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From: Andreas Rossberg <rossberg@mpi-sws.org>
To: Gabriel Scherer <gabriel.scherer@gmail.com>
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Date: Wed, 22 Feb 2012 19:49:01 +0100
Cc: caml List <caml-list@inria.fr>
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Subject: Re: [Caml-list] Re: "module type of" on sub-module of functor result

On Feb 22, 2012, at 18.24 h, Gabriel Scherer wrote:
>> [A(B)] and  [A.B] are syntacticly valid module_expr's but
>> [A(B).C] isn't. Is this because of an inherent limitation in the
>> module system?
>
> I believe so. When you apply a functor, you may get fresh types as a
> result -- the generative (abstract, algebraic) types of the functor
> image. If you write `module M = A(B)`, the fresh types have a clear
> identity: M.t, M.q etc; similarly if you pass A(B) to a functor with
> formal parameter X, it is X.t, X.q etc. But if you write `module M =
> A(B).C`, there is no syntactic way to name the fresh types generated
> by the functor application; in particular, naming them A(B).t would be
> incorrect -- because you could mix types of different applications.
>
> For example, what would be the signature of A(B).C with:
>
>  module B = struct end
>  module A(X : sig end) = struct
>    type t
>    module C = struct type q = t end
>  end
>
> ?

Are you perhaps thinking of SML-style generative functors here?  
Because with Ocaml's applicative functors F(A) in fact always returns  
the same abstract types, and you _can_ actually refer to types via the  
notation F(A).t ;-)

/Andreas