Thank your for your answer, but it does not solve my actual
problem: suppose I have a data-structure with no cons (for example
a matrix). I know how to implement a map function but I don't know
how to type it as a method.

Jean

2009/4/17 Peng Zang <peng.zang@gmail.com>

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> See previous answers to this question:
>
>
> http://groups.google.com/group/fa.caml/browse_thread/thread/9bb6ff19373371fb
>
> I've used the classical solution thus far and have not run into any
> trouble..
>
> Peng
>
> On Friday 17 April 2009 05:37:52 am Jean Balthazar wrote:
> > Dear OCamlers,
> >
> > I try to define a polymorphic container  encapsulating a list. I'd like
> to
> > define
> > a map and a fold method. I have read in the documentation how
> > to specify a polymorphic fold method but I'm unable to transpose
> > for the map method. I include in the code below a tentative: the map1
> > method is not polymorphic. If I uncomment the map2 method, I have the
> error
> >
> > This type scheme cannot quantify 'c :
> > it escapes this scope.
> >
> > Do you have any idea how to solve this problem? And aditionnaly, could
> you
> > explain
> > me why it works for fold and not for map?
> >
> > All the best,
> > Jean
> >
> >
> ***************************************************************************
> >***************************
> >
> > class ['elt] container arg =
> > object
> >
> >   val  _repr = arg
> >
> >   method fold : 'b. ('b -> 'elt -> 'b) -> 'b -> 'b
> >     = fun f accu -> List.fold_left f accu _repr
> >
> >   method map1 f = {< _repr = List.map f _repr >}
> >
> > (*
> >   method map2 : 'c.('elt -> 'c) -> ('c container)
> >     = function f -> new container (List.map f _repr)
> > *)
> > end
> >
> >
> > let l = new container [1;2;3]
> >
> > let fint = l#fold (fun acc x -> x + acc) 0
> > and fstring = l#fold (fun acc x -> (string_of_int x)^acc) "empty"
> >
> >
> > let mint = l#map1 (function x -> 0)
> > let mstring = l#map1 (function x -> "zero")
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