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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
For what it's worth, last time I brought this up, it was mentioned that
some solutions have been proposed to this problem in the Standard ML

Also, here's a link to the previous discussion on google groups.


> I've asked this question before, but never quite got a satisfactory
> answer.  I'm hoping something has changed between now and then (maybe
> with 3.07?).
> 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?
> 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.
> Is there any non-awful way to achieve this?  The absence of this kind of
> feature makes the entire functor system seem much less attractive for
> adding any large-scale genericity.
> Yaron
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