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Unable to define a polymorphic map method
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Date: -- (:)
From: Peng Zang <peng.zang@g...>
Subject: Re: [Caml-list] Unable to define a polymorphic map method
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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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