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<h2 class="section" id="sec209">6.12  Compilation units</h2>
<table class="display dcenter"><tr class="c026"><td class="dcell"><table class="c002 cellpading0"><tr><td class="c025">
<a class="syntax" id="unit-interface"><span class="c014">unit-interface</span></a></td><td class="c022">::=</td><td class="c024"> { <a class="syntax" href="modtypes.html#specification"><span class="c014">specification</span></a>  [<span class="c008">;;</span>] }
 </td></tr>
<tr><td class="c025"> </td></tr>
<tr><td class="c025">
<a class="syntax" id="unit-implementation"><span class="c014">unit-implementation</span></a></td><td class="c022">::=</td><td class="c024"> [ <a class="syntax" href="modules.html#module-items"><span class="c014">module-items</span></a> ]
</td></tr>
</table></td></tr>
</table><p>Compilation units bridge the module system and the separate
compilation system. A compilation unit is composed of two parts: an
interface and an implementation. The interface contains a sequence of
specifications, just as the inside of a <span class="c008">sig</span> … <span class="c008">end</span>
signature expression. The implementation contains a sequence of
definitions and expressions, just as the inside of a
<span class="c008">struct</span> … <span class="c008">end</span> module
expression. A compilation unit also has a name <span class="c014">unit-name</span>, derived
from the names of the files containing the interface and the
implementation (see chapter <a href="comp.html#c%3Acamlc">8</a> for more details). A
compilation unit behaves roughly as the module definition
</p><div class="center">
<span class="c005"><span class="c007">module</span> <span class="c014">unit-name</span> <span class="c007">:</span> <span class="c007">sig</span></span>  <a class="syntax" href="#unit-interface"><span class="c014">unit-interface</span></a> <span class="c005"><span class="c007">end</span> <span class="c007">=</span>
<span class="c007">struct</span></span>  <a class="syntax" href="#unit-implementation"><span class="c014">unit-implementation</span></a> <span class="c008">end</span>
</div><p>A compilation unit can refer to other compilation units by their
names, as if they were regular modules. For instance, if <span class="c007">U</span> is a
compilation unit that defines a type <span class="c007">t</span>, other compilation units can
refer to that type under the name <span class="c007">U.t</span>; they can also refer to <span class="c007">U</span> as
a whole structure. Except for names of other compilation units, a unit
interface or unit implementation must not have any other free variables.
In other terms, the type-checking and compilation of an interface or
implementation proceeds in the initial environment
</p><div class="center">
<span class="c014">name</span><sub>1</sub> <span class="c005"><span class="c007">:</span> <span class="c007">sig</span></span>  <a class="syntax" href="modtypes.html#specification"><span class="c014">specification</span></a><sub>1</sub> <span class="c008">end</span> …
 <span class="c014">name</span><sub><span class="c013">n</span></sub> <span class="c005"><span class="c007">:</span> <span class="c007">sig</span></span>  <a class="syntax" href="modtypes.html#specification"><span class="c014">specification</span></a><sub><span class="c013">n</span></sub> <span class="c008">end</span>
</div><p>
where <span class="c014">name</span><sub>1</sub> …  <span class="c014">name</span><sub><span class="c013">n</span></sub> are the names of the other
compilation units available in the search path (see
chapter <a href="comp.html#c%3Acamlc">8</a> for more details) and <a class="syntax" href="modtypes.html#specification"><span class="c014">specification</span></a><sub>1</sub> …
 <a class="syntax" href="modtypes.html#specification"><span class="c014">specification</span></a><sub><span class="c013">n</span></sub> are their respective interfaces.
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