I am writing a very simple class to handle bidirectional 
(bidi) lists in an object-functional manner. I have come up 
with a curious problem. The following text is the toplevel's 
output when it is passed my code.

#
type 'a bidi = Elem of 'a bidi * 'a * 'a bidi | Null
class ['a, 'b, 'c] bidi_list :
   'a list ->
   object
     method head : 'a bidi
     method left_iter : ('a -> 'b) -> 'b list
     method right_iter : ('a -> 'c) -> 'c list
     method tail : 'a bidi
     val first : 'a bidi
     val last : 'a bidi
   end
#   new bidi_list [1;2;3;4;5];;
- : (int, '_a, '_b) bidi_list = <obj>


The basic idea is that I want to convert a value with type 
'a list to a value belonging to class ['a] bidi_list. Now, 
if I leave 'b and 'c out of the parameter list my code 
refuses to compile, but even when I add 'b and 'c as type 
parameters, the type checker specializes them to '_b and 
'_c, which are not polymorphic.

Is there any way, then, to define polymorphic versions of 
the two iterators?

Alex Baretta

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