[Caml-list] module alias inside class

[Caml-list] module alias inside class

[ygrek]

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?

-- 

Re: [Caml-list] module alias inside class

[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.

On the other hand, "let open" would be much easier to add.

	Jacques Garrigue

AW: [Caml-list] module alias inside class

[Gerd Stolpmann]

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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> 



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