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<h1 class="lined-header"><span>Xerl: two modules</span></h1>
<p class="date">
<span class="year">2013</span>
<span class="day-month">03 Feb</span>
</p>
</header>
<p>Everything is an expression.</p>
<p>This sentence carries profound meaning. We will invoke it many times over the course of these articles.</p>
<p>If everything is an expression, then the language shouldn't have any problem with me defining two modules in the same source file.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#FF6600">mod</font> <font color="#FF6600">first_module</font>
<b><font color="#0000FF">begin</font></b>
<b><font color="#0000FF">end</font></b>
<font color="#FF6600">mod</font> <font color="#FF6600">second_module</font>
<b><font color="#0000FF">begin</font></b>
<b><font color="#0000FF">end</font></b></tt></pre>
</div></div>
<p>Likewise, it shouldn't have any problem with me defining a module inside another module.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#FF6600">mod</font> <font color="#FF6600">out_module</font>
<b><font color="#0000FF">begin</font></b>
<font color="#FF6600">mod</font> <font color="#FF6600">in_module</font>
<b><font color="#0000FF">begin</font></b>
<b><font color="#0000FF">end</font></b>
<b><font color="#0000FF">end</font></b></tt></pre>
</div></div>
<p>Of course, in the context of the Erlang VM, these two snippets are equivalent; there is nothing preventing you from calling the <code>in_module</code> module from any other module. The <code>mod</code> instruction means a module should be created in the Erlang VM, with no concept of scope attached.</p>
<p>Still we need to handle both. To do this we will add a step between the parser and the code generator that will walk over the abstract syntax tree, from here onward shortened as <em>AST</em>, and transform the AST by executing it where applicable.</p>
<p>What happens when you execute a <code>mod</code> instruction? A module is created. Since we are compiling, that simply means the compiler will branch out and create a module.</p>
<p>If everything is an expression, does that mean this will allow me to create modules at runtime using the same syntax? Yes, but let's not get ahead of ourselves yet.</p>
<p>For now we will just iterate over the AST, and will compile a module for each <code>mod</code> found. Modules cannot contain expressions yet, so there's no need to recurse over it at this point. This should solve the compilation of our first snippet.</p>
<p>The <code>compile/1</code> function becomes:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">compile</font></b>(<font color="#009900">Filename</font>) <font color="#990000">-></font>
<b><font color="#000000">io:format</font></b>(<font color="#FF0000">"Compiling ~s...~n"</font>, [<font color="#009900">Filename</font>]),
{<font color="#FF6600">ok</font>, <font color="#009900">Src</font>} <font color="#990000">=</font> <b><font color="#000000">file:read_file</font></b>(<font color="#009900">Filename</font>),
{<font color="#FF6600">ok</font>, <font color="#009900">Tokens</font>, <font color="#990000">_</font>} <font color="#990000">=</font> <b><font color="#000000">xerl_lexer:string</font></b>(<b><font color="#000080">binary_to_list</font></b>(<font color="#009900">Src</font>)),
{<font color="#FF6600">ok</font>, <font color="#009900">Exprs</font>} <font color="#990000">=</font> <b><font color="#000000">xerl_parser:parse</font></b>(<font color="#009900">Tokens</font>),
<b><font color="#000000">execute</font></b>(<font color="#009900">Filename</font>, <font color="#009900">Exprs</font>, [])<font color="#990000">.</font>
<b><font color="#000000">execute</font></b>(<font color="#990000">_</font>, [], <font color="#009900">Modules</font>) <font color="#990000">-></font>
<b><font color="#000000">io:format</font></b>(<font color="#FF0000">"Done...~n"</font>),
{<font color="#FF6600">ok</font>, <b><font color="#000000">lists:reverse</font></b>(<font color="#009900">Modules</font>)};
<b><font color="#000000">execute</font></b>(<font color="#009900">Filename</font>, [<font color="#009900">Expr</font> <font color="#990000">=</font> {<font color="#FF6600">mod</font>, <font color="#990000">_</font>, {<b><font color="#000080">atom</font></b>, <font color="#990000">_</font>, <font color="#009900">Name</font>}, []}|<font color="#009900">Tail</font>], <font color="#009900">Modules</font>) <font color="#990000">-></font>
{<font color="#FF6600">ok</font>, [<font color="#009900">Core</font>]} <font color="#990000">=</font> <b><font color="#000000">xerl_codegen:exprs</font></b>([<font color="#009900">Expr</font>]),
{<font color="#FF6600">ok</font>, [{<font color="#009900">Name</font>, []}]} <font color="#990000">=</font> <b><font color="#000000">core_lint:module</font></b>(<font color="#009900">Core</font>),
<b><font color="#000000">io:format</font></b>(<font color="#FF0000">"~s~n"</font>, [<b><font color="#000000">core_pp:format</font></b>(<font color="#009900">Core</font>)]),
{<font color="#FF6600">ok</font>, <font color="#990000">_</font>, <font color="#009900">Beam</font>} <font color="#990000">=</font> <b><font color="#000000">compile:forms</font></b>(<font color="#009900">Core</font>,
[<b><font color="#000080">binary</font></b>, <font color="#FF6600">from_core</font>, <font color="#FF6600">return_errors</font>, {<font color="#FF6600">source</font>, <font color="#009900">Filename</font>}]),
{<font color="#FF6600">module</font>, <font color="#009900">Name</font>} <font color="#990000">=</font> <b><font color="#000000">code:load_binary</font></b>(<font color="#009900">Name</font>, <font color="#009900">Filename</font>, <font color="#009900">Beam</font>),
<b><font color="#000000">execute</font></b>(<font color="#009900">Filename</font>, <font color="#009900">Tail</font>, [<font color="#009900">Name</font>|<font color="#009900">Modules</font>])<font color="#990000">.</font></tt></pre>
</div></div>
<p>Running this compiler over the first snippet yields the following result:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#009900">Compiling</font> <b><font color="#000000">test</font></b><font color="#990000">/</font><b><font color="#000000">mod_SUITE_data</font></b><font color="#990000">/</font><font color="#FF6600">two_modules</font><font color="#990000">.</font><font color="#FF6600">xerl</font><font color="#990000">...</font>
<font color="#FF6600">module</font> <font color="#FF6600">'first_module'</font> [<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">0</font>,
<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">1</font>]
<font color="#FF6600">attributes</font> []
<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">0</font> <font color="#990000">=</font>
<b><font color="#0000FF">fun</font></b> () <font color="#990000">-></font>
<b><font color="#000080">call</font></b> <font color="#FF6600">'erlang'</font><font color="#990000">:</font><font color="#FF6600">'get_module_info'</font>
(<font color="#FF6600">'first_module'</font>)
<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">1</font> <font color="#990000">=</font>
<b><font color="#0000FF">fun</font></b> (<font color="#009900">Key</font>) <font color="#990000">-></font>
<b><font color="#000080">call</font></b> <font color="#FF6600">'erlang'</font><font color="#990000">:</font><font color="#FF6600">'get_module_info'</font>
(<font color="#FF6600">'first_module'</font>, <font color="#009900">Key</font>)
<b><font color="#0000FF">end</font></b>
<font color="#FF6600">module</font> <font color="#FF6600">'second_module'</font> [<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">0</font>,
<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">1</font>]
<font color="#FF6600">attributes</font> []
<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">0</font> <font color="#990000">=</font>
<b><font color="#0000FF">fun</font></b> () <font color="#990000">-></font>
<b><font color="#000080">call</font></b> <font color="#FF6600">'erlang'</font><font color="#990000">:</font><font color="#FF6600">'get_module_info'</font>
(<font color="#FF6600">'second_module'</font>)
<font color="#FF6600">'module_info'</font><font color="#990000">/</font><font color="#993399">1</font> <font color="#990000">=</font>
<b><font color="#0000FF">fun</font></b> (<font color="#009900">Key</font>) <font color="#990000">-></font>
<b><font color="#000080">call</font></b> <font color="#FF6600">'erlang'</font><font color="#990000">:</font><font color="#FF6600">'get_module_info'</font>
(<font color="#FF6600">'second_module'</font>, <font color="#009900">Key</font>)
<b><font color="#0000FF">end</font></b>
<font color="#009900">Done</font><font color="#990000">...</font>
{<font color="#FF6600">ok</font>,[<font color="#FF6600">first_module</font>,<font color="#FF6600">second_module</font>]}</tt></pre>
</div></div>
<p>Everything looks fine. And we can check that the two modules have been loaded into the VM:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#993399">9</font><font color="#990000">></font> <font color="#FF6600">m</font>(<font color="#FF6600">first_module</font>)<font color="#990000">.</font>
<font color="#009900">Module</font> <font color="#FF6600">first_module</font> <font color="#FF6600">compiled</font><font color="#990000">:</font> <font color="#009900">Date</font><font color="#990000">:</font> <font color="#009900">February</font> <font color="#993399">2</font> <font color="#993399">2013</font>, <font color="#009900">Time</font><font color="#990000">:</font> <font color="#993399">14.56</font>
<font color="#009900">Compiler</font> <font color="#FF6600">options</font><font color="#990000">:</font> [<font color="#FF6600">from_core</font>]
<font color="#009900">Object</font> <b><font color="#000000">file: test</font></b><font color="#990000">/</font><b><font color="#000000">mod_SUITE_data</font></b><font color="#990000">/</font><font color="#FF6600">two_modules</font><font color="#990000">.</font><font color="#FF6600">xerl</font>
<font color="#009900">Exports</font><font color="#990000">:</font>
<b><font color="#000080">module_info</font></b><font color="#990000">/</font><font color="#993399">0</font>
<b><font color="#000080">module_info</font></b><font color="#990000">/</font><font color="#993399">1</font>
<font color="#FF6600">ok</font>
<font color="#993399">10</font><font color="#990000">></font> <font color="#FF6600">m</font>(<font color="#FF6600">second_module</font>)<font color="#990000">.</font>
<font color="#009900">Module</font> <font color="#FF6600">second_module</font> <font color="#FF6600">compiled</font><font color="#990000">:</font> <font color="#009900">Date</font><font color="#990000">:</font> <font color="#009900">February</font> <font color="#993399">2</font> <font color="#993399">2013</font>, <font color="#009900">Time</font><font color="#990000">:</font> <font color="#993399">14.56</font>
<font color="#009900">Compiler</font> <font color="#FF6600">options</font><font color="#990000">:</font> [<font color="#FF6600">from_core</font>]
<font color="#009900">Object</font> <b><font color="#000000">file: test</font></b><font color="#990000">/</font><b><font color="#000000">mod_SUITE_data</font></b><font color="#990000">/</font><font color="#FF6600">two_modules</font><font color="#990000">.</font><font color="#FF6600">xerl</font>
<font color="#009900">Exports</font><font color="#990000">:</font>
<b><font color="#000080">module_info</font></b><font color="#990000">/</font><font color="#993399">0</font>
<b><font color="#000080">module_info</font></b><font color="#990000">/</font><font color="#993399">1</font>
<font color="#FF6600">ok</font></tt></pre>
</div></div>
<p>So far so good!</p>
<p>What about the second snippet? It brings up many questions. What happens once a <code>mod</code> expression has been executed at compile time? If it's an expression then it has to have a result, right? Right. We are still a bit lacking with expressions for now, though, so let's get back to it after we add more.</p>
<ul><li><a href="https://github.com/extend/xerl/blob/0.2/">View the source</a>
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