From: Markus Mottl <mottl@miss.wu-wien.ac.at>
Message-Id: <199901182037.VAA04696@miss.wu-wien.ac.at>
Subject: Re: Problem binding type parameters in modules + subtyping and inheritance
To: Jerome.Vouillon@inria.fr (Jerome Vouillon)
Date: Mon, 18 Jan 1999 21:37:20 +0100 (MET)
In-Reply-To: <19990118201616.45587@pauillac.inria.fr> from "Jerome Vouillon" at Jan 18, 99 08:16:16 pm
Hello,
> Another solution is to turn "empty" into a function :
> module type FOO1 = sig
> type 'a foo
> val empty : unit -> 'a foo
> end;;
> module Bar : FOO1 = struct
> type 'a foo = 'a list Lazy.t
> let empty () = lazy []
> end;;
Would certainly also work. But I guess it is better to stay with the
functor - modules parameterized with a new value are not so common as
function invocations. Thus, I won't have to change this much code porting
Okasaki's sources.
> -----------------------------------------------------------
> Actually, it is still possible to make the coercion when the type of
> self only occur in covariant position (in particular, only on the
> right sides of arrows) :
> class c = object (self)
> method m = self
> end;;
> class d = object (self)
> inherit c
> method n = 1
> end;;
> let x = (new d :> c);;
>
> There is a difficulty with "binary methods", where self type occurs in
> contravariant position :
> class c = object (self : 'a)
> method x = 1
> method compare (o : 'a) = self#x = o#x
> end;;
> class d = object
> inherit c
> method y = 1
> end;;
> let x = (new d :> c);; (* Fails *)
Unfortunately, it's the binary methods I need...
> However, you can coerce to a common supertype of c and d which has no
> method "compare" :
> class type c' = object
> method x : int
> end;;
> let l = [(new c :> c'); (new d :> c')];; (* Succeeds *)
That's the problem:
Actually the only method I would need is "compare" ;-)
Regards,
Markus
-- Markus Mottl, mottl@miss.wu-wien.ac.at, http://miss.wu-wien.ac.at/~mottl
This archive was generated by hypermail 2b29 : Sun Jan 02 2000 - 11:58:18 MET