Re: [Caml-list] shortcut to omit the functor keyword

[Andreas Rossberg <rossberg@mpi-sws.org>]

On Mar 25, 2011, at 19.43 h, Joel Reymont wrote:
> Why does the shortcut to omit the functor keyword only exists in  
> module definitions but not in signatures?
>
> For example, why do
>
> 	module type A = sig end;;
> 	module type B = sig end;;
>
> 	module type C = functor (X : A) -> functor (Y : B) -> sig end;;
>
> and not
>
> 	module type C (X : A) (Y : B) = sig end;;

That syntax would suggest that C is a parameterized signature that you  
could use like

   module M : C(X)(Y)

But that's something entirely different from a functor signature.

> the latter would be consistent with
>
> 	module C (X : A) (Y : B) = struct end;;

No, it wouldn't - I'd say you are making a category error. It's easy  
to see in terms of core types and values, where the latter is  
analogous to

   let c x y = ...

while the former would be more like

   type ('a, 'b) c = ...

and *not* at all

   val c : 'a -> 'b -> ...

That is, a parameterized signature would correspond to a type  
constructor, while a functor signature corresponds to a function type.

Hope that helps,
/Andreas