Modules within classes

Modules within classes

[Martin Hofmann]

Dear all, 

Can someone explain what is wrong here and perhaps suggest a workaround? 

class ['a] test ( l : 'a list) = 
object 
   method result = 
     let module M = 
      struct
        type t = 'a      
        let compare x y =
          if x < y then -1 else if x > y then 1 else 0
      end
  in
    let module MSet = Set.Make(M)
    in  MSet.elements (List.fold_right MSet.add l MSet.empty) 
end;;

OCaml says: "Unbound type parameter 'a". 

Our aim is of course not to program sorting the umpteenth time but to 
have a means for using Pervasives.Set within a parametrised class.

The problem seems to be that although "class ['a]" binds 'a its use is 
nevertheless disallowed in the scope of the class declaration. 

Many thanks in advance!

Martin Hofmann (on behalf of Oliver Friedmann)

Re: [Caml-list] Modules within classes

[Jonathan Roewen]

It's the same at normal scope:

type t = 'a;; doesn't compile.

you need: type 'a t = 'a, which isn't compatible with input type for Set.

I think the Set module is intentionally monomorphic...

Jonathan

On 9/13/06, Martin Hofmann <mhofmann@tcs.ifi.lmu.de> wrote:
> Dear all,
>
> Can someone explain what is wrong here and perhaps suggest a workaround?
>
> class ['a] test ( l : 'a list) =
> object
>   method result =
>     let module M =
>      struct
>        type t = 'a
>        let compare x y =
>          if x < y then -1 else if x > y then 1 else 0
>      end
>  in
>    let module MSet = Set.Make(M)
>    in  MSet.elements (List.fold_right MSet.add l MSet.empty)
> end;;
>
> OCaml says: "Unbound type parameter 'a".
>
> Our aim is of course not to program sorting the umpteenth time but to
> have a means for using Pervasives.Set within a parametrised class.
>
> The problem seems to be that although "class ['a]" binds 'a its use is
> nevertheless disallowed in the scope of the class declaration.
>
> Many thanks in advance!
>
> Martin Hofmann (on behalf of Oliver Friedmann)
>
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Re: [Caml-list] Modules within classes

[Jonathan Roewen]

As for a workaround, use List.sort. I presume there's a real reason to
want to use Set rather than the contrived example given though... if
not, then there's your solution (=

Jonathan

PS: The order of traversing the list doesn't seem to be of much
significance, so List.fold_left might be better suited.

On 9/13/06, Jonathan Roewen <jonathan.roewen@gmail.com> wrote:
> It's the same at normal scope:
>
> type t = 'a;; doesn't compile.
>
> you need: type 'a t = 'a, which isn't compatible with input type for Set.
>
> I think the Set module is intentionally monomorphic...
>
> Jonathan
>
> On 9/13/06, Martin Hofmann <mhofmann@tcs.ifi.lmu.de> wrote:
> > Dear all,
> >
> > Can someone explain what is wrong here and perhaps suggest a workaround?
> >
> > class ['a] test ( l : 'a list) =
> > object
> >   method result =
> >     let module M =
> >      struct
> >        type t = 'a
> >        let compare x y =
> >          if x < y then -1 else if x > y then 1 else 0
> >      end
> >  in
> >    let module MSet = Set.Make(M)
> >    in  MSet.elements (List.fold_right MSet.add l MSet.empty)
> > end;;
> >
> > OCaml says: "Unbound type parameter 'a".
> >
> > Our aim is of course not to program sorting the umpteenth time but to
> > have a means for using Pervasives.Set within a parametrised class.
> >
> > The problem seems to be that although "class ['a]" binds 'a its use is
> > nevertheless disallowed in the scope of the class declaration.
> >
> > Many thanks in advance!
> >
> > Martin Hofmann (on behalf of Oliver Friedmann)
> >
> > _______________________________________________
> > Caml-list mailing list. Subscription management:
> > http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list
> > Archives: http://caml.inria.fr
> > Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> > Bug reports: http://caml.inria.fr/bin/caml-bugs
> >
>

Re: [Caml-list] Modules within classes

[Pietro Abate]

On Wed, Sep 13, 2006 at 09:44:30AM +0200, Martin Hofmann wrote:
> Can someone explain what is wrong here and perhaps suggest a workaround? 

maybe you can try to encapsulate your class in a functor ? 
but I guess this doesn't  really solve your problem...

module Make(T : sig type a end) = struct
    class test ( l : T.a list) =
    object
       method result =
         let module M =
          struct
            type t = T.a
            let compare x y =
              if x < y then -1 else if x > y then 1 else 0
          end
      in
        let module MSet = Set.Make(M)
        in  MSet.elements (List.fold_right MSet.add l MSet.empty)
    end;;
end

:)
p

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