Applying Logical Knowledge

Control Structures

Other control constructs can sometimes be more efficient, but can sometimes cause loss of readability:

or - THIS ; THAT
if-then-else - COND -> THEN ; ELSE

price(apple, 10).
price(orange, 10).

?- price(X, 10).
X = apple ;
X = orange ;
no

price(X, 10) :-
  ( X = apple ; X = orange ).

?- price(X, 10).
X = apple ;
X = orange ;
no

price(apple, 10).
price(orange, 20).

?- price(orange, X).
X = 20
yes

price(Item, Price) :-
  ( Item = apple ->
       Price = 10
	   ;
	   (Item = orange ->
	      Price = 20
		  ;
		  fail)
   ).

?- price(orange, X).
X = 20
yes