I have written this little program:
married(guy1, fem1).
married(guy2, fem2).
married_to(X,Y):-!, married(X,Y); married(Y,X).
Prints:
X = guy1,
Y = fem1 ;
X = guy2,
Y = fem2.
My purpose was to print the married couples but one time each. The above rule works for me, but I can't understand why! How does this work?
What's the difference with that:
married_to(X,Y):- married(X,Y); married(Y,X).
This prints:
X = guy1,
Y = fem1 ;
X = guy2,
Y = fem2 ;
X = fem1,
Y = guy1 ;
X = fem2,
Y = guy2.
The cut commits execution to the first branch on the disjunction. Thus your rule:
married_to(X,Y):-!, married(X,Y); married(Y,X).
is operationally the same as:
married_to(X,Y):-!, married(X,Y).
And, given that there are no other clauses for the married_to /2 predicate, equivalent to:
married_to(X,Y):- married(X,Y).
imagine you have the equivalent definition married_to(X,Y):- true, !, married(X,Y) ; married(Y,X).. If true were to fail, then we'd jump to the alternative, but since it's not going to, we're firmly on the consequent branch (of the disjunction).
IOW your code is
married_to(X,Y):- ( !, married(X,Y) ) ; ( married(Y,X) ).
and not
married_to(X,Y):- !, ( married(X,Y) ; married(Y,X) ).
It is actually equivalent to
married_to(X,Y):- !, married(X,Y).
married_to(X,Y):- married(Y,X).
Perhaps with this it is easier to get a "feeling" for what is going on here. Obviously, the second clause of the predicate is cut off. :) We're committed to the first clause, the second doesn't get a chance to work at all.
