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Message-ID: <3D2E6A3F.70001@uiowa.edu>
Date: Fri, 12 Jul 2002 00:33:51 -0500
From: Brian Smith <brian-l-smith@uiowa.edu>
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To: OCaml Mailing list <caml-list@inria.fr>
Subject: [Caml-list] Syntax for multiple subtyping constraints?
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Hello everbody,

Please consider the following:

# class virtual x = object method virtual foo : int    end
   class virtual y = object method virtual bar : string end
   class z =
     object
      method foo = 2
      method bar = "there"
     end;;

How can I assert that class "z" is supposed to be a subtype of class x 
and class y? In other words, how can I assert that (some_z : z :> x) and 
(some_z : z :> y) will always be valid? I tried:

# class z =
     object (self : #x, self : #y)
      method foo = 2
      method bar = "there"
     end;;
Characters 29-30:
     object (self : #x, self : #y)
                      ^
Syntax error

But, it only works for one supertype, e.g.:

class z =
   object (self : #x) (* or (self : #y) *)
    method foo = 2
    method bar = "there"
   end;;

I know that I could just leave it out and everything would work, but I 
want to specify that z is supposed to be a subtype of both x and y so 
that, if I screw up the definition of class z, the compiler will give me 
an error. For example, if I didn't define the method "bar" in class "z", 
I want to get the error:

   This class  type should be virtual
   The following methods are undefined : bar

Thanks,
Brian

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