fjh at cs.mu.oz.au
Thu Sep 25 12:42:13 AEST 1997
Tomas By, you wrote:
> Suppose I have the following types:
> :- type a(X,Y) ---> x(X) ; y(Y) ; z(int).
A type like that, where the type parameters X and Y are used in different
constructors, is a little bit odd; I haven't come across any like
that myself, except in our artificially constructed test suite, so if this
is from a real example, I'd be interested in knowing what the real example
> and I want a single procedure 'p/1' that handles things of the types:
> but no others.
Why don't you want p/1 to handle things of other types?
> Is this possible? How do I declare it?
Tyson already suggested one solution.
Another way of doing it is to define a type `ij' as the union of `i' and `j',
and use `inst' declarations to define `i' and `j' as "subtypes" of `ij'.
Then you can declare p/1 as simply
:- pred p(a(ij, mn)).
and give it either just
:- mode p(in, in).
or if you prefer
:- mode p(in(i), in(m)).
:- mode p(in(i), in(n)).
:- mode p(in(j), in(m)).
:- mode p(in(j), in(n)).
(Which is more efficient will depend on the details of p/1.)
This approach of using insts for subtyping has the drawback that
I explained in mail to mercury-users the other day.
When we have typeclasses implemented, you could declare a typeclass ij
and make i and j instances of it. Whether or not that is an appropriate
solution depends on whether you want to access `i' and `j' as concrete
types or only via an abstract interface. That will depend on your answer
to my earlier question "why don't you want p/1 to handle things of other
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