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reference_manual.html
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reference_manual.html
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<!DOCTYPE HTML>
<html lang="en">
<head>
<!-- Autogenerated from on Mon Dec 12 00:44:42 2011 -->
<meta charset="UTF-8">
<title>MoonScript v0.2.0 - Language Guide</title>
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</head><body>
<div class="header">
<h1>MoonScript v0.2.0 - Language Guide</h1>
</div>
<div class="clearfix index">
<div class="column">
<h3>On This Page</h3>
<ul>
<li><a href="#the_language">The Language</a></li>
<ul>
<li><a href="#assignment">Assignment</a></li>
<li><a href="#update_assignment">Update Assignment</a></li>
<li><a href="#comments_">Comments </a></li>
<li><a href="#literals_and_operators">Literals & Operators</a></li>
<li><a href="#function_literals">Function Literals</a></li>
<ul>
<li><a href="#fat_arrows">Fat Arrows</a></li>
<li><a href="#argument_defaults">Argument Defaults</a></li>
<li><a href="#considerations">Considerations</a></li>
<li><a href="#multiline_arguments">Multi-line arguments</a></li>
</ul>
<li><a href="#table_literals">Table Literals</a></li>
<li><a href="#comprehensions">Comprehensions</a></li>
<ul>
<li><a href="#list_comprehensions">List Comprehensions</a></li>
<li><a href="#table_comprehensions">Table Comprehensions</a></li>
<li><a href="#slicing">Slicing</a></li>
</ul>
<li><a href="#for_loop">For Loop</a></li>
<li><a href="#while_loop">While Loop</a></li>
<li><a href="#conditionals">Conditionals</a></li>
<li><a href="#line_decorators">Line Decorators</a></li>
<li><a href="#switch">Switch</a></li>
<li><a href="#object_oriented_programming">Object Oriented Programming</a></li>
<ul>
<li><a href="#inheritance">Inheritance</a></li>
<li><a href="#super">Super</a></li>
<li><a href="#types">Types</a></li>
</ul>
<li><a href="#export_statement">Export Statement</a></li>
<ul>
<li><a href="#export_all_and_export_proper">Export All & Export Proper</a></li>
</ul>
<li><a href="#import_statement">Import Statement</a></li>
<li><a href="#with_statement">With Statement</a></li>
<li><a href="#function_stubs">Function Stubs</a></li>
<li><a href="#the_using_clause_controlling_destructive_assignment">The Using Clause; Controlling Destructive Assignment</a></li>
</ul>
<li><a href="#moonscript_api">MoonScript API</a></li>
<ul>
<li><a href="#moonscript_module"><code>moonscript</code> Module</a></li>
<li><a href="#error_rewriting">Error Rewriting</a></li>
<li><a href="#programmatically_compiling">Programmatically Compiling</a></li>
</ul>
<li><a href="#command_line_use">Command Line Use</a></li>
<ul>
<li><a href="#moon"><code>moon</code></a></li>
<li><a href="#moonc"><code>moonc</code></a></li>
</ul>
<li><a href="#license_mit">License (MIT)</a></li>
</ul>
</div>
<div class="column">
<h3>All Pages</h3>
<ul>
<li><a href="./reference_manual.html" class="current">Language Guide</a></li>
<li><a href="./standard_lib.html">Standard Library</a></li>
</ul>
</div>
</div>
<div class="main">
<p>MoonScript is a programming language that compiles to
<a href="http://www.lua.org">Lua</a>. This guide expects the reader to have basic
familiarity with Lua. For each code snippet below, the MoonScript is on the
left and the compiled Lua is on right right.</p>
</div><h1><a name="the_language"></a>The Language</h1><div class="main">
<h2><a name="assignment"></a>Assignment</h2>
<p>Unlike Lua, there is no <code>local</code> keyword. All assignments to names that are not
already defined will be declared as local to the scope of that declaration. If
you wish to create a global variable it must be done using the <code>export</code>
keyword.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">hello</span> <span class="l_symbol">=</span> <span class="l_string">"world"</span>
<span class="l_atom">a</span>,<span class="l_atom">b</span>,<span class="l_atom">c</span> <span class="l_symbol">=</span> <span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">hello</span> <span class="l_symbol">=</span> <span class="l_string">"world"</span>
<span class="l_keyword">local</span> <span class="l_atom">a</span>, <span class="l_atom">b</span>, <span class="l_atom">c</span> <span class="l_symbol">=</span> <span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span></code></pre></td></tr>
</tbody></table>
<h2><a name="update_assignment"></a>Update Assignment</h2>
<p><code>+=</code>, <code>-=</code>, <code>/=</code>, <code>*=</code>, <code>%=</code>, <code>..=</code> operators have been added for updating a value by
a certain amount. They are aliases for their expanded equivalents.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_number">0</span>
<span class="l_atom">x</span> <span class="l_symbol">+=</span> <span class="l_number">10</span>
<span class="l_atom">s</span> <span class="l_symbol">=</span> <span class="l_string">"hello "</span>
<span class="l_atom">s</span> <span class="l_bold_symbol">.</span><span class="l_bold_symbol">.</span><span class="l_symbol">=</span> <span class="l_string">"world"</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_number">0</span>
<span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_atom">x</span> + <span class="l_number">10</span>
<span class="l_keyword">local</span> <span class="l_atom">s</span> <span class="l_symbol">=</span> <span class="l_string">"hello "</span>
<span class="l_atom">s</span> <span class="l_symbol">=</span> <span class="l_atom">s</span> <span class="l_symbol">.</span><span class="l_symbol">.</span> <span class="l_string">"world"</span></code></pre></td></tr>
</tbody></table>
<h2><a name="comments_"></a>Comments </h2>
<p>Like Lua, comments start with <code>--</code> and continue to the end of the line.
Comments are not written to the output.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_comment">-- I am a comment</span></code></pre></td>
<td><pre><code class="lua-code"></code></pre></td></tr>
</tbody></table>
<h2><a name="literals_and_operators"></a>Literals & Operators</h2>
<p>MoonScript supports all the same primitive literals as Lua and uses the same
syntax. This applies to numbers, strings, booleans, and <code>nil</code>.</p>
<p>MoonScript also supports all the same binary and unary operators. Additionally
<code>!=</code> is as an alias for <code>~=</code>.</p>
<h2><a name="function_literals"></a>Function Literals</h2>
<p>All functions are created using a function expression. A simple function is
denoted using the arrow: <code>-></code></p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">my_function</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">-></span>
<span class="l_atom">my_function</span>() <span class="l_comment">-- call the empty function</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">my_function</span>
<span class="l_atom">my_function</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>() <span class="l_keyword">end</span>
<span class="l_atom">my_function</span>()</code></pre></td></tr>
</tbody></table>
<p>The body of the function can either be one statement placed directly after the
arrow, or it can be a series of statements indented on the following lines:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">func_a</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">-></span> <span class="l_builtins">print</span> <span class="l_string">"hello world"</span>
<span class="l_atom">func_b</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">-></span>
<span class="l_atom">value</span> <span class="l_symbol">=</span> <span class="l_number">100</span>
<span class="l_builtins">print</span> <span class="l_string">"The value:"</span>, <span class="l_atom">value</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">func_a</span>
<span class="l_atom">func_a</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>()
<span class="l_keyword">return</span> <span class="l_builtins">print</span>(<span class="l_string">"hello world"</span>)
<span class="l_keyword">end</span>
<span class="l_keyword">local</span> <span class="l_atom">func_b</span>
<span class="l_atom">func_b</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>()
<span class="l_keyword">local</span> <span class="l_atom">value</span> <span class="l_symbol">=</span> <span class="l_number">100</span>
<span class="l_keyword">return</span> <span class="l_builtins">print</span>(<span class="l_string">"The value:"</span>, <span class="l_atom">value</span>)
<span class="l_keyword">end</span></code></pre></td></tr>
</tbody></table>
<p>If a function has no arguments, it can be called using the <code>!</code> operator,
instead of empty parentheses. The <code>!</code> invocation is the preferred way to call
functions with no arguments.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">func_a</span><span class="l_symbol">!</span>
<span class="l_atom">func_b</span>()</code></pre></td>
<td><pre><code class="lua-code"><span class="l_atom">func_a</span>()
<span class="l_atom">func_b</span>()</code></pre></td></tr>
</tbody></table>
<p>Functions with arguments can be created by preceding the arrow with a list of
argument names in parentheses:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">sum</span> <span class="l_symbol">=</span> (<span class="l_atom">x</span>, <span class="l_atom">y</span>) <span class="l_fn_symbol">-></span> <span class="l_builtins">print</span> <span class="l_string">"sum"</span>, <span class="l_atom">x</span> + <span class="l_atom">y</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">sum</span>
<span class="l_atom">sum</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>(<span class="l_atom">x</span>, <span class="l_atom">y</span>)
<span class="l_keyword">return</span> <span class="l_builtins">print</span>(<span class="l_string">"sum"</span>, <span class="l_atom">x</span> + <span class="l_atom">y</span>)
<span class="l_keyword">end</span></code></pre></td></tr>
</tbody></table>
<p>Functions can be called by listing the values of the arguments after the name
of the variable where the function is stored. When chaining together function
calls, the arguments are applied to the closest function to the left.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">sum</span> <span class="l_number">10</span>, <span class="l_number">20</span>
<span class="l_builtins">print</span> <span class="l_atom">sum</span> <span class="l_number">10</span>, <span class="l_number">20</span>
<span class="l_atom">a</span> <span class="l_atom">b</span> <span class="l_atom">c</span> <span class="l_string">"a"</span>, <span class="l_string">"b"</span>, <span class="l_string">"c"</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_atom">sum</span>(<span class="l_number">10</span>, <span class="l_number">20</span>)
<span class="l_builtins">print</span>(<span class="l_atom">sum</span>(<span class="l_number">10</span>, <span class="l_number">20</span>))
<span class="l_atom">a</span>(<span class="l_atom">b</span>(<span class="l_atom">c</span>(<span class="l_string">"a"</span>, <span class="l_string">"b"</span>, <span class="l_string">"c"</span>)))</code></pre></td></tr>
</tbody></table>
<p>In order to avoid ambiguity in when calling functions, parentheses can also be
used to surround the arguments. This is required here in order to make sure the
right arguments get sent to the right functions.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_builtins">print</span> <span class="l_string">"x:"</span>, <span class="l_atom">sum</span>(<span class="l_number">10</span>, <span class="l_number">20</span>), <span class="l_string">"y:"</span>, <span class="l_atom">sum</span>(<span class="l_number">30</span>, <span class="l_number">40</span>)</code></pre></td>
<td><pre><code class="lua-code"><span class="l_builtins">print</span>(<span class="l_string">"x:"</span>, <span class="l_atom">sum</span>(<span class="l_number">10</span>, <span class="l_number">20</span>), <span class="l_string">"y:"</span>, <span class="l_atom">sum</span>(<span class="l_number">30</span>, <span class="l_number">40</span>))</code></pre></td></tr>
</tbody></table>
<p>Functions will coerce the last statement in their body into a return statement,
this is called implicit return:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">sum</span> <span class="l_symbol">=</span> (<span class="l_atom">x</span>, <span class="l_atom">y</span>) <span class="l_fn_symbol">-></span> <span class="l_atom">x</span> + <span class="l_atom">y</span>
<span class="l_builtins">print</span> <span class="l_string">"The sum is "</span>, <span class="l_atom">sum</span> <span class="l_number">10</span>, <span class="l_number">20</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">sum</span>
<span class="l_atom">sum</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>(<span class="l_atom">x</span>, <span class="l_atom">y</span>)
<span class="l_keyword">return</span> <span class="l_atom">x</span> + <span class="l_atom">y</span>
<span class="l_keyword">end</span>
<span class="l_builtins">print</span>(<span class="l_string">"The sum is "</span>, <span class="l_atom">sum</span>(<span class="l_number">10</span>, <span class="l_number">20</span>))</code></pre></td></tr>
</tbody></table>
<p>And if you need to explicitly return, you can use the <code>return</code> keyword:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">sum</span> <span class="l_symbol">=</span> (<span class="l_atom">x</span>, <span class="l_atom">y</span>) <span class="l_fn_symbol">-></span> <span class="l_keyword">return</span> <span class="l_atom">x</span> + <span class="l_atom">y</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">sum</span>
<span class="l_atom">sum</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>(<span class="l_atom">x</span>, <span class="l_atom">y</span>)
<span class="l_keyword">return</span> <span class="l_atom">x</span> + <span class="l_atom">y</span>
<span class="l_keyword">end</span></code></pre></td></tr>
</tbody></table>
<p>Just like in Lua, functions can return multiple values. The last statement must
be a list of values separated by commas:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">mystery</span> <span class="l_symbol">=</span> (<span class="l_atom">x</span>, <span class="l_atom">y</span>) <span class="l_fn_symbol">-></span> <span class="l_atom">x</span> + <span class="l_atom">y</span>, <span class="l_atom">x</span> - <span class="l_atom">y</span>
<span class="l_atom">a</span>,<span class="l_atom">b</span> <span class="l_symbol">=</span> <span class="l_atom">mystery</span> <span class="l_number">10</span>, <span class="l_number">20</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">mystery</span>
<span class="l_atom">mystery</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>(<span class="l_atom">x</span>, <span class="l_atom">y</span>)
<span class="l_keyword">return</span> <span class="l_atom">x</span> + <span class="l_atom">y</span>, <span class="l_atom">x</span> - <span class="l_atom">y</span>
<span class="l_keyword">end</span>
<span class="l_keyword">local</span> <span class="l_atom">a</span>, <span class="l_atom">b</span> <span class="l_symbol">=</span> <span class="l_atom">mystery</span>(<span class="l_number">10</span>, <span class="l_number">20</span>)</code></pre></td></tr>
</tbody></table>
<h3><a name="fat_arrows"></a>Fat Arrows</h3>
<p>Because it is an idiom in Lua to send an object as the first argument when
calling a method, a special syntax is provided for creating functions which
automatically includes a <code>self</code> argument.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">func</span> <span class="l_symbol">=</span> (<span class="l_atom">num</span>) <span class="l_fn_symbol">=></span> <span class="l_self">self</span><span class="l_bold_symbol">.</span><span class="l_atom">value</span> + <span class="l_atom">num</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">func</span>
<span class="l_atom">func</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>(<span class="l_atom">self</span>, <span class="l_atom">num</span>)
<span class="l_keyword">return</span> <span class="l_atom">self</span><span class="l_symbol">.</span><span class="l_atom">value</span> + <span class="l_atom">num</span>
<span class="l_keyword">end</span></code></pre></td></tr>
</tbody></table>
<h3><a name="argument_defaults"></a>Argument Defaults</h3>
<p>It is possible to provide default values for the arguments of a function. An
argument is determined to be empty if it’s value is <code>nil</code>. Any <code>nil</code> arguments
that have a default value will be replace before the body of the function is run.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">my_function</span> <span class="l_symbol">=</span> (<span class="l_atom">name</span><span class="l_symbol">=</span><span class="l_string">"something"</span>, <span class="l_atom">height</span><span class="l_symbol">=</span><span class="l_number">100</span>) <span class="l_fn_symbol">-></span>
<span class="l_builtins">print</span> <span class="l_string">"Hello I am"</span>, <span class="l_atom">name</span>
<span class="l_builtins">print</span> <span class="l_string">"My height is"</span>, <span class="l_atom">height</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">my_function</span>
<span class="l_atom">my_function</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>(<span class="l_atom">name</span>, <span class="l_atom">height</span>)
<span class="l_keyword">if</span> <span class="l_atom">name</span> <span class="l_symbol">=</span><span class="l_symbol">=</span> <span class="l_special">nil</span> <span class="l_keyword">then</span>
<span class="l_atom">name</span> <span class="l_symbol">=</span> <span class="l_string">"something"</span>
<span class="l_keyword">end</span>
<span class="l_keyword">if</span> <span class="l_atom">height</span> <span class="l_symbol">=</span><span class="l_symbol">=</span> <span class="l_special">nil</span> <span class="l_keyword">then</span>
<span class="l_atom">height</span> <span class="l_symbol">=</span> <span class="l_number">100</span>
<span class="l_keyword">end</span>
<span class="l_builtins">print</span>(<span class="l_string">"Hello I am"</span>, <span class="l_atom">name</span>)
<span class="l_keyword">return</span> <span class="l_builtins">print</span>(<span class="l_string">"My height is"</span>, <span class="l_atom">height</span>)
<span class="l_keyword">end</span></code></pre></td></tr>
</tbody></table>
<p>An argument default value expression is evaluated in the body of the function
in the order of the argument declarations. For this reason default values have
access to previously declared arguments.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">some_args</span> <span class="l_symbol">=</span> (<span class="l_atom">x</span><span class="l_symbol">=</span><span class="l_number">100</span>, <span class="l_atom">y</span><span class="l_symbol">=</span><span class="l_atom">x</span>+<span class="l_number">1000</span>) <span class="l_fn_symbol">-></span>
<span class="l_builtins">print</span> <span class="l_atom">x</span> + <span class="l_atom">y</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">some_args</span>
<span class="l_atom">some_args</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">function</span>(<span class="l_atom">x</span>, <span class="l_atom">y</span>)
<span class="l_keyword">if</span> <span class="l_atom">x</span> <span class="l_symbol">=</span><span class="l_symbol">=</span> <span class="l_special">nil</span> <span class="l_keyword">then</span>
<span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_number">100</span>
<span class="l_keyword">end</span>
<span class="l_keyword">if</span> <span class="l_atom">y</span> <span class="l_symbol">=</span><span class="l_symbol">=</span> <span class="l_special">nil</span> <span class="l_keyword">then</span>
<span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_atom">x</span> + <span class="l_number">1000</span>
<span class="l_keyword">end</span>
<span class="l_keyword">return</span> <span class="l_builtins">print</span>(<span class="l_atom">x</span> + <span class="l_atom">y</span>)
<span class="l_keyword">end</span></code></pre></td></tr>
</tbody></table>
<h3><a name="considerations"></a>Considerations</h3>
<p>Because of the expressive parentheses-less way of calling functions, some
restrictions must be put in place to avoid parsing ambiguity involving
whitespace.</p>
<p>The minus sign plays two roles, a unary negation operator and a binary
subtraction operator. In order to force subtraction a space must be placed
after the <code>-</code> operator. In order to force a negation, no space must follow
the <code>-</code>. Consider the examples below.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">a</span> <span class="l_symbol">=</span> <span class="l_atom">x</span> - <span class="l_number">10</span>
<span class="l_atom">b</span> <span class="l_symbol">=</span> <span class="l_atom">x</span><span class="l_number">-10</span>
<span class="l_atom">c</span> <span class="l_symbol">=</span> <span class="l_atom">x</span> -<span class="l_atom">y</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">a</span> <span class="l_symbol">=</span> <span class="l_atom">x</span> - <span class="l_number">10</span>
<span class="l_keyword">local</span> <span class="l_atom">b</span> <span class="l_symbol">=</span> <span class="l_atom">x</span>(<span class="l_number">-10</span>)
<span class="l_keyword">local</span> <span class="l_atom">c</span> <span class="l_symbol">=</span> <span class="l_atom">x</span>(-<span class="l_atom">y</span>)</code></pre></td></tr>
</tbody></table>
<p>The precedence of the first argument of a function call can also be controlled
using whitespace if the argument is a literal string.In Lua, it is common to
leave off parentheses when calling a function with a single string literal.</p>
<p>When there is no space between a variable and a string literal, the
function call takes precedence over any following expressions. No other
arguments can be passed to the function when it is called this way.</p>
<p>Where there is a space following a variable and a string literal, the function
call acts as show above. The string literal belongs to any following
expressions (if they exist), which serves as the argument list.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_atom">func</span><span class="l_string">"hello"</span> + <span class="l_number">100</span>
<span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_atom">func</span> <span class="l_string">"hello"</span> + <span class="l_number">100</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_atom">func</span>(<span class="l_string">"hello"</span>) + <span class="l_number">100</span>
<span class="l_keyword">local</span> <span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_atom">func</span>(<span class="l_string">"hello"</span> + <span class="l_number">100</span>)</code></pre></td></tr>
</tbody></table>
<h3><a name="multiline_arguments"></a>Multi-line arguments</h3>
<p>When calling functions that take a large number of arguments, it is convenient
to split the argument list over multiple lines. Because of the white-space
sensitive nature of the language, care must be taken when splitting up the
argument list.</p>
<p>If an argument list is to be continued onto the next line, the current line
must end in a comma. And the following line must be indented more than the
current indentation. Once indented, all other argument lines must be at the
same level of indentation to be part of the argument list</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">my_func</span> <span class="l_number">5</span>,<span class="l_number">4</span>,<span class="l_number">3</span>,
<span class="l_number">8</span>,<span class="l_number">9</span>,<span class="l_number">10</span>
<span class="l_atom">cool_func</span> <span class="l_number">1</span>,<span class="l_number">2</span>,
<span class="l_number">3</span>,<span class="l_number">4</span>,
<span class="l_number">5</span>,<span class="l_number">6</span>,
<span class="l_number">7</span>,<span class="l_number">8</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_atom">my_func</span>(<span class="l_number">5</span>, <span class="l_number">4</span>, <span class="l_number">3</span>, <span class="l_number">8</span>, <span class="l_number">9</span>, <span class="l_number">10</span>)
<span class="l_atom">cool_func</span>(<span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span>, <span class="l_number">4</span>, <span class="l_number">5</span>, <span class="l_number">6</span>, <span class="l_number">7</span>, <span class="l_number">8</span>)</code></pre></td></tr>
</tbody></table>
<p>This type of invocation can be nested. The level of indentation is used to
determine to which function the arguments belong to.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">my_func</span> <span class="l_number">5</span>,<span class="l_number">6</span>,<span class="l_number">7</span>,
<span class="l_number">6</span>, <span class="l_atom">another_func</span> <span class="l_number">6</span>,<span class="l_number">7</span>,<span class="l_number">8</span>,
<span class="l_number">9</span>,<span class="l_number">1</span>,<span class="l_number">2</span>,
<span class="l_number">5</span>,<span class="l_number">4</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_atom">my_func</span>(<span class="l_number">5</span>, <span class="l_number">6</span>, <span class="l_number">7</span>, <span class="l_number">6</span>, <span class="l_atom">another_func</span>(<span class="l_number">6</span>, <span class="l_number">7</span>, <span class="l_number">8</span>, <span class="l_number">9</span>, <span class="l_number">1</span>, <span class="l_number">2</span>), <span class="l_number">5</span>, <span class="l_number">4</span>)</code></pre></td></tr>
</tbody></table>
<p>Because <a href="#table_literals">tables</a> also use the comma as a delimiter, this
indentation syntax is helpful for letting values be part of the argument list
instead of being part of the table.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span>
<span class="l_number">1</span>,<span class="l_number">2</span>,<span class="l_number">3</span>,<span class="l_number">4</span>, <span class="l_atom">a_func</span> <span class="l_number">4</span>,<span class="l_number">5</span>,
<span class="l_number">5</span>,<span class="l_number">6</span>,
<span class="l_number">8</span>,<span class="l_number">9</span>,<span class="l_number">10</span>
<span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_number">1</span>,
<span class="l_number">2</span>,
<span class="l_number">3</span>,
<span class="l_number">4</span>,
<span class="l_atom">a_func</span>(<span class="l_number">4</span>, <span class="l_number">5</span>, <span class="l_number">5</span>, <span class="l_number">6</span>),
<span class="l_number">8</span>,
<span class="l_number">9</span>,
<span class="l_number">10</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>Although uncommon, notice how we can give a deeper indentation for function
arguments if we know we will be using a lower indentation futher on.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span> <span class="l_atom">my_func</span> <span class="l_number">1</span>,<span class="l_number">2</span>,<span class="l_number">3</span>,
<span class="l_number">4</span>,<span class="l_number">5</span>,
<span class="l_number">5</span>,<span class="l_number">6</span>,<span class="l_number">7</span>
<span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_atom">my_func</span>(<span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span>, <span class="l_number">4</span>, <span class="l_number">5</span>),
<span class="l_number">5</span>,
<span class="l_number">6</span>,
<span class="l_number">7</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>The same thing can be done with other block level statements like
<a href="#conditionals">conditionals</a>. We can use indentation level to determine what
statement a value belongs to:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_keyword">if</span> <span class="l_atom">func</span> <span class="l_number">1</span>,<span class="l_number">2</span>,<span class="l_number">3</span>,
<span class="l_string">"hello"</span>,
<span class="l_string">"world"</span>
<span class="l_builtins">print</span> <span class="l_string">"hello"</span>
<span class="l_builtins">print</span> <span class="l_string">"I am inside if"</span>
<span class="l_keyword">if</span> <span class="l_atom">func</span> <span class="l_number">1</span>,<span class="l_number">2</span>,<span class="l_number">3</span>,
<span class="l_string">"hello"</span>,
<span class="l_string">"world"</span>
<span class="l_builtins">print</span> <span class="l_string">"hello"</span>
<span class="l_builtins">print</span> <span class="l_string">"I am inside if"</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">if</span> <span class="l_atom">func</span>(<span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span>, <span class="l_string">"hello"</span>, <span class="l_string">"world"</span>) <span class="l_keyword">then</span>
<span class="l_builtins">print</span>(<span class="l_string">"hello"</span>)
<span class="l_builtins">print</span>(<span class="l_string">"I am inside if"</span>)
<span class="l_keyword">end</span>
<span class="l_keyword">if</span> <span class="l_atom">func</span>(<span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span>, <span class="l_string">"hello"</span>, <span class="l_string">"world"</span>) <span class="l_keyword">then</span>
<span class="l_builtins">print</span>(<span class="l_string">"hello"</span>)
<span class="l_builtins">print</span>(<span class="l_string">"I am inside if"</span>)
<span class="l_keyword">end</span></code></pre></td></tr>
</tbody></table>
<h2><a name="table_literals"></a>Table Literals</h2>
<p>Like in Lua, tables are delimited in curly braces.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">some_values</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span> <span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span>, <span class="l_number">4</span> <span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">some_values</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_number">1</span>,
<span class="l_number">2</span>,
<span class="l_number">3</span>,
<span class="l_number">4</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>Unlike Lua, assigning a value to a key in a table is done with <code>:</code> (instead of
<code>=</code>).</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">some_values</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span>
<span class="l_table_key">name</span><span class="l_bold_symbol">:</span> <span class="l_string">"Bill"</span>,
<span class="l_table_key">age</span><span class="l_bold_symbol">:</span> <span class="l_number">200</span>,
<span class="l_fn_symbol">[</span><span class="l_string">"favorite food"</span><span class="l_fn_symbol">]</span><span class="l_bold_symbol">:</span> <span class="l_string">"rice"</span>
<span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">some_values</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_atom">name</span> <span class="l_symbol">=</span> <span class="l_string">"Bill"</span>,
<span class="l_atom">age</span> <span class="l_symbol">=</span> <span class="l_number">200</span>,
[<span class="l_string">"favorite food"</span>] <span class="l_symbol">=</span> <span class="l_string">"rice"</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>The curly braces can be left off if a single table of key value pairs is being
assigned.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">profile</span> <span class="l_symbol">=</span>
<span class="l_table_key">height</span><span class="l_bold_symbol">:</span> <span class="l_string">"4 feet"</span>,
<span class="l_table_key">shoe_size</span><span class="l_bold_symbol">:</span> <span class="l_number">13</span>,
<span class="l_table_key">favorite_foods</span><span class="l_bold_symbol">:</span> <span class="l_fn_symbol">{</span><span class="l_string">"ice cream"</span>, <span class="l_string">"donuts"</span><span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">profile</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_atom">height</span> <span class="l_symbol">=</span> <span class="l_string">"4 feet"</span>,
<span class="l_atom">shoe_size</span> <span class="l_symbol">=</span> <span class="l_number">13</span>,
<span class="l_atom">favorite_foods</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_string">"ice cream"</span>,
<span class="l_string">"donuts"</span>
<span class="l_symbol">}</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>Newlines can be used to delimit values instead of a comma (or both):</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">values</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span>
<span class="l_number">1</span>,<span class="l_number">2</span>,<span class="l_number">3</span>,<span class="l_number">4</span>
<span class="l_number">5</span>,<span class="l_number">6</span>,<span class="l_number">7</span>,<span class="l_number">8</span>
<span class="l_table_key">name</span><span class="l_bold_symbol">:</span> <span class="l_string">"superman"</span>
<span class="l_table_key">occupation</span><span class="l_bold_symbol">:</span> <span class="l_string">"crime fighting"</span>
<span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">values</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_number">1</span>,
<span class="l_number">2</span>,
<span class="l_number">3</span>,
<span class="l_number">4</span>,
<span class="l_number">5</span>,
<span class="l_number">6</span>,
<span class="l_number">7</span>,
<span class="l_number">8</span>,
<span class="l_atom">name</span> <span class="l_symbol">=</span> <span class="l_string">"superman"</span>,
<span class="l_atom">occupation</span> <span class="l_symbol">=</span> <span class="l_string">"crime fighting"</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>When creating a single line table literal, the curly braces can also be left
off:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">my_function</span> <span class="l_table_key">dance</span><span class="l_bold_symbol">:</span> <span class="l_string">"Tango"</span>, <span class="l_table_key">partner</span><span class="l_bold_symbol">:</span> <span class="l_string">"none"</span>
<span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_table_key">type</span><span class="l_bold_symbol">:</span> <span class="l_string">"dog"</span>, <span class="l_table_key">legs</span><span class="l_bold_symbol">:</span> <span class="l_number">4</span>, <span class="l_table_key">tails</span><span class="l_bold_symbol">:</span> <span class="l_number">1</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_atom">my_function</span>(<span class="l_symbol">{</span>
<span class="l_atom">dance</span> <span class="l_symbol">=</span> <span class="l_string">"Tango"</span>,
<span class="l_atom">partner</span> <span class="l_symbol">=</span> <span class="l_string">"none"</span>
<span class="l_symbol">}</span>)
<span class="l_keyword">local</span> <span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_atom">type</span> <span class="l_symbol">=</span> <span class="l_string">"dog"</span>,
<span class="l_atom">legs</span> <span class="l_symbol">=</span> <span class="l_number">4</span>,
<span class="l_atom">tails</span> <span class="l_symbol">=</span> <span class="l_number">1</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>The keys of a table literal can be language keywords without being escaped:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">tbl</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span>
<span class="l_keyword">do</span><span class="l_bold_symbol">:</span> <span class="l_string">"something"</span>
<span class="l_table_key">end</span><span class="l_bold_symbol">:</span> <span class="l_string">"hunger"</span>
<span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">tbl</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
[<span class="l_string">"do"</span>] <span class="l_symbol">=</span> <span class="l_string">"something"</span>,
[<span class="l_string">"end"</span>] <span class="l_symbol">=</span> <span class="l_string">"hunger"</span>
<span class="l_symbol">}</span></code></pre></td></tr>
</tbody></table>
<p>If you are constructing a table out of variables and wish the keys to be the
same as the variable names, then the <code>:</code> prefix operator can be used:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">hair</span> <span class="l_symbol">=</span> <span class="l_string">"golden"</span>
<span class="l_atom">height</span> <span class="l_symbol">=</span> <span class="l_number">200</span>
<span class="l_atom">person</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span> <span class="l_bold_symbol">:</span><span class="l_atom">hair</span>, <span class="l_bold_symbol">:</span><span class="l_atom">height</span>, <span class="l_table_key">shoe_size</span><span class="l_bold_symbol">:</span> <span class="l_number">40</span> <span class="l_fn_symbol">}</span>
<span class="l_atom">print_table</span> <span class="l_bold_symbol">:</span><span class="l_atom">hair</span>, <span class="l_bold_symbol">:</span><span class="l_atom">height</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">hair</span> <span class="l_symbol">=</span> <span class="l_string">"golden"</span>
<span class="l_keyword">local</span> <span class="l_atom">height</span> <span class="l_symbol">=</span> <span class="l_number">200</span>
<span class="l_keyword">local</span> <span class="l_atom">person</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_atom">hair</span> <span class="l_symbol">=</span> <span class="l_atom">hair</span>,
<span class="l_atom">height</span> <span class="l_symbol">=</span> <span class="l_atom">height</span>,
<span class="l_atom">shoe_size</span> <span class="l_symbol">=</span> <span class="l_number">40</span>
<span class="l_symbol">}</span>
<span class="l_atom">print_table</span>(<span class="l_symbol">{</span>
<span class="l_atom">hair</span> <span class="l_symbol">=</span> <span class="l_atom">hair</span>,
<span class="l_atom">height</span> <span class="l_symbol">=</span> <span class="l_atom">height</span>
<span class="l_symbol">}</span>)</code></pre></td></tr>
</tbody></table>
<h2><a name="comprehensions"></a>Comprehensions</h2>
<p>Compiling provide a convenient syntax for constructing a new table by iterating
over some existing object and applying an expression to its values. There are
two kinds of comprehensions: list comprehensions and table comprehensions. They
both produce Lua tables; <em>list comprehensions</em> accumulate values into an
array-like table, and <em>table comprehensions</em> let you set both the key and the
value on each iteration.</p>
<h3><a name="list_comprehensions"></a>List Comprehensions</h3>
<p>The following creates a copy of the <code>items</code> table but with all the values
doubled.</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">items</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span> <span class="l_number">1</span>, <span class="l_number">2</span>, <span class="l_number">3</span>, <span class="l_number">4</span> <span class="l_fn_symbol">}</span>
<span class="l_atom">doubled</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">[</span><span class="l_atom">item</span> <span class="l_symbol">*</span> <span class="l_number">2</span> <span class="l_keyword">for</span> <span class="l_atom">i</span>, <span class="l_atom">item</span> <span class="l_keyword">in</span> <span class="l_builtins">ipairs</span> <span class="l_atom">items</span><span class="l_fn_symbol">]</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">items</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_number">1</span>,
<span class="l_number">2</span>,
<span class="l_number">3</span>,
<span class="l_number">4</span>
<span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">doubled</span> <span class="l_symbol">=</span> (<span class="l_fn_symbol">function</span>()
<span class="l_keyword">local</span> <span class="l_atom">_accum_0</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span> <span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_number">0</span>
<span class="l_keyword">for</span> <span class="l_atom">i</span>, <span class="l_atom">item</span> <span class="l_atom">in</span> <span class="l_builtins">ipairs</span>(<span class="l_atom">items</span>) <span class="l_keyword">do</span>
<span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_atom">_len_0</span> + <span class="l_number">1</span>
<span class="l_atom">_accum_0</span>[<span class="l_atom">_len_0</span>] <span class="l_symbol">=</span> <span class="l_atom">item</span> * <span class="l_number">2</span>
<span class="l_keyword">end</span>
<span class="l_keyword">return</span> <span class="l_atom">_accum_0</span>
<span class="l_keyword">end</span>)()</code></pre></td></tr>
</tbody></table>
<p>The items included in the new table can be restricted with a <code>when</code> clause:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">iter</span> <span class="l_symbol">=</span> <span class="l_builtins">ipairs</span> <span class="l_atom">items</span>
<span class="l_atom">slice</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">[</span><span class="l_atom">item</span> <span class="l_keyword">for</span> <span class="l_atom">i</span>, <span class="l_atom">item</span> <span class="l_keyword">in</span> <span class="l_atom">iter</span> <span class="l_keyword">when</span> <span class="l_atom">i</span> <span class="l_symbol">></span> <span class="l_number">1</span> <span class="l_keyword">and</span> <span class="l_atom">i</span> <span class="l_symbol"><</span> <span class="l_number">3</span><span class="l_fn_symbol">]</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">iter</span> <span class="l_symbol">=</span> <span class="l_builtins">ipairs</span>(<span class="l_atom">items</span>)
<span class="l_keyword">local</span> <span class="l_atom">slice</span> <span class="l_symbol">=</span> (<span class="l_fn_symbol">function</span>()
<span class="l_keyword">local</span> <span class="l_atom">_accum_0</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span> <span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_number">0</span>
<span class="l_keyword">for</span> <span class="l_atom">i</span>, <span class="l_atom">item</span> <span class="l_atom">in</span> <span class="l_atom">iter</span> <span class="l_keyword">do</span>
<span class="l_keyword">if</span> <span class="l_atom">i</span> > <span class="l_number">1</span> <span class="l_keyword">and</span> <span class="l_atom">i</span> < <span class="l_number">3</span> <span class="l_keyword">then</span>
<span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_atom">_len_0</span> + <span class="l_number">1</span>
<span class="l_atom">_accum_0</span>[<span class="l_atom">_len_0</span>] <span class="l_symbol">=</span> <span class="l_atom">item</span>
<span class="l_keyword">end</span>
<span class="l_keyword">end</span>
<span class="l_keyword">return</span> <span class="l_atom">_accum_0</span>
<span class="l_keyword">end</span>)()</code></pre></td></tr>
</tbody></table>
<p>Because it is common to iterate over the values of a numerically indexed table,
an <code>*</code> operator is introduced. The doubled example can be rewritten as:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">doubled</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">[</span><span class="l_atom">item</span> <span class="l_symbol">*</span> <span class="l_number">2</span> <span class="l_keyword">for</span> <span class="l_atom">item</span> <span class="l_keyword">in</span> <span class="l_symbol">*</span><span class="l_atom">items</span><span class="l_fn_symbol">]</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">doubled</span> <span class="l_symbol">=</span> (<span class="l_fn_symbol">function</span>()
<span class="l_keyword">local</span> <span class="l_atom">_accum_0</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span> <span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_number">0</span>
<span class="l_keyword">local</span> <span class="l_atom">_list_0</span> <span class="l_symbol">=</span> <span class="l_atom">items</span>
<span class="l_keyword">for</span> <span class="l_atom">_index_0</span> <span class="l_symbol">=</span> <span class="l_number">1</span>, #<span class="l_atom">_list_0</span> <span class="l_keyword">do</span>
<span class="l_keyword">local</span> <span class="l_atom">item</span> <span class="l_symbol">=</span> <span class="l_atom">_list_0</span>[<span class="l_atom">_index_0</span>]
<span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_atom">_len_0</span> + <span class="l_number">1</span>
<span class="l_atom">_accum_0</span>[<span class="l_atom">_len_0</span>] <span class="l_symbol">=</span> <span class="l_atom">item</span> * <span class="l_number">2</span>
<span class="l_keyword">end</span>
<span class="l_keyword">return</span> <span class="l_atom">_accum_0</span>
<span class="l_keyword">end</span>)()</code></pre></td></tr>
</tbody></table>
<p>The <code>for</code> and <code>when</code> clauses can be chained as much as desired. The only
requirement is that a comprehension has at least one <code>for</code> clause.</p>
<p>Using multiple <code>for</code> clauses is the same as using nested loops:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">x_coords</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span><span class="l_number">4</span>, <span class="l_number">5</span>, <span class="l_number">6</span>, <span class="l_number">7</span><span class="l_fn_symbol">}</span>
<span class="l_atom">y_coords</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span><span class="l_number">9</span>, <span class="l_number">2</span>, <span class="l_number">3</span><span class="l_fn_symbol">}</span>
<span class="l_atom">points</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">[</span><span class="l_fn_symbol">{</span><span class="l_atom">x</span>,<span class="l_atom">y</span><span class="l_fn_symbol">}</span> <span class="l_keyword">for</span> <span class="l_atom">x</span> <span class="l_keyword">in</span> <span class="l_symbol">*</span><span class="l_atom">x_coords</span> <span class="l_keyword">for</span> <span class="l_atom">y</span> <span class="l_keyword">in</span> <span class="l_symbol">*</span><span class="l_atom">y_coords</span><span class="l_fn_symbol">]</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">x_coords</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_number">4</span>,
<span class="l_number">5</span>,
<span class="l_number">6</span>,
<span class="l_number">7</span>
<span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">y_coords</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_number">9</span>,
<span class="l_number">2</span>,
<span class="l_number">3</span>
<span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">points</span> <span class="l_symbol">=</span> (<span class="l_fn_symbol">function</span>()
<span class="l_keyword">local</span> <span class="l_atom">_accum_0</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span> <span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_number">0</span>
<span class="l_keyword">local</span> <span class="l_atom">_list_0</span> <span class="l_symbol">=</span> <span class="l_atom">x_coords</span>
<span class="l_keyword">for</span> <span class="l_atom">_index_0</span> <span class="l_symbol">=</span> <span class="l_number">1</span>, #<span class="l_atom">_list_0</span> <span class="l_keyword">do</span>
<span class="l_keyword">local</span> <span class="l_atom">x</span> <span class="l_symbol">=</span> <span class="l_atom">_list_0</span>[<span class="l_atom">_index_0</span>]
<span class="l_keyword">local</span> <span class="l_atom">_list_1</span> <span class="l_symbol">=</span> <span class="l_atom">y_coords</span>
<span class="l_keyword">for</span> <span class="l_atom">_index_0</span> <span class="l_symbol">=</span> <span class="l_number">1</span>, #<span class="l_atom">_list_1</span> <span class="l_keyword">do</span>
<span class="l_keyword">local</span> <span class="l_atom">y</span> <span class="l_symbol">=</span> <span class="l_atom">_list_1</span>[<span class="l_atom">_index_0</span>]
<span class="l_atom">_len_0</span> <span class="l_symbol">=</span> <span class="l_atom">_len_0</span> + <span class="l_number">1</span>
<span class="l_atom">_accum_0</span>[<span class="l_atom">_len_0</span>] <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_atom">x</span>,
<span class="l_atom">y</span>
<span class="l_symbol">}</span>
<span class="l_keyword">end</span>
<span class="l_keyword">end</span>
<span class="l_keyword">return</span> <span class="l_atom">_accum_0</span>
<span class="l_keyword">end</span>)()</code></pre></td></tr>
</tbody></table>
<h3><a name="table_comprehensions"></a>Table Comprehensions</h3>
<p>The syntax for table comprehensions is very similar, differing by using <code>{</code> and
<code>}</code> and taking two values from each iteration.</p>
<p>This example copies the key-value table <code>thing</code>:</p>
<table width="100%" cellpadding="1" cellspacing="0"><tbody><tr class="code-header"><td>moonscript</td>
<td>lua</td></tr>
<tr><td width="50%" class="code-border"><pre><code class="moon-code"><span class="l_atom">thing</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span>
<span class="l_table_key">color</span><span class="l_bold_symbol">:</span> <span class="l_string">"red"</span>
<span class="l_table_key">name</span><span class="l_bold_symbol">:</span> <span class="l_string">"fast"</span>
<span class="l_table_key">width</span><span class="l_bold_symbol">:</span> <span class="l_number">123</span>
<span class="l_fn_symbol">}</span>
<span class="l_atom">thing_copy</span> <span class="l_symbol">=</span> <span class="l_fn_symbol">{</span><span class="l_atom">k</span>,<span class="l_atom">v</span> <span class="l_keyword">for</span> <span class="l_atom">k</span>,<span class="l_atom">v</span> <span class="l_keyword">in</span> <span class="l_builtins">pairs</span> <span class="l_atom">thing</span><span class="l_fn_symbol">}</span></code></pre></td>
<td><pre><code class="lua-code"><span class="l_keyword">local</span> <span class="l_atom">thing</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span>
<span class="l_atom">color</span> <span class="l_symbol">=</span> <span class="l_string">"red"</span>,
<span class="l_atom">name</span> <span class="l_symbol">=</span> <span class="l_string">"fast"</span>,
<span class="l_atom">width</span> <span class="l_symbol">=</span> <span class="l_number">123</span>
<span class="l_symbol">}</span>
<span class="l_keyword">local</span> <span class="l_atom">thing_copy</span> <span class="l_symbol">=</span> (<span class="l_fn_symbol">function</span>()
<span class="l_keyword">local</span> <span class="l_atom">_tbl_0</span> <span class="l_symbol">=</span> <span class="l_symbol">{</span> <span class="l_symbol">}</span>
<span class="l_keyword">for</span> <span class="l_atom">k</span>, <span class="l_atom">v</span> <span class="l_atom">in</span> <span class="l_builtins">pairs</span>(<span class="l_atom">thing</span>) <span class="l_keyword">do</span>
<span class="l_atom">_tbl_0</span>[<span class="l_atom">k</span>] <span class="l_symbol">=</span> <span class="l_atom">v</span>
<span class="l_keyword">end</span>
<span class="l_keyword">return</span> <span class="l_atom">_tbl_0</span>
<span class="l_keyword">end</span>)()</code></pre></td></tr>
</tbody></table>
<p>Table comprehensions, like list comprehensions, also support multiple <code>for</code> and
<code>when</code> clauses. In this example we use a <code>where</code> clause to prevent the value
associated with the <code>color</code> key from being copied.</p>