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POOcaml Functors, Modules and Submodules
 

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    <p>Apologies for posting such long, non-compilable code. But despite reading several questions and answers on stackoverflow on ocaml's functors, I don't get how to solve this:</p> <p>Assume I have a very abstract data structure:</p> <h3><code>ads.mli</code></h3> <pre><code>module type ENTRY = sig type t val get_index : t -&gt; int val compare : t -&gt; t -&gt; int end module type T = sig type entry type t val create : unit -&gt; t val insert : entry -&gt; t -&gt; t val delete : entry -&gt; t -&gt; t end </code></pre> <p>Based on this, I can make concrete data structures on these abstract implementation by passing a functor. For example I made:</p> <h3><code>concrete_ads.mli</code></h3> <pre><code>module Make (Entry: Ads.ENTRY) : (ads.T with type entry = Entry.t) </code></pre> <p>This work, I can now use my implementation in other source-files, for example like this:</p> <pre><code>module AT = Concrete_ads.Make( type t = int * int;; let get_index = fst;; let to_string = (fun (x,y) -&gt; Printf "%i, %i" x y);; end);; </code></pre> <p>And, then, use the implemenation like:</p> <pre><code>let at = AT.create () in let ati = AT.insert (1,2) at in let atd = AT.delete (1,2) ati in </code></pre> <p>... etc.</p> <p>Now, I want write several functions that operate on these data structures in a seperate sourcefile, and they should be accesible from outside. But, I do not know how to declare the type of these functions. Something like this:</p> <h3><code>search.mli</code></h3> <pre><code>val search : Int -&gt; Ads.T -&gt; int list </code></pre> <p>But, when compiling I get: </p> <pre><code> Failure: "invalid long identifier type" </code></pre> <p>I, then, thought I need to specifically declare the module of adt as a submodule in <code>search.mli</code>, something like:</p> <h3><code>search.mli</code></h3> <pre><code>module AD = Ads;; ... val search : Int -&gt; AD.T -&gt; int list </code></pre> <p>But, I get:</p> <pre><code>Parse error: [module_declaration] expected after [a_UIDENT] (in [sig_item]) </code></pre> <p>What am I missing here ? I feel I either fail with the syntax, or did not fully grasp the concept of Functors, Modules and Submodules ...</p> <p><strong>Edit</strong> Thank you so much for your explanation, gasche! With your example I was able to write what I inteded. I'll post it here for clarification, since there seems to be alot of confusion about functors in ocaml. </p> <p>In fact I wanted to make the function abstract with respect to <code>Ads.T</code>, but require a specific type for <code>Ads.T.t</code>. </p> <p>I now have <code>search.mli</code>:</p> <pre><code>module Make (T : Ads.T with type entry = int * int) : sig val search : T.t -&gt; int -&gt; int end;; </code></pre> <p>And, in <code>search.ml</code>:</p> <pre><code>module Make (T : Ads.T with type entry = int * int) : sig val search : T.t -&gt; int -&gt; int end = struct (* actual implementation of search *) end;; </code></pre> <p>And it worked exactly as I intended.</p>
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