Question

I’ve read about it in a book but it wasn’t explained at all. I also never saw it in a program. Is part of Prolog syntax? What’s it for? Do you use it?

Answer 1

It represents implication. The righthand side is only executed if the lefthand side is true. Thus, if you have this code,

implication(X) :-
  (X = a ->
    write('Argument a received.'), nl
  ; X = b ->
    write('Argument b received.'), nl
  ;
    write('Received unknown argument.'), nl
  ).

Then it will write different things depending on it argument:

?- implication(a).
Argument a received.
true.

?- implication(b).
Argument b received.
true.

?- implication(c).
Received unknown argument.
true.

(link to documentation.)