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<title>8.6 Recovering the type of a module</title>
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<h2 class="section" id="s:module-type-of"><a class="section-anchor" href="#s:module-type-of" aria-hidden="true"></a>8.6 Recovering the type of a module</h2>
<p><a id="hevea_manual.kwd216"></a>
<a id="hevea_manual.kwd217"></a>
<a id="hevea_manual.kwd218"></a>
<a id="hevea_manual.kwd219"></a></p><p>(Introduced in OCaml 3.12)</p><div class="syntax"><table class="display dcenter"><tr class="c019"><td class="dcell"><table class="c001 cellpading0"><tr><td class="c018">
<a class="syntax" href="modtypes.html#module-type"><span class="c010">module-type</span></a></td><td class="c015">::=</td><td class="c017">
...
</td></tr>
<tr><td class="c018"> </td><td class="c015">∣</td><td class="c017"> <span class="c004">module</span> <span class="c004">type</span> <span class="c004">of</span> <a class="syntax" href="modules.html#module-expr"><span class="c010">module-expr</span></a>
</td></tr>
</table></td></tr>
</table></div><p>The construction <span class="c002"><span class="c003">module</span> <span class="c003">type</span> <span class="c003">of</span></span> <a class="syntax" href="modules.html#module-expr"><span class="c010">module-expr</span></a> expands to the module type
(signature or functor type) inferred for the module expression <a class="syntax" href="modules.html#module-expr"><span class="c010">module-expr</span></a>.
To make this module type reusable in many situations, it is
intentionally not strengthened: abstract types and datatypes are not
explicitly related with the types of the original module.
For the same reason, module aliases in the inferred type are expanded.</p><p>A typical use, in conjunction with the signature-level <span class="c004">include</span>
construct, is to extend the signature of an existing structure.
In that case, one wants to keep the types equal to types in the
original module. This can done using the following idiom.
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> <span class="ocamlkeyword">type</span> MYHASH = <span class="ocamlkeyword">sig</span>
<span class="ocamlkeyword">include</span> <span class="ocamlkeyword">module</span> <span class="ocamlkeyword">type</span> <span class="ocamlkeyword">of</span> <span class="ocamlkeyword">struct</span> <span class="ocamlkeyword">include</span> Hashtbl <span class="ocamlkeyword">end</span>
<span class="ocamlkeyword">val</span> replace: ('a, 'b) t -> 'a -> 'b -> unit
<span class="ocamlkeyword">end</span></div></div>
</div><p>
The signature <span class="c003">MYHASH</span> then contains all the fields of the signature
of the module <span class="c003">Hashtbl</span> (with strengthened type definitions), plus the
new field <span class="c003">replace</span>. An implementation of this signature can be
obtained easily by using the <span class="c004">include</span> construct again, but this
time at the structure level:
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> MyHash : MYHASH = <span class="ocamlkeyword">struct</span>
<span class="ocamlkeyword">include</span> Hashtbl
<span class="ocamlkeyword">let</span> replace t k v = remove t k; add t k v
<span class="ocamlkeyword">end</span></div></div>
</div><p>Another application where the absence of strengthening comes handy, is
to provide an alternative implementation for an existing module.
</p><div class="caml-example verbatim">
<div class="ocaml">
<div class="pre caml-input"> <span class="ocamlkeyword">module</span> MySet : <span class="ocamlkeyword">module</span> <span class="ocamlkeyword">type</span> <span class="ocamlkeyword">of</span> Set = <span class="ocamlkeyword">struct</span>
…
<span class="ocamlkeyword">end</span></div></div>
</div><p>
This idiom guarantees that <span class="c003">Myset</span> is compatible with Set, but allows
it to represent sets internally in a different way.</p>
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