Unable to define a polymorphic map method

Unable to define a polymorphic map method

[Jean Balthazar]

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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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Re: [Caml-list] Unable to define a polymorphic map method

[Peng Zang]

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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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Re: [Caml-list] Unable to define a polymorphic map method

[Jean Balthazar]

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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>

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> 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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> Version: GnuPG v2.0.7 (GNU/Linux)
>
> iD4DBQFJ6G+ffIRcEFL/JewRAmmdAKCYZabKzST8EgFhirfKumt0AwbP5ACYq4Qf
> BIX7A9TN5v4zce0seJbMEA==
> =mhdV
> -----END PGP SIGNATURE-----
>

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