More Prolog

Here are some Prolog programs (written in Strawberry Prolog) and their output:

program
father(avraham, yitzchak).
father(yitzchak, yaakov).

ancestor(X, Y) :- father(X,Y).
ancestor(X, Y) :- father(X,Z), ancestor(Z, Y).

?- father(yitzchak, X), write(X), write("\n").

Output:
Compiling the file:
c:\father
0 errors, 0 warnings.

yaakov
Yes.

program
add1(X, Y) :- Y is X + 1.

?- add1(4, 5).

output
Yes.

program
add1(X, Y) :- Y is X + 1.

?- add1(4, 6).

output
No.

program
add1(X, Y) :- Y is X + 1.

?- add1(4, X), write(X), write("\n").

output
5
Yes.

program
add(X, Y, Z) :- Z is X + Y.

?- add(4, 5, X), write(X), write("\n").

output
9
Yes.

program
add(X, Y, Z) :- Z is X + Y.

?- add(4, "hello", X), write(X), write("\n").

output
4hello
Yes.

program
add(X, Y, Z) :- Z is X + Y.

?- add(4, Y, X), write(X), write("\n").

output
Compiling the file:
c:\father
0 errors, 0 warnings.

Runtime Error at step 3:
The argument 2 of "+"
is "var" but has to be "str" or "int" or "float".

No.

program
add(X, Y, Z) :- Z is X + Y.

?- add(4, 5, 9), write(X), write("\n").

output
_9
Yes.

program
father(avraham, yitzchak).
father(yitzchak, yaakov).

ancestor(X, Y) :- father(X,Y).
ancestor(X, Y) :- father(X,Z), ancestor(Z, Y).

?- ancestor(avraham, X), write(X), write("\n").

output (selecting Run, then Next Answer)
Compiling the file:
C:\father
0 errors, 0 warnings.

yitzchak
Yes.
yaakov
Yes.

program
abs(X, Y) :- X = Y, X >= 0.

?- abs(4, T), write(T), nl.

output
4
Yes.

program
abs(X, Y) :- X = Y, X >= 0.

?- abs(-4, T), write(T), nl.

output
No.

program
abs(X, Y) :- Y = X, X >= 0.
abs(X, Y) :- Y = 0 - X.

?- abs(-4, T), write(T), nl.

output
4
Yes.

program:
append([], L, L).
append([H | T] , L2, [H | L3] ) :- append(T, L2, L3).

1) empty list + L = L
2) recursion step
appending a list [H | T] to any list L2 produces the list [H | L3], on condition that L3 is formed by appending T to L2.

try out an example.