[m-users.] Multi predicate of entire enum

[Julien Fischer]

Hi,

On Sun, 19 Jul 2020, Ace wrote:

> So say I have some enum:
> 
> `:- type ex ---> a; b; c.`
> 
> And I want to generate a function similar to:
> 
> ```
> :- pred ex_mul(ex::out) is multi.
> ex_mul(a).
> ex_mul(b).
> ex_mul(c).
> ```
> 
> Any kind of standard library predicate I can use so I don't have to
> add a new case to the predicate for every instance of the enum?

You could use RTTI to implement such a predicate, although it will be more
expensive than than the simple form above, e.g. something like:

     :- pred ex_mul(ex::out) is multi.

     ex_mul(V) :-
         TypeDesc = type_desc.type_of(V),
         NumFunctors = construct.det_num_functors(TypeDesc),
         ( if
            int.nondet_int_in_range(0, NumFunctors - 1, FunctorNum),
            Univ = construct.construct(TypeDesc, FunctorNum, [])
         then
            univ.det_univ_to_type(Univ, V)
         else
            error("should not get here")
        ).

Julien.