five literal strings below denote the same string: 

     a = 'alo\n123"'
     a = "alo\n123\""
     a = '\97lo\10\04923"'
     a = [[alo
     123"]]
     a = [==[
     alo
     123"]==]

A numerical constant can be written with an optional fractional part and an optional decimal exponent, marked by a letter 'e' or 'E'. Lua also accepts hexadecimal constants, which start with 0x or 0X. Hexadecimal constants also accept an optional fractional part plus an optional binary exponent, marked by a letter 'p' or 'P'. Examples of valid numerical constants are 

     3     3.0     3.1416     314.16e-2     0.31416E1
     0xff  0x0.1E  0xA23p-4   0X1.921FB54442D18P+1

A comment starts with a double hyphen (--) anywhere outside a string. If the text immediately after -- is not an opening long bracket, the comment is a short comment, which runs until the end of the line. Otherwise, it is a long comment, which runs until the corresponding closing long bracket. Long comments are frequently used to disable code temporarily.

3.2 – Variables

Variables are places that store values. There are three kinds of variables in Lua: global variables, local variables, and table fields.

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): 

	var ::= Name

Name denotes identifiers, as defined in §3.1.

Any variable name is assumed to be global unless explicitly declared as a local (see §3.3.7). Local variables are lexically scoped: local variables can be freely accessed by functions defined inside their scope (see §3.5).

Before the first assignment to a variable, its value is nil.

Square brackets are used to index a table: 

	var ::= prefixexp ‘[’ exp ‘]’

The meaning of accesses to table fields can be changed via metatables. An access to an indexed variable t[i] is equivalent to a call get_table_event(t,i). (See §2.4 for a complete description of the get_table_event function. This function is not defined or callable in Lua. We use it here only for explanatory purposes.)

The syntax var.Name is just syntactic sugar for var["Name"]: 

	var ::= prefixexp ‘.’ Name

An access to a global variable x is equivalent to _ENV.x. Due to the way that chunks are compiled, _ENV is never a global name (see §2.2).

3.3 – Statements

Lua 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.

3.3.1 – Blocks

A block is a list of statements, which are executed sequentially: 

	block ::= {stat}

Lua has empty statements that allow you to separate statements with semicolons, start a block with a semicolon or write two semicolons in sequence: 

	stat ::= ‘;’

A block can be explicitly delimited to produce a single statement: 

	stat ::= do block end

Explicit blocks are useful to control the scope of variable declarations.

3.3.2 – Chunks

The unit of compilation of Lua is called a chunk. Syntactically, a chunk is simply a block: 

	chunk ::= block

Lua handles a chunk as the body of an anonymous function with a variable number of arguments (see §3.4.10). As such, chunks can define local variables, receive arguments, and return values. Moreover, such anonymous function is compiled as in the scope of an external local variable called _ENV (see §2.2). The resulting function always has _ENV as its only upvalue, even if it does not use that variable.

A chunk can be stored in a file or in a string inside the host program. To execute a chunk, Lua first precompiles the chunk into instructions for a virtual machine, and then it executes the compiled code with an interpreter for the virtual machine.

3.3.3 – Assignment

Lua 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: 

	stat ::= varlist ‘=’ explist
	varlist ::= var {‘,’ var}
	explist ::= exp {‘,’ exp}

Expressions are discussed in §3.4.

Before the assignment, the list of values is adjusted 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 nil'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 §3.4).

The assignment statement first evaluates all its expressions and only then are the assignments performed. Thus the code 

     i = 3
     i, a[i] = i+1, 20

sets a[3] to 20, without affecting a[4] because the i in a[i] is evaluated (to 3) before it is assigned 4. Similarly, the line 

     x, y = y, x

exchanges the values of x and y, and 

     x, y, z = y, z, x

cyclically permutes the values of x, y, and z.

The meaning of assignments to global variables and table fields can be changed via metatables. An assignment to an indexed variable t[i] = val is equivalent to settable_event(t,i,val). (See §2.4 for a complete description of the settable_event function. This function is not defined or callable in Lua. We use it here only for explanatory purposes.)

An assignment to a global variable x = val is equivalent to the assignment _ENV.x = val (see §2.2).

3.3.4 – Control Structures

The control structures if, while, and repeat have the usual meaning and familiar syntax: 

	stat ::= while exp do block end
	stat ::= repeat block until exp
	stat ::= if exp then block {elseif exp then block} [else block] end

Lua also has a for statement (see §3.3.5).

The condition expression of a control structure must return a boolean. This is unlike Lua and is intended to catch programming errors more quickly.

In the repeat–until loop, the inner block does not end at the until keyword, but only after the condition. So, the condition can refer to local variables declared inside the loop block.

The break statement terminates the execution of a while, repeat, or for loop, skipping to the next statement after the loop: 

	stat ::= break

A break ends the innermost enclosing loop.

The return statement is used to return values from a function or a chunk (which is a function in disguise). Functions can return more than one value, so the syntax for the return statement is 

	stat ::= return [explist] [‘;’]

3.3.5 – For Statement

The for statement works over functions, called iterators. On each iteration, the iterator function is called to produce a new value, stopping when this new value is nil. The for loop has the following syntax: 

	stat ::= for namelist in explist do block end
	namelist ::= Name {‘,’ Name}

A for statement like 

     for var_1, ···, var_n in expression do block end

is equivalent to the code: 

     do
       local f = expression
       while true do
         local var_1, ···, var_n = f()
         if var_1 == nil then break end
         block
       end
     end

Note the following:

Lua also has a numeric for statement which Luan does not support. Instead, Luan offers the range function (inspired by Python) which does the same thing without adding to the syntax of the language.

3.3.6 – Function Calls as Statements

To allow possible side-effects, function calls can be executed as statements: 

	stat ::= functioncall

In this case, all returned values are thrown away. Function calls are explained in §3.4.9.

3.3.7 – Local Declarations

Local variables can be declared anywhere inside a block. The declaration can include an initial assignment: 

	stat ::= local namelist [‘=’ explist]

If present, an initial assignment has the same semantics of a multiple assignment (see §3.3.3). Otherwise, all variables are initialized with nil.

A chunk is also a block (see §3.3.2), and so local variables can be declared in a chunk outside any explicit block.

The visibility rules for local variables are explained in §3.5.

3.3.8 – Or/And Statements

An or or and expression is also considered a statement. This is new for Luan and doesn't exist in Lua.

For example, consider a function do_something that returns a boolean indicating whether it succeeded or failed. You can then do: 

	do_something() or error "didn't work"

3.4 – Expressions

The basic expressions in Luan are the following: 

	exp ::= prefixexp
	exp ::= nil | false | true
	exp ::= Number
	exp ::= String
	exp ::= functiondef
	exp ::= tableconstructor
	exp ::= ‘...’
	exp ::= exp binop exp
	exp ::= unop exp
	prefixexp ::= var | functioncall | ‘(’ exp ‘)’

Numbers and literal strings are explained in §3.1; variables are explained in §3.2; function definitions are explained in §3.4.10; function calls are explained in §3.4.9; table constructors are explained in §3.4.8. Vararg expressions, denoted by three dots ('...'), can only be used when directly inside a vararg function; they are explained in §3.4.10.

Binary operators comprise arithmetic operators (see §3.4.1), relational operators (see §3.4.3), logical operators (see §3.4.4), and the concatenation operator (see §3.4.5). Unary operators comprise the unary minus (see §3.4.1), the unary not (see §3.4.4), and the unary length operator (see §3.4.6).

Both function calls and vararg expressions can result in multiple values. If a function call is used as a statement (see §3.3.6), 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, Lua adjusts the result list to one element, either discarding all values except the first one or adding a single nil if there are no values.

Here are some examples: 

     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

Any expression enclosed in parentheses always results in only one value. Thus, (f(x,y,z)) is always a single value, even if f returns several values. (The value of (f(x,y,z)) is the first value returned by f or nil if f does not return any values.)

3.4.1 – Arithmetic Operators

Luan supports the usual arithmetic operators: the binary + (addition), - (subtraction), * (multiplication), / (division), % (modulo), and ^ (exponentiation); and unary - (mathematical negation). If the operands are numbers, or strings that can be converted to numbers (see §3.4.2), then all operations have the usual meaning. Exponentiation works for any exponent. For instance, x^(-0.5) computes the inverse of the square root of x. Modulo is defined as 

     a % b == a - Math.floor(a/b)*b

That is, it is the remainder of a division that rounds the quotient towards minus infinity.

3.4.2 – Coercion

Luan provides automatic conversion between string and number values at run time. Any arithmetic operation applied to a string tries to convert this string to a number, following the rules of the Lua lexer. (The string may have leading and trailing spaces and a sign.) Conversely, whenever a number is used where a string is expected, the number is converted to a string, in a reasonable format. For complete control over how numbers are converted to strings, use the format function from the string library (see string.format).

3.4.3 – Relational Operators

The relational operators in Luan are 

     ==    ~=    <     >     <=    >=

These operators always result in false or true.

Equality (==) first compares the type of its operands. If the types are different, then the result is false. Otherwise, the values of the operands are compared. Numbers and strings are compared in the usual way. Tables, userdata, and threads are compared by reference: two objects are considered equal only if they are the same object. Every time you create a new object (a table, userdata, or thread), this new object is different from any previously existing object. Closures with the same reference are always equal. Closures with any detectable difference (different behavior, different definition) are always different.

You can change the way that Luan compares tables by using the "eq" metamethod (see §2.4).

The conversion rules of §3.4.2 do not apply to equality comparisons. Thus, "0"==0 evaluates to false, and t[0] and t["0"] denote different entries in a table.

The operator ~= is exactly the negation of equality (==).

The order operators work as follows. If both arguments are numbers, then they are compared as such. Otherwise, if both arguments are strings, then their values are compared according to the current locale. Otherwise, Lua tries to call the "lt" or the "le" metamethod (see §2.4). A comparison a > b is translated to b < a and a >= b is translated to b <= a.

3.4.4 – Logical Operators

The logical operators in Lua are and, or, and not. Like the control structures (see §3.3.4), all logical operators consider both false and nil as false and anything else as true.

The negation operator not always returns false or true. The conjunction operator and returns its first argument if this value is false or nil; otherwise, and returns its second argument. The disjunction operator or returns its first argument if this value is different from nil and false; otherwise, or returns its second argument. Both and and or use short-cut evaluation; that is, the second operand is evaluated only if necessary. Here are some examples: 

     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

(In this manual, --> indicates the result of the preceding expression.)

3.4.5 – Concatenation

The string concatenation operator in Luan is denoted by two dots ('..'). If both operands are strings or numbers, then they are converted to strings according to the rules mentioned in §3.4.2. Otherwise, the __concat metamethod is called (see §2.4).

3.4.6 – The Length Operator

The length operator is denoted by the unary prefix operator #. The length of a string is its number of bytes (that is, the usual meaning of string length when each character is one byte).

A program can modify the behavior of the length operator for any value but strings through the __len metamethod (see §2.4).

Unless a __len metamethod is given, the length of a table t is only defined if the table is a sequence, that is, the set of its positive numeric keys is equal to {1..n} for some integer n. In that case, n is its length. Note that a table like 

     {10, 20, nil, 40}

is not a sequence, because it has the key 4 but does not have the key 3. (So, there is no n such that the set {1..n} is equal to the set of positive numeric keys of that table.) Note, however, that non-numeric keys do not interfere with whether a table is a sequence.

3.4.7 – Precedence

Operator precedence in Luan follows the table below, from lower to higher priority: 

     or
     and
     <     >     <=    >=    ~=    ==
     ..
     +     -
     *     /     %
     not   #     - (unary)
     ^

As usual, you can use parentheses to change the precedences of an expression. The concatenation ('..') and exponentiation ('^') operators are right associative. All other binary operators are left associative.

3.4.8 – Table Constructors

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 

	tableconstructor ::= ‘{’ [fieldlist] ‘}’
	fieldlist ::= field {fieldsep field} [fieldsep]
	field ::= ‘[’ exp ‘]’ ‘=’ exp | Name ‘=’ exp | exp
	fieldsep ::= ‘,’ | ‘;’

Each field of the form [exp1] = exp2 adds to the new table an entry with key exp1 and value exp2. A field of the form name = exp is equivalent to ["name"] = exp. Finally, fields of the form exp are equivalent to [i] = exp, where i are consecutive numerical integers, starting with 1. Fields in the other formats do not affect this counting. For example, 

     a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }

is equivalent to 

     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

If the last field in the list has the form exp and the expression is a function call or a vararg expression, then all values returned by this expression enter the list consecutively (see §3.4.9).

The field list can have an optional trailing separator, as a convenience for machine-generated code.

3.4.9 – Function Calls

A function call in Luan has the following syntax: 

	functioncall ::= prefixexp args

In a function call, first prefixexp and args are evaluated. If the value of prefixexp has type function, 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 §2.4).

Lua supports a special function call for "methods" like obj:fn(args) . Luan does not support this.

Arguments have the following syntax: 

	args ::= ‘(’ [explist] ‘)’
	args ::= tableconstructor
	args ::= String

All argument expressions are evaluated before the call. A call of the form f{fields} is syntactic sugar for f({fields}); that is, the argument list is a single new table. A call of the form f'string' (or f"string" or f[[string]]) is syntactic sugar for f('string'); that is, the argument list is a single literal string.

A call of the form return functioncall is called a tail call. Luan implements proper tail calls (or proper tail recursion): 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 return 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: 

     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

3.4.10 – Function Definitions

The syntax for function definition is 

	functiondef ::= function funcbody
	funcbody ::= ‘(’ [parlist] ‘)’ block end

The following syntactic sugar simplifies function definitions: 

	stat ::= function funcname funcbody
	stat ::= local function Name funcbody
	funcname ::= Name {‘.’ Name}

The statement 

     function f () body end

translates to 

     f = function () body end

The statement 

     function t.a.b.c.f () body end

translates to 

     t.a.b.c.f = function () body end

The statement 

     local function f () body end

translates to 

     local f; f = function () body end

not to 

     local f = function () body end

(This only makes a difference when the body of the function contains references to f.)

A function definition is an executable expression, whose value has type function. When Luan precompiles a chunk, all its function bodies are precompiled too. Then, whenever Luan executes the function definition, the function is instantiated (or closed). This function instance (or closure) is the final value of the expression.

Parameters act as local variables that are initialized with the argument values: 

	parlist ::= namelist [‘,’ ‘...’] | ‘...’

When a function is called, the list of arguments is adjusted to the length of the list of parameters, unless the function is a vararg function, which is indicated by three dots ('...') 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 vararg expression, 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).

As an example, consider the following definitions: 

     function f(a, b) end
     function g(a, b, ...) end
     function r() return 1,2,3 end

Then, we have the following mapping from arguments to parameters and to the vararg expression: 

     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

Results are returned using the return statement (see §3.3.4). If control reaches the end of a function without encountering a return statement, then the function returns with no results.

3.5 – Visibility Rules

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: 

     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)

Notice that, in a declaration like local x = x, the new x being declared is not in scope yet, and so the second x refers to the outside variable.

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 upvalue, or external local variable, inside the inner function.

Notice that each execution of a local statement defines new local variables. Consider the following example: 

     a = {}
     local x = 20
     for i=1,10 do
       local y = 0
       a[i] = function () y=y+1; return x+y end
     end

The loop creates ten closures (that is, ten instances of the anonymous function). Each of these closures uses a different y variable, while all of them share the same x.

4 – The Application Program Interface

In the Lua documentation, this section described the C API for Lua. Obviously this is not relevant for Luan. The implementation of Luan is radically different from Lua and will be documented eventually in Javadoc. So this section is just a placeholder so that Luan documentation can match Lua's documentation.

5 – The Auxiliary Library

Like the previous section, this section is specific to Lua and is not relevant to Luan. So this section is just a placeholder.

6 – Standard Libraries

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., type and getmetatable); 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., table.sort).

All libraries are implemented through the official C API and are provided as separate C modules. Currently, Lua has the following standard libraries:

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.

To have access to these libraries, the C host program should call the luaL_openlibs function, which opens all standard libraries. Alternatively, the host program can open them individually by using luaL_requiref to call luaopen_base (for the basic library), luaopen_package (for the package library), luaopen_coroutine (for the coroutine library), luaopen_string (for the string library), luaopen_table (for the table library), luaopen_math (for the mathematical library), luaopen_bit32 (for the bit library), luaopen_io (for the I/O library), luaopen_os (for the Operating System library), and luaopen_debug (for the debug library). These functions are declared in lualib.h.

6.1 – Basic Functions

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.

assert (v [, message])

Issues an error when the value of its argument v is false (i.e., nil or false); otherwise, returns all its arguments. message is an error message; when absent, it defaults to "assertion failed!"

collectgarbage ([opt [, arg]])

This function is a generic interface to the garbage collector. It performs different functions according to its first argument, opt:

dofile ([filename])

Opens the named file and executes its contents as a Lua chunk. When called without arguments, dofile executes the contents of the standard input (stdin). Returns all values returned by the chunk. In case of errors, dofile propagates the error to its caller (that is, dofile does not run in protected mode).

error (message [, level])

Terminates the last protected function called and returns message as the error message. Function error never returns.

Usually, error adds some information about the error position at the beginning of the message, if the message is a string. The level argument specifies how to get the error position. With level 1 (the default), the error position is where the error function was called. Level 2 points the error to where the function that called error was called; and so on. Passing a level 0 avoids the addition of error position information to the message.

_G

A global variable (not a function) that holds the global environment (see §2.2). Lua itself does not use this variable; changing its value does not affect any environment, nor vice-versa.

getmetatable (object)

If object does not have a metatable, returns nil. Otherwise, if the object's metatable has a "__metatable" field, returns the associated value. Otherwise, returns the metatable of the given object.

ipairs (t)

If t has a metamethod __ipairs, calls it with t as argument and returns the first three results from the call.

Otherwise, returns three values: an iterator function, the table t, and 0, so that the construction 

     for i,v in ipairs(t) do body end

will iterate over the pairs (1,t[1]), (2,t[2]), ..., up to the first integer key absent from the table.

load (ld [, source [, mode [, env]]])

Loads a chunk.

If ld is a string, the chunk is this string. If ld is a function, load calls it repeatedly to get the chunk pieces. Each call to ld must return a string that concatenates with previous results. A return of an empty string, nil, or no value signals the end of the chunk.

If there are no syntactic errors, returns the compiled chunk as a function; otherwise, returns nil plus the error message.

If the resulting function has upvalues, the first upvalue is set to the value of env, if that parameter is given, or to the value of the global environment. (When you load a main chunk, the resulting function will always have exactly one upvalue, the _ENV variable (see §2.2). When you load a binary chunk created from a function (see string.dump), the resulting function can have arbitrary upvalues.)

source is used as the source of the chunk for error messages and debug information (see §4.9). When absent, it defaults to ld, if ld is a string, or to "=(load)" otherwise.

The string mode controls whether the chunk can be text or binary (that is, a precompiled chunk). It may be the string "b" (only binary chunks), "t" (only text chunks), or "bt" (both binary and text). The default is "bt".

loadfile ([filename [, mode [, env]]])

Similar to load, but gets the chunk from file filename or from the standard input, if no file name is given.

next (table [, index])

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. next returns the next index of the table and its associated value. When called with nil as its second argument, next returns an initial index and its associated value. When called with the last index, or with nil in an empty table, next returns nil. If the second argument is absent, then it is interpreted as nil. In particular, you can use next(t) to check whether a table is empty.

The order in which the indices are enumerated is not specified, even for numeric indices. (To traverse a table in numeric order, use a numerical for.)

The behavior of next 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.

pairs (t)

If t has a metamethod __pairs, calls it with t as argument and returns the first three results from the call.

Otherwise, returns three values: the next function, the table t, and nil, so that the construction 

     for k,v in pairs(t) do body end

will iterate over all key–value pairs of table t.

See function next for the caveats of modifying the table during its traversal.

pcall (f [, arg1, ···])

Calls function f with the given arguments in protected mode. This means that any error inside f is not propagated; instead, pcall 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, pcall also returns all results from the call, after this first result. In case of any error, pcall returns false plus the error message.

print (···)

Receives any number of arguments and prints their values to stdout, using the tostring function to convert each argument to a string. print 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 string.format and io.write.

rawequal (v1, v2)

Checks whether v1 is equal to v2, without invoking any metamethod. Returns a boolean.

rawget (table, index)

Gets the real value of table[index], without invoking any metamethod. table must be a table; index may be any value.

rawlen (v)

Returns the length of the object v, which must be a table or a string, without invoking any metamethod. Returns an integer number.

rawset (table, index, value)

Sets the real value of table[index] to value, without invoking any metamethod. table must be a table, index any value different from nil and NaN, and value any Lua value.

This function returns table.

select (index, ···)

If index is a number, returns all arguments after argument number index; a negative number indexes from the end (-1 is the last argument). Otherwise, index must be the string "#", and select returns the total number of extra arguments it received.

setmetatable (table, metatable)

Sets the metatable for the given table. (You cannot change the metatable of other types from Lua, only from C.) If metatable is nil, removes the metatable of the given table. If the original metatable has a "__metatable" field, raises an error.

This function returns table.

tonumber (e [, base])

When called with no base, tonumber tries to convert its argument to a number. If the argument is already a number or a string convertible to a number (see §3.4.2), then tonumber returns this number; otherwise, it returns nil.

When called with base, then e should 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 'A' (in either upper or lower case) represents 10, 'B' represents 11, and so forth, with 'Z' representing 35. If the string e is not a valid numeral in the given base, the function returns nil.

tostring (v)

Receives a value of any type and converts it to a string in a reasonable format. (For complete control of how numbers are converted, use string.format.)

If the metatable of v has a "__tostring" field, then tostring calls the corresponding value with v as argument, and uses the result of the call as its result.

type (v)

Returns the type of its only argument, coded as a string. The possible results of this function are "nil" (a string, not the value nil), "number", "string", "boolean", "table", "function", "thread", and "userdata".

_VERSION

A global variable (not a function) that holds a string containing the current interpreter version. The current contents of this variable is "Lua 5.2".

xpcall (f, msgh [, arg1, ···])

This function is similar to pcall, except that it sets a new message handler msgh.

6.2 – Coroutine Manipulation

The operations related to coroutines comprise a sub-library of the basic library and come inside the table coroutine. See §2.6 for a general description of coroutines.

coroutine.create (f)

Creates a new coroutine, with body f. f must be a Lua function. Returns this new coroutine, an object with type "thread".

coroutine.resume (co [, val1, ···])

Starts or continues the execution of coroutine co. The first time you resume a coroutine, it starts running its body. The values val1, ... are passed as the arguments to the body function. If the coroutine has yielded, resume restarts it; the values val1, ... are passed as the results from the yield.

If the coroutine runs without any errors, resume returns true plus any values passed to yield (if the coroutine yields) or any values returned by the body function (if the coroutine terminates). If there is any error, resume returns false plus the error message.

coroutine.running ()

Returns the running coroutine plus a boolean, true when the running coroutine is the main one.

coroutine.status (co)

Returns the status of coroutine co, as a string: "running", if the coroutine is running (that is, it called status); "suspended", if the coroutine is suspended in a call to yield, or if it has not started running yet; "normal" if the coroutine is active but not running (that is, it has resumed another coroutine); and "dead" if the coroutine has finished its body function, or if it has stopped with an error.

coroutine.wrap (f)

Creates a new coroutine, with body f. f 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 resume. Returns the same values returned by resume, except the first boolean. In case of error, propagates the error.

coroutine.yield (···)

Suspends the execution of the calling coroutine. Any arguments to yield are passed as extra results to resume.

6.3 – Modules

The package library provides basic facilities for loading modules in Lua. It exports one function directly in the global environment: require. Everything else is exported in a table package.

require (modname)

Loads the given module. The function starts by looking into the package.loaded table to determine whether modname is already loaded. If it is, then require returns the value stored at package.loaded[modname]. Otherwise, it tries to find a loader for the module.

To find a loader, require is guided by the package.searchers sequence. By changing this sequence, we can change how require looks for a module. The following explanation is based on the default configuration for package.searchers.

First require queries package.preload[modname]. If it has a value, this value (which should be a function) is the loader. Otherwise require searches for a Lua loader using the path stored in package.path. If that also fails, it searches for a C loader using the path stored in package.cpath. If that also fails, it tries an all-in-one loader (see package.searchers).

Once a loader is found, require calls the loader with two arguments: modname 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, require assigns the returned value to package.loaded[modname]. If the loader does not return a non-nil value and has not assigned any value to package.loaded[modname], then require assigns true to this entry. In any case, require returns the final value of package.loaded[modname].

If there is any error loading or running the module, or if it cannot find any loader for the module, then require raises an error.

package.config

A string describing some compile-time configurations for packages. This string is a sequence of lines:

package.cpath

The path used by require to search for a C loader.

Lua initializes the C path package.cpath in the same way it initializes the Lua path package.path, using the environment variable LUA_CPATH_5_2 or the environment variable LUA_CPATH or a default path defined in luaconf.h.

package.loaded

A table used by require to control which modules are already loaded. When you require a module modname and package.loaded[modname] is not false, require simply returns the value stored there.

This variable is only a reference to the real table; assignments to this variable do not change the table used by require.

package.loadlib (libname, funcname)

Dynamically links the host program with the C library libname.

If funcname is "*", 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 funcname inside the library and returns this function as a C function. So, funcname must follow the lua_CFunction prototype (see lua_CFunction).

This is a low-level function. It completely bypasses the package and module system. Unlike require, it does not perform any path searching and does not automatically adds extensions. libname must be the complete file name of the C library, including if necessary a path and an extension. funcname must be the exact name exported by the C library (which may depend on the C compiler and linker used).

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 dlfcn standard).

package.path

The path used by require to search for a Lua loader.

At start-up, Lua initializes this variable with the value of the environment variable LUA_PATH_5_2 or the environment variable LUA_PATH or with a default path defined in luaconf.h, if those environment variables are not defined. Any ";;" in the value of the environment variable is replaced by the default path.

package.preload

A table to store loaders for specific modules (see require).

This variable is only a reference to the real table; assignments to this variable do not change the table used by require.

package.searchers

A table used by require to control how to load modules.

Each entry in this table is a searcher function. When looking for a module, require calls each of these searchers in ascending order, with the module name (the argument given to require) as its sole parameter. The function can return another function (the module loader) plus an extra value that will be passed to that loader, or a string explaining why it did not find that module (or nil if it has nothing to say).

Lua initializes this table with four searcher functions.

The first searcher simply looks for a loader in the package.preload table.

The second searcher looks for a loader as a Lua library, using the path stored at package.path. The search is done as described in function package.searchpath.

The third searcher looks for a loader as a C library, using the path given by the variable package.cpath. Again, the search is done as described in function package.searchpath. For instance, if the C path is the string 

     "./?.so;./?.dll;/usr/local/?/init.so"

the searcher for module foo will try to open the files ./foo.so, ./foo.dll, and /usr/local/foo/init.so, 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 "luaopen_" 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 prefix up to (and including) the first hyphen is removed. For instance, if the module name is a.v1-b.c, the function name will be luaopen_b_c.

The fourth searcher tries an all-in-one loader. It searches the C path for a library for the root name of the given module. For instance, when requiring a.b.c, it will search for a C library for a. If found, it looks into it for an open function for the submodule; in our example, that would be luaopen_a_b_c. With this facility, a package can pack several C submodules into one single library, with each submodule keeping its original open function.

All searchers except the first one (preload) return as the extra value the file name where the module was found, as returned by package.searchpath. The first searcher returns no extra value.

package.searchpath (name, path [, sep [, rep]])

Searches for the given name in the given path.

A path is a string containing a sequence of templates separated by semicolons. For each template, the function replaces each interrogation mark (if any) in the template with a copy of name wherein all occurrences of sep (a dot, by default) were replaced by rep (the system's directory separator, by default), and then tries to open the resulting file name.

For instance, if the path is the string 

     "./?.lua;./?.lc;/usr/local/?/init.lua"

the search for the name foo.a will try to open the files ./foo/a.lua, ./foo/a.lc, and /usr/local/foo/a/init.lua, in that order.

Returns the resulting name of the first file that it can open in read mode (after closing the file), or nil plus an error message if none succeeds. (This error message lists all file names it tried to open.)

6.4 – String Manipulation

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.

The string library provides all its functions inside the table string. It also sets a metatable for strings where the __index field points to the string table. Therefore, you can use the string functions in object-oriented style. For instance, string.byte(s,i) can be written as s:byte(i).

The string library assumes one-byte character encodings.

string.byte (s [, i [, j]])

Returns the internal numerical codes of the characters s[i], s[i+1], ..., s[j]. The default value for i is 1; the default value for j is i. These indices are corrected following the same rules of function string.sub.

Numerical codes are not necessarily portable across platforms.

string.char (···)

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.

Numerical codes are not necessarily portable across platforms.

string.dump (function)

Returns a string containing a binary representation of the given function, so that a later load on this string returns a copy of the function (but with new upvalues).

string.find (s, pattern [, init [, plain]])

Looks for the first match of pattern in the string s. If it finds a match, then find returns the indices of s where this occurrence starts and ends; otherwise, it returns nil. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative. A value of true as a fourth, optional argument plain turns off the pattern matching facilities, so the function does a plain "find substring" operation, with no characters in pattern being considered magic. Note that if plain is given, then init must be given as well.

If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.

string.format (formatstring, ···)

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 ANSI C function sprintf. The only differences are that the options/modifiers *, h, L, l, n, and p are not supported and that there is an extra option, q. The q 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 

     string.format('%q', 'a string with "quotes" and \n new line')

may produce the string: 

     "a string with \"quotes\" and \
      new line"

Options A and a (when available), E, e, f, G, and g all expect a number as argument. Options c, d, i, o, u, X, and x also expect a number, but the range of that number may be limited by the underlying C implementation. For options o, u, X, and x, the number cannot be negative. Option q expects a string; option s expects a string without embedded zeros. If the argument to option s is not a string, it is converted to one following the same rules of tostring.

string.gmatch (s, pattern)

Returns an iterator function that, each time it is called, returns the next captures from pattern over the string s. If pattern specifies no captures, then the whole match is produced in each call.

As an example, the following loop will iterate over all the words from string s, printing one per line: 

     s = "hello world from Lua"
     for w in string.gmatch(s, "%a+") do
       print(w)
     end

The next example collects all pairs key=value from the given string into a table: 

     t = {}
     s = "from=world, to=Lua"
     for k, v in string.gmatch(s, "(%w+)=(%w+)") do
       t[k] = v
     end

For this function, a caret '^' at the start of a pattern does not work as an anchor, as this would prevent the iteration.

string.gsub (s, pattern, repl [, n])

Returns a copy of s in which all (or the first n, if given) occurrences of the pattern have been replaced by a replacement string specified by repl, which can be a string, a table, or a function. gsub also returns, as its second value, the total number of matches that occurred. The name gsub comes from Global SUBstitution.

If repl is a string, then its value is used for replacement. The character % works as an escape character: any sequence in repl of the form %d, with d between 1 and 9, stands for the value of the d-th captured substring. The sequence %0 stands for the whole match. The sequence %% stands for a single %.

If repl is a table, then the table is queried for every match, using the first capture as the key.

If repl is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order.

In any case, if the pattern specifies no captures, then it behaves as if the whole pattern was inside a capture.

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 false or nil, then there is no replacement (that is, the original match is kept in the string).

Here are some examples: 

     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="5.2"}
     x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
     --> x="lua-5.2.tar.gz"

string.len (s)

Receives a string and returns its length. The empty string "" has length 0. Embedded zeros are counted, so "a\000bc\000" has length 5.

string.lower (s)

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.

string.match (s, pattern [, init])

Looks for the first match of pattern in the string s. If it finds one, then match returns the captures from the pattern; otherwise it returns nil. If pattern specifies no captures, then the whole match is returned. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative.

string.rep (s, n [, sep])

Returns a string that is the concatenation of n copies of the string s separated by the string sep. The default value for sep is the empty string (that is, no separator).

string.reverse (s)

Returns a string that is the string s reversed.

string.sub (s, i [, j])

Returns the substring of s that starts at i and continues until j; i and j can be negative. If j is absent, then it is assumed to be equal to -1 (which is the same as the string length). In particular, the call string.sub(s,1,j) returns a prefix of s with length j, and string.sub(s, -i) returns a suffix of s with length i.

If, after the translation of negative indices, i is less than 1, it is corrected to 1. If j is greater than the string length, it is corrected to that length. If, after these corrections, i is greater than j, the function returns the empty string.

string.upper (s)

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.

6.4.1 – Patterns

Character Class:

A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:

For all classes represented by single letters (%a, %c, etc.), the corresponding uppercase letter represents the complement of the class. For instance, %S represents all non-space characters.

The definitions of letter, space, and other character groups depend on the current locale. In particular, the class [a-z] may not be equivalent to %l.

Pattern Item:

A pattern item can be

Pattern:

A pattern is a sequence of pattern items. A caret '^' at the beginning of a pattern anchors the match at the beginning of the subject string. A '$' at the end of a pattern anchors the match at the end of the subject string. At other positions, '^' and '$' have no special meaning and represent themselves.

Captures:

A pattern can contain sub-patterns enclosed in parentheses; they describe captures. When a match succeeds, the substrings of the subject string that match captures are stored (captured) for future use. Captures are numbered according to their left parentheses. For instance, in the pattern "(a*(.)%w(%s*))", the part of the string matching "a*(.)%w(%s*)" is stored as the first capture (and therefore has number 1); the character matching "." is captured with number 2, and the part matching "%s*" has number 3.

As a special case, the empty capture () captures the current string position (a number). For instance, if we apply the pattern "()aa()" on the string "flaaap", there will be two captures: 3 and 5.

6.5 – Table Manipulation

This library provides generic functions for table manipulation. It provides all its functions inside the table table.

Remember that, whenever an operation needs the length of a table, the table should be a proper sequence or have a __len metamethod (see §3.4.6). All functions ignore non-numeric keys in tables given as arguments.

For performance reasons, all table accesses (get/set) performed by these functions are raw.

table.concat (list [, sep [, i [, j]]])

Given a list where all elements are strings or numbers, returns the string list[i]..sep..list[i+1] ··· sep..list[j]. The default value for sep is the empty string, the default for i is 1, and the default for j is #list. If i is greater than j, returns the empty string.

table.insert (list, [pos,] value)

Inserts element value at position pos in list, shifting up the elements list[pos], list[pos+1], ···, list[#list]. The default value for pos is #list+1, so that a call table.insert(t,x) inserts x at the end of list t.

table.pack (···)

Returns a new table with all parameters stored into keys 1, 2, etc. and with a field "n" with the total number of parameters. Note that the resulting table may not be a sequence.

table.remove (list [, pos])

Removes from list the element at position pos, returning the value of the removed element. When pos is an integer between 1 and #list, it shifts down the elements list[pos+1], list[pos+2], ···, list[#list] and erases element list[#list]; The index pos can also be 0 when #list is 0, or #list + 1; in those cases, the function erases the element list[pos].

The default value for pos is #list, so that a call table.remove(t) removes the last element of list t.

table.sort (list [, comp])

Sorts list elements in a given order, in-place, from list[1] to list[#list]. If comp 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 not comp(list[i+1],list[i]) will be true after the sort). If comp is not given, then the standard Lua operator < is used instead.

The sort algorithm is not stable; that is, elements considered equal by the given order may have their relative positions changed by the sort.

table.unpack (list [, i [, j]])

Returns the elements from the given table. This function is equivalent to 

     return list[i], list[i+1], ···, list[j]

By default, i is 1 and j is #list.

6.6 – Mathematical Functions

This library is an interface to the standard C math library. It provides all its functions inside the table math.

math.abs (x)

Returns the absolute value of x.

math.acos (x)

Returns the arc cosine of x (in radians).

math.asin (x)

Returns the arc sine of x (in radians).

math.atan (x)

Returns the arc tangent of x (in radians).

math.atan2 (y, x)

Returns the arc tangent of y/x (in radians), but uses the signs of both parameters to find the quadrant of the result. (It also handles correctly the case of x being zero.)

math.ceil (x)

Returns the smallest integer larger than or equal to x.

math.cos (x)

Returns the cosine of x (assumed to be in radians).

math.cosh (x)

Returns the hyperbolic cosine of x.

math.deg (x)

Returns the angle x (given in radians) in degrees.

math.exp (x)

Returns the value ex.

math.floor (x)

Returns the largest integer smaller than or equal to x.

math.fmod (x, y)

Returns the remainder of the division of x by y that rounds the quotient towards zero.

math.frexp (x)

Returns m and e such that x = m2e, e is an integer and the absolute value of m is in the range [0.5, 1) (or zero when x is zero).

math.huge

The value HUGE_VAL, a value larger than or equal to any other numerical value.

math.ldexp (m, e)

Returns m2e (e should be an integer).

math.log (x [, base])

Returns the logarithm of x in the given base. The default for base is e (so that the function returns the natural logarithm of x).

math.max (x, ···)

Returns the maximum value among its arguments.

math.min (x, ···)

Returns the minimum value among its arguments.

math.modf (x)

Returns two numbers, the integral part of x and the fractional part of x.

math.pi

The value of π.

math.pow (x, y)

Returns xy. (You can also use the expression x^y to compute this value.)

math.rad (x)

Returns the angle x (given in degrees) in radians.

math.random ([m [, n]])

This function is an interface to the simple pseudo-random generator function rand provided by Standard C. (No guarantees can be given for its statistical properties.)

When called without arguments, returns a uniform pseudo-random real number in the range [0,1). When called with an integer number m, math.random returns a uniform pseudo-random integer in the range [1, m]. When called with two integer numbers m and n, math.random returns a uniform pseudo-random integer in the range [m, n].

math.randomseed (x)

Sets x as the "seed" for the pseudo-random generator: equal seeds produce equal sequences of numbers.

math.sin (x)

Returns the sine of x (assumed to be in radians).

math.sinh (x)

Returns the hyperbolic sine of x.

math.sqrt (x)

Returns the square root of x. (You can also use the expression x^0.5 to compute this value.)

math.tan (x)

Returns the tangent of x (assumed to be in radians).

math.tanh (x)

Returns the hyperbolic tangent of x.

6.7 – Bitwise Operations

This library provides bitwise operations. It provides all its functions inside the table bit32.

Unless otherwise stated, all functions accept numeric arguments in the range (-251,+251); each argument is normalized to the remainder of its division by 232 and truncated to an integer (in some unspecified way), so that its final value falls in the range [0,232 - 1]. Similarly, all results are in the range [0,232 - 1]. Note that bit32.bnot(0) is 0xFFFFFFFF, which is different from -1.

bit32.arshift (x, disp)

Returns the number x shifted disp bits to the right. The number disp may be any representable integer. Negative displacements shift to the left.

This shift operation is what is called arithmetic shift. Vacant bits on the left are filled with copies of the higher bit of x; vacant bits on the right are filled with zeros. In particular, displacements with absolute values higher than 31 result in zero or 0xFFFFFFFF (all original bits are shifted out).

bit32.band (···)

Returns the bitwise and of its operands.

bit32.bnot (x)

Returns the bitwise negation of x. For any integer x, the following identity holds: 

     assert(bit32.bnot(x) == (-1 - x) % 2^32)

bit32.bor (···)

Returns the bitwise or of its operands.

bit32.btest (···)

Returns a boolean signaling whether the bitwise and of its operands is different from zero.

bit32.bxor (···)

Returns the bitwise exclusive or of its operands.

bit32.extract (n, field [, width])

Returns the unsigned number formed by the bits field to field + width - 1 from n. Bits are numbered from 0 (least significant) to 31 (most significant). All accessed bits must be in the range [0, 31].

The default for width is 1.

bit32.replace (n, v, field [, width])

Returns a copy of n with the bits field to field + width - 1 replaced by the value v. See bit32.extract for details about field and width.

bit32.lrotate (x, disp)

Returns the number x rotated disp bits to the left. The number disp may be any representable integer.

For any valid displacement, the following identity holds: 

     assert(bit32.lrotate(x, disp) == bit32.lrotate(x, disp % 32))

In particular, negative displacements rotate to the right.

bit32.lshift (x, disp)

Returns the number x shifted disp bits to the left. The number disp may be any representable integer. Negative displacements shift to the right. In any direction, vacant bits are filled with zeros. In particular, displacements with absolute values higher than 31 result in zero (all bits are shifted out).

For positive displacements, the following equality holds: 

     assert(bit32.lshift(b, disp) == (b * 2^disp) % 2^32)

bit32.rrotate (x, disp)

Returns the number x rotated disp bits to the right. The number disp may be any representable integer.

For any valid displacement, the following identity holds: 

     assert(bit32.rrotate(x, disp) == bit32.rrotate(x, disp % 32))

In particular, negative displacements rotate to the left.

bit32.rshift (x, disp)

Returns the number x shifted disp bits to the right. The number disp may be any representable integer. Negative displacements shift to the left. In any direction, vacant bits are filled with zeros. In particular, displacements with absolute values higher than 31 result in zero (all bits are shifted out).

For positive displacements, the following equality holds: 

     assert(bit32.rshift(b, disp) == math.floor(b % 2^32 / 2^disp))

This shift operation is what is called logical shift.

6.8 – Input and Output Facilities

The I/O library provides two different styles for file manipulation. The first one uses implicit file descriptors; 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 descriptors.

When using implicit file descriptors, all operations are supplied by table io. When using explicit file descriptors, the operation io.open returns a file descriptor and then all operations are supplied as methods of the file descriptor.

The table io also provides three predefined file descriptors with their usual meanings from C: io.stdin, io.stdout, and io.stderr. The I/O library never closes these files.

Unless otherwise stated, all I/O functions return nil 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 nil 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 errno.

io.close ([file])

Equivalent to file:close(). Without a file, closes the default output file.

io.flush ()

Equivalent to io.output():flush().

io.input ([file])

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.

In case of errors this function raises the error, instead of returning an error code.

io.lines ([filename ···])

Opens the given file name in read mode and returns an iterator function that works like file:lines(···) over the opened file. When the iterator function detects the end of file, it returns nil (to finish the loop) and automatically closes the file.

The call io.lines() (with no file name) is equivalent to io.input():lines(); 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.

In case of errors this function raises the error, instead of returning an error code.

io.open (filename [, mode])

This function opens a file, in the mode specified in the string mode. It returns a new file handle, or, in case of errors, nil plus an error message.

The mode string can be any of the following:

The mode string can also have a 'b' at the end, which is needed in some systems to open the file in binary mode.

io.output ([file])

Similar to io.input, but operates over the default output file.

io.popen (prog [, mode])

This function is system dependent and is not available on all platforms.

Starts program prog in a separated process and returns a file handle that you can use to read data from this program (if mode is "r", the default) or to write data to this program (if mode is "w").

io.read (···)

Equivalent to io.input():read(···).

io.tmpfile ()

Returns a handle for a temporary file. This file is opened in update mode and it is automatically removed when the program ends.

io.type (obj)

Checks whether obj is a valid file handle. Returns the string "file" if obj is an open file handle, "closed file" if obj is a closed file handle, or nil if obj is not a file handle.

io.write (···)

Equivalent to io.output():write(···).

file:close ()

Closes file. Note that files are automatically closed when their handles are garbage collected, but that takes an unpredictable amount of time to happen.

When closing a file handle created with io.popen, file:close returns the same values returned by os.execute.

file:flush ()

Saves any written data to file.

file:lines (···)

Returns an iterator function that, each time it is called, reads the file according to the given formats. When no format is given, uses "*l" as a default. As an example, the construction 

     for c in file:lines(1) do body end

will iterate over all characters of the file, starting at the current position. Unlike io.lines, this function does not close the file when the loop ends.

In case of errors this function raises the error, instead of returning an error code.

file:read (···)

Reads the file file, 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 nil if it cannot read data with the specified format. When called without formats, it uses a default format that reads the next line (see below).

The available formats are

file:seek ([whence [, offset]])

Sets and gets the file position, measured from the beginning of the file, to the position given by offset plus a base specified by the string whence, as follows:

In case of success, seek returns the final file position, measured in bytes from the beginning of the file. If seek fails, it returns nil, plus a string describing the error.

The default value for whence is "cur", and for offset is 0. Therefore, the call file:seek() returns the current file position, without changing it; the call file:seek("set") sets the position to the beginning of the file (and returns 0); and the call file:seek("end") sets the position to the end of the file, and returns its size.

file:setvbuf (mode [, size])

Sets the buffering mode for an output file. There are three available modes:

For the last two cases, size specifies the size of the buffer, in bytes. The default is an appropriate size.

file:write (···)

Writes the value of each of its arguments to file. The arguments must be strings or numbers.

In case of success, this function returns file. Otherwise it returns nil plus a string describing the error.

6.9 – Operating System Facilities

This library is implemented through table os.

os.clock ()

Returns an approximation of the amount in seconds of CPU time used by the program.

os.date ([format [, time]])

Returns a string or a table containing date and time, formatted according to the given string format.

If the time argument is present, this is the time to be formatted (see the os.time function for a description of this value). Otherwise, date formats the current time.

If format starts with '!', then the date is formatted in Coordinated Universal Time. After this optional character, if format is the string "*t", then date returns a table with the following fields: year (four digits), month (1–12), day (1–31), hour (0–23), min (0–59), sec (0–61), wday (weekday, Sunday is 1), yday (day of the year), and isdst (daylight saving flag, a boolean). This last field may be absent if the information is not available.

If format is not "*t", then date returns the date as a string, formatted according to the same rules as the ANSI C function strftime.

When called without arguments, date returns a reasonable date and time representation that depends on the host system and on the current locale (that is, os.date() is equivalent to os.date("%c")).

On non-Posix systems, this function may be not thread safe because of its reliance on C function gmtime and C function localtime.

os.difftime (t2, t1)

Returns the number of seconds from time t1 to time t2. In POSIX, Windows, and some other systems, this value is exactly t2-t1.

os.execute ([command])

This function is equivalent to the ANSI C function system. It passes command to be executed by an operating system shell. Its first result is true if the command terminated successfully, or nil otherwise. After this first result the function returns a string and a number, as follows:

When called without a command, os.execute returns a boolean that is true if a shell is available.

os.exit ([code [, close])

Calls the ANSI C function exit to terminate the host program. If code is true, the returned status is EXIT_SUCCESS; if code is false, the returned status is EXIT_FAILURE; if code is a number, the returned status is this number. The default value for code is true.

If the optional second argument close is true, closes the Lua state before exiting.

os.getenv (varname)

Returns the value of the process environment variable varname, or nil if the variable is not defined.

os.remove (filename)

Deletes the file (or empty directory, on POSIX systems) with the given name. If this function fails, it returns nil, plus a string describing the error and the error code.

os.rename (oldname, newname)

Renames file or directory named oldname to newname. If this function fails, it returns nil, plus a string describing the error and the error code.

os.setlocale (locale [, category])

Sets the current locale of the program. locale is a system-dependent string specifying a locale; category is an optional string describing which category to change: "all", "collate", "ctype", "monetary", "numeric", or "time"; the default category is "all". The function returns the name of the new locale, or nil if the request cannot be honored.

If locale is the empty string, the current locale is set to an implementation-defined native locale. If locale is the string "C", the current locale is set to the standard C locale.

When called with nil as the first argument, this function only returns the name of the current locale for the given category.

This function may be not thread safe because of its reliance on C function setlocale.

os.time ([table])

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 year, month, and day, and may have fields hour (default is 12), min (default is 0), sec (default is 0), and isdst (default is nil). For a description of these fields, see the os.date function.

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 time can be used only as an argument to os.date and os.difftime.

os.tmpname ()

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.

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).

When possible, you may prefer to use io.tmpfile, which automatically removes the file when the program ends.

6.10 – The Debug Library

This library provides the functionality of the debug interface (§4.9) 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.

All functions in this library are provided inside the debug 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.

debug.debug ()

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 cont finishes this function, so that the caller continues its execution.

Note that commands for debug.debug are not lexically nested within any function and so have no direct access to local variables.

debug.gethook ([thread])

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 debug.sethook function).

debug.getinfo ([thread,] f [, what])

Returns a table with information about a function. You can give the function directly or you can give a number as the value of f, which means the function running at level f of the call stack of the given thread: level 0 is the current function (getinfo itself); level 1 is the function that called getinfo (except for tail calls, which do not count on the stack); and so on. If f is a number larger than the number of active functions, then getinfo returns nil.

The returned table can contain all the fields returned by lua_getinfo, with the string what describing which fields to fill in. The default for what is to get all information available, except the table of valid lines. If present, the option 'f' adds a field named func with the function itself. If present, the option 'L' adds a field named activelines with the table of valid lines.

For instance, the expression debug.getinfo(1,"n").name returns a table with a name for the current function, if a reasonable name can be found, and the expression debug.getinfo(print) returns a table with all available information about the print function.

debug.getlocal ([thread,] f, local)

This function returns the name and the value of the local variable with index local of the function at level f of the stack. This function accesses not only explicit local variables, but also parameters, temporaries, etc.

The first parameter or local variable has index 1, and so on, until the last active variable. Negative indices refer to vararg parameters; -1 is the first vararg parameter. The function returns nil if there is no variable with the given index, and raises an error when called with a level out of range. (You can call debug.getinfo to check whether the level is valid.)

Variable names starting with '(' (open parenthesis) represent internal variables (loop control variables, temporaries, varargs, and C function locals).

The parameter f may also be a function. In that case, getlocal returns only the name of function parameters.

debug.getmetatable (value)

Returns the metatable of the given value or nil if it does not have a metatable.

debug.getregistry ()

Returns the registry table (see §4.5).

debug.getupvalue (f, up)

This function returns the name and the value of the upvalue with index up of the function f. The function returns nil if there is no upvalue with the given index.

debug.getuservalue (u)

Returns the Lua value associated to u. If u is not a userdata, returns nil.

debug.sethook ([thread,] hook, mask [, count])

Sets the given function as a hook. The string mask and the number count describe when the hook will be called. The string mask may have the following characters, with the given meaning:

With a count different from zero, the hook is called after every count instructions.

When called without arguments, debug.sethook turns off the hook.

When the hook is called, its first parameter is a string describing the event that has triggered its call: "call" (or "tail call"), "return", "line", and "count". For line events, the hook also gets the new line number as its second parameter. Inside a hook, you can call getinfo with level 2 to get more information about the running function (level 0 is the getinfo function, and level 1 is the hook function).

debug.setlocal ([thread,] level, local, value)

This function assigns the value value to the local variable with index local of the function at level level of the stack. The function returns nil if there is no local variable with the given index, and raises an error when called with a level out of range. (You can call getinfo to check whether the level is valid.) Otherwise, it returns the name of the local variable.

See debug.getlocal for more information about variable indices and names.

debug.setmetatable (value, table)

Sets the metatable for the given value to the given table (which can be nil). Returns value.

debug.setupvalue (f, up, value)

This function assigns the value value to the upvalue with index up of the function f. The function returns nil if there is no upvalue with the given index. Otherwise, it returns the name of the upvalue.

debug.setuservalue (udata, value)

Sets the given value as the Lua value associated to the given udata. value must be a table or nil; udata must be a full userdata.

Returns udata.

debug.traceback ([thread,] [message [, level]])

If message is present but is neither a string nor nil, this function returns message without further processing. Otherwise, it returns a string with a traceback of the call stack. An optional message string is appended at the beginning of the traceback. An optional level number tells at which level to start the traceback (default is 1, the function calling traceback).

debug.upvalueid (f, n)

Returns an unique identifier (as a light userdata) for the upvalue numbered n from the given function.

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.

debug.upvaluejoin (f1, n1, f2, n2)

Make the n1-th upvalue of the Lua closure f1 refer to the n2-th upvalue of the Lua closure f2.

7 – Lua Standalone

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 lua, is provided with the standard distribution. The standalone interpreter includes all standard libraries, including the debug library. Its usage is: 

     lua [options] [script [args]]

The options are:

After handling its options, lua runs the given script, passing to it the given args as string arguments. When called without arguments, lua behaves as lua -v -i when the standard input (stdin) is a terminal, and as lua - otherwise.

When called without option -E, the interpreter checks for an environment variable LUA_INIT_5_2 (or LUA_INIT if it is not defined) before running any argument. If the variable content has the format @filename, then lua executes the file. Otherwise, lua executes the string itself.

When called with option -E, besides ignoring LUA_INIT, Lua also ignores the values of LUA_PATH and LUA_CPATH, setting the values of package.path and package.cpath with the default paths defined in luaconf.h.

All options are handled in order, except -i and -E. For instance, an invocation like 

     $ lua -e'a=1' -e 'print(a)' script.lua

will first set a to 1, then print the value of a, and finally run the file script.lua with no arguments. (Here $ is the shell prompt. Your prompt may be different.)

Before starting to run the script, lua collects all arguments in the command line in a global table called arg. The script name is stored at 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 the options) go to negative indices. For instance, in the call 

     $ lua -la b.lua t1 t2

the interpreter first runs the file a.lua, then creates a table 

     arg = { [-2] = "lua", [-1] = "-la",
             [0] = "b.lua",
             [1] = "t1", [2] = "t2" }

and finally runs the file b.lua. The script is called with arg[1], arg[2], ... as arguments; it can also access these arguments with the vararg expression '...'.

In interactive mode, if you write an incomplete statement, the interpreter waits for its completion by issuing a different prompt.

In case of unprotected errors in the script, the interpreter reports the error to the standard error stream. If the error object is a string, the interpreter adds a stack traceback to it. Otherwise, if the error object has a metamethod __tostring, the interpreter calls this metamethod to produce the final message. Finally, if the error object is nil, the interpreter does not report the error.

When finishing normally, the interpreter closes its main Lua state (see lua_close). The script can avoid this step by calling os.exit to terminate.

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 #. Therefore, Lua scripts can be made into executable programs by using chmod +x and the #! form, as in 

     #!/usr/local/bin/lua

(Of course, the location of the Lua interpreter may be different in your machine. If lua is in your PATH, then 

     #!/usr/bin/env lua

is a more portable solution.)

8 – Incompatibilities with the Previous Version

Here we list the incompatibilities that you may find when moving a program from Lua 5.1 to Lua 5.2. You can avoid some incompatibilities by compiling Lua with appropriate options (see file luaconf.h). However, all these compatibility options will be removed in the next version of Lua. Similarly, all features marked as deprecated in Lua 5.1 have been removed in Lua 5.2.

8.1 – Changes in the Language

8.2 – Changes in the Libraries

8.3 – Changes in the API

9 – The Complete Syntax of Lua

Here is the complete syntax of Lua in extended BNF. (It does not describe operator precedences.) 


	chunk ::= block

	block ::= {stat} [retstat]

	stat ::=  ‘;’ |
		 varlist ‘=’ explist |
		 functioncall |
		 label |
		 break |
		 goto Name |
		 do block end |
		 while exp do block end |
		 repeat block until exp |
		 if exp then block {elseif exp then block} [else block] end |
		 for Name ‘=’ exp ‘,’ exp [‘,’ exp] do block end |
		 for namelist in explist do block end |
		 function funcname funcbody |
		 local function Name funcbody |
		 local namelist [‘=’ explist]

	retstat ::= return [explist] [‘;’]

	label ::= ‘::’ Name ‘::’

	funcname ::= Name {‘.’ Name} [‘:’ Name]

	varlist ::= var {‘,’ var}

	var ::=  Name | prefixexp ‘[’ exp ‘]’ | prefixexp ‘.’ Name

	namelist ::= Name {‘,’ Name}

	explist ::= exp {‘,’ exp}

	exp ::=  nil | false | true | Number | String | ‘...’ | functiondef |
		 prefixexp | tableconstructor | exp binop exp | unop exp

	prefixexp ::= var | functioncall | ‘(’ exp ‘)’

	functioncall ::=  prefixexp args | prefixexp ‘:’ Name args

	args ::=  ‘(’ [explist] ‘)’ | tableconstructor | String

	functiondef ::= function funcbody

	funcbody ::= ‘(’ [parlist] ‘)’ block end

	parlist ::= namelist [‘,’ ‘...’] | ‘...’

	tableconstructor ::= ‘{’ [fieldlist] ‘}’

	fieldlist ::= field {fieldsep field} [fieldsep]

	field ::= ‘[’ exp ‘]’ ‘=’ exp | Name ‘=’ exp | exp

	fieldsep ::= ‘,’ | ‘;’

	binop ::= ‘+’ | ‘-’ | ‘*’ | ‘/’ | ‘^’ | ‘%’ | ‘..’ |
		 ‘<’ | ‘<=’ | ‘>’ | ‘>=’ | ‘==’ | ‘~=’ |
		 and | or

	unop ::= ‘-’ | not | ‘#’

Last update: Thu Mar 21 13:01:53 BRT 2013