Re: [Caml-list] Syntax for multiple subtyping constraints?

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Brian Smith <brian-l-smith@uiowa.edu>

> 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?

You have many ways to force (self : #x) and (self : #y).

class z = object ((self : #x) : #y)
  method foo = 2
  method bar = "there"
end
class z = object (self : 'a)
  constraint 'a = #x
  constraint 'a = #y
  method foo = 2
  method bar = "there"
end
class z = object
  inherit x
  inherit y
  method foo = 2
  method bar = "there"
end

The last one uses the fact x and y are virtual classes, rather than
class types.

Note that none of these guarantee a subtyping relation, only a
matching one. Fortunately, as long as the type of self does not appear
in a contravariant position in x or y, this relation is included in
subtyping.

Jacques Garrigue
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