AW: [Caml-list] module alias inside class

[Gerd Stolpmann <info@gerd-stolpmann.de>]

Am 03.07.2013 07:09:49 schrieb(en) Jacques Garrigue:
> On 2013/07/03, at 0:09, ygrek <ygrekheretix@gmail.com> wrote:
> 
> > Dear list,
> >
> > Just a short question tonight, why is the following code rejected?
> >
> > class z = let module A = String in object end;;
> >
> > Is there some deep reason or it was simply overlooked?
> 
> 
> This would be very nice to able to do that.
> Unfortunately, the typing of classes is a rather complex process,
> which seems difficult to integrate with local module definitions,
> except maybe in a very restrictive way.
> There are also deep questions, such as what to do in presence
> of inheritance.

I guess the main problem is that types from A wouldn't be allowed to  
appear in class types (type escapes definition scope) - unless the  
notion of class types were extended so that such modules are bound  
(e.g. class type z = object method foo : A.t end with module A :  
TYPE_OF_A). But that mixes classes with modules conceptually (well, not  
that we have that already).

Just trying:

# class x =
     let s = (module String : Set.OrderedType) in
     object
       method compare = let module S = (val s) in S.compare
     end;;
Error: This expression has type S.t -> S.t -> int
        but an expression was expected of type S.t -> S.t -> int
        The type constructor S.t would escape its scope

Gerd


> 
> On the other hand, "let open" would be much easier to add.
> 	Jacques Garrigue
> 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
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