discuss.visual-prolog.com

[Steve Lympany]

Hi Thomas,
Just an observation.
The following compiles without problem:

domains
    dom=one;two.
predicates
   test:(dom).
clauses
   test(DOM):-
      if DOM=one then
         RET=1
      elseif DOM=two then
         RET=2
      end if.

- the if-then-else correctly recognises all domain options to make the predicate procedural, and there is no need for a "catch-all" else statement.

However for a predicate function:

predicates
   test:(dom)->integer.
clauses
   test(DOM)=RET:-
      if DOM=one then
         RET=1
      elseif DOM=two then
         RET=2
      end if.

the compiler complains:

error c998: The variable 'RET' can be both free and bound

So it needs a final "else" to fix it:

      else
         RET=3

But maybe this is quite tricky for the compiler to resolve.
regards
Steve

[Thomas Linder Puls]

Hi, Steve.

The compiler does not even attempt to make such judgements for elseif.

The reason the predicates works, while the function doesn't is that you use "RET" after the if-construction in function but not in the predicate.

Completeness analysis is made plain pattern matching using and and or:

clauses
    test(DOM) :-
       (
            DOM=one, RET=1
        or
            DOM=two, RET=2
        ),
        stdio::write(Ret).

and for pattern matching in arguments:

clauses
    test(one) :-
        RET=1,
        stdio::write(Ret).
    test(two) :-
        RET=1,
        stdio::write(Ret).