[mercury-users] discriminant types, generators

[Matthias Kretschmer]

Hello!

I have the following problem, want to do the following task:

having some discriminant type, say
:- type mytype ---> a ; b ; c ; d.

maybe used by some other type, say
:- type othertype ---> someint(int) ; intandmytype(int, mytype).

Now I want to define some predicate that can be used to generate a list 
of intandmytype(2, X) for all X in mytype.

I thought of something like this:
:- pred mypred(othertype).
:- mode mypred(out) is multi.
mypred(someint(2, _)).

and using solutions/2 from std_util for generating a list using mypred.

but the compiler complains about a mode error. The only solution I come 
up with, was some extra predicate, lets say
:- pred is_mytype(mytype).
:- pred is_mytype(out) is multi.
is_mytype(a). is_mytype(b). is_mytype(c). is_mytype(d).

but I don't really like to have to do this this way. Is there any other 
solution?

--
Greetings
Matthias Kretschmer

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