Ref polymorphism restriction

[Ohad Rodeh <orodeh@cs.huji.ac.il>]

List,
  The following program does not compile, with a 
rather strange error message. 

module type SIG = 
  sig
    val l : '_a list ref
    val f : '_a -> unit
  end

module M : SIG = 
  struct
    let l = ref [] 
      
    let f a = l:= a::!l
  end


Modules do not match:
  sig val l : '_a list ref val f : '_a -> unit end
is not included in
  SIG
Values do not match:
  val l : '_a list ref
is not included in
  val l : 'a list ref

Note that the signature for list l IS '_a list ref. 

I suppose this is due to the restriction on polymorphic references. 
Is the OCaml restriction such that references may only be local to
a single module? Is there a different restriction? 

	Ohad.

----------------------------------------------------------
orodeh@cs.huji.ac.il
www.cs.huji.ac.il/~orodeh