[mercury-users] polymorphism

[Tyson Richard DOWD]

Tomas By wrote:
> Hi,
> 
> Suppose I have the following types:
> 
>    :- type a(X,Y) ---> x(X) ; y(Y) ; z(int).
> 
>    :- type i ---> [...].
>    :- type j ---> [...].
> 
>    :- type m ---> [...].
>    :- type n ---> [...].
> 
> and I want a single procedure 'p/1' that handles things of the types:
> 
>    a(i,m)
>    a(i,n)
>    a(j,m)
>    a(j,n)

You have 4 different types here. If you want a single predicate to
handle these cases, you'll need to put wrappers around them.

Either:
	Create a new type

	:- type b ---> w(a(i,m)) ; x(a(i,n)) ; y(a(j,m)) ; z(a(j,n)).

	So p operates on b

Or:	Create two new types:

	:- type c ---> q(i) ; r(j).
	:- type d ---> s(n) ; t(m).

	So p operates on a(c,d).

	Which one you choose really depends on the problem domain - if
	the operations involving i and j are separate to m an n (as it
	appears to be because of the definition of a), then the second
	alternative may be more natural.

I think there are other alternatives that involve unimplemented
features...

-- 
       Tyson Dowd           #          Another great idea from the 
                            #            people who brought you
      trd at .cs.mu.oz.au      #               Beer Milkshakes!
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