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0 pointsoldsecondhand4y ago0 comments

In Prolog :- is the if operator.

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3 comments · 1 top-level

gumby4y ago· 2 in thread

Its use is kind of a code smell, and I believe it was a relatively (prolog is old) late addition.

In any case, I wrote "doesn't need", though perhaps you consider that hair splitting.

I think you misinterpreted that. Prolog does has an if operator -> as in ( P -> If ; Else ) yes, but oldsecondhand said :-

"predicateA is true" if "clauseB is true" and "clauseC is true".

or in prolog

  predicateA :-
    clauseB,
    clauseC.

so :- is if, just in it's own, Prolog-y way.

Is that an "if", though? In C/C++, you could write that as:

  if (clauseB && clauseC)
    predicateA = true;
  else
    predicateA = false;

which is clearly an "if". Or you could write it as:

  predicateA = claseB && clauseC;

which is not an "if" at all, but just a boolean calculation. (Unless you regard all boolean calculations as "if"s in disguise...)

The prolog version seems to me to be more in the spirit of the second C version.

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