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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