Re: [Caml-list] polymorphic lists, existential types and asorted other hattery

[Julien Moutinho <julien.moutinho@gmail.com>]

On Tue, Nov 13, 2007 at 12:27:07PM -0500, Peng Zang wrote:
> Is there a way to create lists in which the elements may be of
> differing types but which all have some set of operations defined
> (eg. tostr) in common?
> [...]

Objects may be of interest to you:

% cat file.ml
class ['e] skel (e: 'e) =
  object
    val mutable e = e
    method get = e
    method set x = e <- x
  end

class virtual ['e] xs
  (to_string: 'e -> string) =
  object (self)
    method virtual get : 'e
    method coerce = (self :> < show : string >)
    method show = to_string self#get
  end

class oint (e: 'e) =
  object
    inherit ['e] skel e
    inherit ['e] xs string_of_int
  end
class ofloat (e: 'e) =
  object
    inherit ['e] skel e
    inherit ['e] xs string_of_float
  end

let xs =
  [ (new oint 1)#coerce
  ; (new ofloat 2.0)#coerce
  ; (new oint 3 :> < show : string >)
  ; (object method show = "soleil" end)
  ]
;;

List.iter (fun o -> print_endline o#show) xs

% ocaml file.ml
1
2.
3
soleil

% ocamlc -i file.ml
class ['a] skel :
  'a -> object val mutable e : 'a method get : 'a method set : 'a -> unit end
class virtual ['a] xs :
  ('a -> string) ->
  object
    method coerce : < show : string >
    method virtual get : 'a
    method show : string
  end
class oint :
  int ->
  object
    val mutable e : int
    method coerce : < show : string >
    method get : int
    method set : int -> unit
    method show : string
  end
class ofloat :
  float ->
  object
    val mutable e : float
    method coerce : < show : string >
    method get : float
    method set : float -> unit
    method show : string
  end
val xs : < show : string > list

HTH,
  Julien.