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update documentation for luan changes
author Franklin Schmidt <fschmidt@gmail.com>
date Fri, 22 May 2015 16:08:23 -0600
parents 7bc63886d4f2
children 0dfc01d8d42d
comparison
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511:e3fb9768dbb3 512:d96944467ffc
230 230
231 231
232 <h3 <%=heading_options%> ><a name="env">Environments</a></h3> 232 <h3 <%=heading_options%> ><a name="env">Environments</a></h3>
233 233
234 <p> 234 <p>
235 The environment of a chunk starts with only two local variables: <tt><a href="#require">require</a></tt> and <tt><a href="#java">java</a></tt>. These are functions are used to load and access libraries and other modules. All other variables must be added to the environment using <a href="http://localhost:8080/manual.html#local_stmt">local declarations</a>.
236
237 <p>
235 As will be discussed in <a href="#vars">Variables</a> and <a href=#assignment">Assignment</a>, 238 As will be discussed in <a href="#vars">Variables</a> and <a href=#assignment">Assignment</a>,
236 any reference to a free name 239 any reference to a free name
237 (that is, a name not bound to any declaration) <tt>var</tt> 240 (that is, a name not bound to any declaration) <tt>var</tt>
238 is syntactically translated to <tt>_ENV.var</tt>. 241 can be syntactically translated to <tt>_ENV.var</tt> if <tt>_ENV</tt> is defined.
239 Moreover, every chunk is compiled in the scope of
240 an external local variable named <tt>_ENV</tt> (see <a href="#chunks">Chunks</a>),
241 so <tt>_ENV</tt> itself is never a free name in a chunk.
242
243
244 <p>
245 Despite the existence of this external <tt>_ENV</tt> variable and
246 the translation of free names,
247 <tt>_ENV</tt> is a completely regular name.
248 In particular,
249 you can define new variables and parameters with that name.
250 Each reference to a free name uses the <tt>_ENV</tt> that is
251 visible at that point in the program,
252 following the usual visibility rules of Luan (see <a href="#visibility">Visibility Rules</a>).
253
254
255 <p>
256 Any table used as the value of <tt>_ENV</tt> is called an <i>environment</i>.
257
258
259 <p>
260 When Luan loads a chunk,
261 the default value for its <tt>_ENV</tt> is an empty table.
262
263 <p>
264 Luan also provides all chunks with two other local values: <tt>require</tt> and <tt>java</tt>. These are functions used to load and access libraries and other modules.
265
266
267 242
268 243
269 <h3 <%=heading_options%> ><a name="error">Error Handling</a></h3> 244 <h3 <%=heading_options%> ><a name="error">Error Handling</a></h3>
270 245
271 <p> 246 <p>
721 <p> 696 <p>
722 Variables are places that store values. 697 Variables are places that store values.
723 There are three kinds of variables in Luan: 698 There are three kinds of variables in Luan:
724 global variables, local variables, and table fields. 699 global variables, local variables, and table fields.
725 700
726
727 <p> 701 <p>
728 A single name can denote a global variable or a local variable 702 A single name can denote a global variable or a local variable
729 (or a function's formal parameter, 703 (or a function's formal parameter,
730 which is a particular kind of local variable): 704 which is a particular kind of local variable):
731 705
734 </pre></tt></p> 708 </pre></tt></p>
735 709
736 <p> 710 <p>
737 Name denotes identifiers, as defined in <a href="#lex">Lexical Conventions</a>. 711 Name denotes identifiers, as defined in <a href="#lex">Lexical Conventions</a>.
738 712
739 713 <p>
740 <p>
741 Any variable name is assumed to be global unless explicitly declared
742 as a local (see <a href="#local_stmt">Local Declarations</a>).
743 Local variables are <i>lexically scoped</i>: 714 Local variables are <i>lexically scoped</i>:
744 local variables can be freely accessed by functions 715 local variables can be freely accessed by functions
745 defined inside their scope (see <a href="#visibility">Visibility Rules</a>). 716 defined inside their scope (see <a href="#visibility">Visibility Rules</a>).
746 717
747 718
748 <p> 719 <p>
749 Before the first assignment to a variable, its value is <b>nil</b>. 720 Before the first assignment to a variable, its value is <b>nil</b>.
750
751 721
752 <p> 722 <p>
753 Square brackets are used to index a table: 723 Square brackets are used to index a table:
754 724
755 <p><tt><pre> 725 <p><tt><pre>
773 <p><tt><pre> 743 <p><tt><pre>
774 var ::= prefixexp &lsquo;<b>.</b>&rsquo; Name 744 var ::= prefixexp &lsquo;<b>.</b>&rsquo; Name
775 </pre></tt></p> 745 </pre></tt></p>
776 746
777 <p> 747 <p>
778 An access to a global variable <tt>x</tt> 748 Global variables are not available by default. To enable global variable, you must define <tt>_ENV</tt> as a local variable whose value is a table. If <tt>_ENV</tt> is not defined, then an unrecognized variable name will produce a compile error. If <tt>_ENV</tt> is defined then an access to an unrecognized variable name will be consider a global variable. So then an acces to global variable <tt>x</tt>
779 is equivalent to <tt>_ENV.x</tt>. 749 is equivalent to <tt>_ENV.x</tt>.
780 Due to the way that chunks are compiled, 750 Due to the way that chunks are compiled,
781 <tt>_ENV</tt> is never a global name (see <a href="#env">Environments</a>). 751 <tt>_ENV</tt> is never a global name (see <a href="#env">Environments</a>).
782 752
783 753
941 911
942 <p> 912 <p>
943 An assignment to a global name <tt>x = val</tt> 913 An assignment to a global name <tt>x = val</tt>
944 is equivalent to the assignment 914 is equivalent to the assignment
945 <tt>_ENV.x = val</tt> (see <a href="#env">Environments</a>). 915 <tt>_ENV.x = val</tt> (see <a href="#env">Environments</a>).
946 916 Global names are only available when <tt>_ENV</tt> is defined.
947 917
948 918
949 919
950 <h4 <%=heading_options%> ><a name="control">Control Structures</a></h4> 920 <h4 <%=heading_options%> ><a name="control">Control Structures</a></h4>
951 921
1820 1790
1821 <h3 <%=heading_options%> ><a name="default_lib">Default Environment</a></h3> 1791 <h3 <%=heading_options%> ><a name="default_lib">Default Environment</a></h3>
1822 1792
1823 <p> 1793 <p>
1824 These are provided by default as local variables for any Luan code as described in <a href="#env">Environments</a>. 1794 These are provided by default as local variables for any Luan code as described in <a href="#env">Environments</a>.
1825
1826
1827 <h4 <%=heading_options%> ><a name="_ENV"><tt>_ENV</tt></a></h4>
1828
1829 <p>
1830 This is a table that holds the global variables of a module as described in <a href="#env">Environments</a>.
1831 1795
1832 1796
1833 <h4 <%=heading_options%> ><a name="require"><tt>java ()</tt></a></h4> 1797 <h4 <%=heading_options%> ><a name="require"><tt>java ()</tt></a></h4>
1834 1798
1835 <p> 1799 <p>