[m-users.] Syntax error at token 'is': operator precedence error

[Volker Wysk]

Hi.

I'm trying to define a predicate, which takes another predicate of arity 0
as its first argument:

:- pred issat(pred, bool).
:- mode issat(pred is semidet, out) is det.

issat(Pred, Issat) :-
    ( if Pred then Issat = yes
              else Issat = no
    ).


I get this error:

test.m:030: Syntax error at token 'is': operator precedence error.


Why? It works for arity > 0, but how to define it for arity 0?

Cheers,
Volker
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