[m-dev.] Foreign type compare and unification

[Ralph Becket]

Peter Schachte, Tuesday, 21 May 2002:
> 
> FWIW, I think simplicity and generality, in that order, are the key
> attributes of elegance.  If a simple scheme will allow users to define
> their own classes to which all types belong, but which allow default
> methods to be overridden, I think that would be more elegant than a
> fixed list.

I was talking about this yesterday with Mark.  One idea was that we
allow declarations of the form

	:- default_instance c(T) where [...].

which would essentially make all types T an instance of type class c/1,
and provide default method implementations (an explicit c(T') instance
declaration for T' overriding the default_instance declaration for T.)

Mark objected that this would be bad in that the behaviour of a program
could change if someone later added an explicit c(T') instance
declaration.  However, this will happen anyway if we get the hoped-for
dynamic type class casts and use them in generic predicates (e.g.
to_doc/[1,2] in pprint.)

- Ralph
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