Re: [Caml-list] Re: "module type of" on sub-module of functor result

[Gabriel Scherer <gabriel.scherer@gmail.com>]

> [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

?

2012/2/22 Till Varoquaux <till@pps.jussieu.fr>:
> 2012/2/22 Milan Stanojević <milanst@gmail.com>d
>> On Tue, Feb 21, 2012 at 3:32 PM, Hezekiah M. Carty <hez@0ok.org> wrote:
>>> On Tue, Feb 21, 2012 at 2:37 PM, Ashish Agarwal <agarwal1975@gmail.com> wrote:
>>>> I must've accidentally deleted part of my email before hitting send. The
>>>> point was to make the first code sample compile after removing the commented
>>>> line. But that is not allowed; I get a syntax error:
>>>>
>>>> $ ocamlfind ocamlc -c -package batteries a.ml
>>>> File "a.ml", line 6, characters 38-39:
>>>> Error: Syntax error: 'end' expected
>>>> File "a.ml", line 4, characters 16-19:
>>>> Error: This 'sig' might be unmatched
>>>>
>>>> I'm wondering if there is a better solution than my second code sample.
>>>>
>>
>> I would consider this a bug. I think than any module expression than
>> can be used with "include" should be usable with "include module type
>> of"
>>
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>
> Milan is right: the language grammar does say that both [include] and
> [module type of] should work on module_expr. However, based upon the
> manual(*),  [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?
>
> [*]http://caml.inria.fr/pub/docs/manual-ocaml/manual019.html#module-expr
>
>
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