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[Caml-list] Parameterizing multiple modules using functors
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Date: -- (:)
From: Yaron Minsky <yminsky@c...>
Subject: Re: [Caml-list] Parameterizing multiple modules using functors
>> Here's the issue.  Consider a project consisting of multiple modules
>> with lots of interdependencies.   Imagine then that I want to add some
>> flexibility to the system by abstracting out some of the structure of
>> the system using a functor.  Thus, I start out with modules A, B, C,
>> and I want to parameterize all these modules, let's say by the same
>> module X.   So what I want is a version of the program where I can use
>> A.F(X) intead of A, B.F(X) instead of B, and so on.  The problem is
>> that I want to allow references between these modules.  Thus, B.F(X)
>> needs access to A.F(X), and C.F(X) needs access to both A.F(X) and
>> B.F(X).  The question is, is there a reasonable way to achieve this?
>
> There are two approaches, but I'm not sure you'll find them
> "reasonable". One is as you described:
>
>> I can think of an unreasonable way.  I can write the relevant functors
>> so that they can be embedded in a unified module U as follows:
>>
>> module U =
>> struct
>>    module A = A.F(X)
>>    module B = B.F(A,X)
>>    module C = C.F(A,B,X)
>> end
>>
>> In other words, B and C and require in their functors explicit
>> arguments corresponding to the other modulse they need.  This works,
>> but it's awfully ugly, not least because you have to write down some
>> awful module signatures in B and C to make this work.
>
> The other is to write:
>
> module F (X: SIGX) =
>   struct
>     module A = ...
>     module B = ...
>     module C = ...
>   end
>
> This way, B and C automatically reference A as instantiated for X. You
> do lose separate compilation of A, B and C, though.  You can still put
> A, B and C in separate files and generate the code above from the files,
> perhaps with the use of a literate programming tool, but the generated
> file will have to be recompiled entirely every time one of the
> sub-modules change.
>
> - Xavier Leroy
>
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