typeclasses in OCaml

[Peng Zang <peng.zang@gmail.com>]

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Hi,

I have been using the Object system in OCaml for the equivalent functionality 
of haskell typeclasses for some time.  It allows nice things like:

  class type ['a] foldable = object
    method fold : 'z. ('z -> 'a -> 'z) -> 'z -> 'z
  end

  let forall (f:'a -> bool) (obj:'a #foldable) = 
    obj#fold (fun acc x -> acc && f x) true


Recently however, I tried to make a "mappable" class.  Something like:

  class type ['a] mappable = object('self)
    method map : 'z. ('a -> 'z) -> 'z 'self
  end

Which of course is invalid syntax.  However, I think the point is clear.  I 
want to specify the type of all container objects that can map their 
contents.  How do you specify this in OCaml?

I realize there are some inherent issues, eg. polymorphic classes are 
invarient.  So I'm willing to accept some limited use of magic as long as it 
can be hidden away and offer a type-safe interface.  Nevertheless, after half 
a day of trying, I cannot seem to figure this out.  I don't even know how to 
specify:

  "  a 'z version of 'self  "

In a syntactically correct way.  Is this possible?  Thank in advance,

Peng
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