Prolog, using expressions
I'm trying to learn SWI prolog, but my simple program fails when I believe it should succeed.
%My code:
orthogonal((X1,Y1,Z1),(X2,Y2,Z2)) :- (X1*X2)+(Y1*Y2)+(Z1*Z2)==0.
integerVector开发者_开发知识库((X,Y,Z)) :- integer(X),integer(Y),integer(Z).
?-orthogonal((1,0,0),(0,0,1)).
I press compile Buffer in the pseudoemacs window and the output is:
% [PATH].pl compiled 0.00 sec, 136 bytes
ERROR: emacs_prolog_mode ->error_at_location: Argument 1 (int): `int' expected, found `@11470948?start'
ERROR: emacs_prolog_mode ->error_at_location: Argument 1 (int): `int' expected, found `@11470948?start'
Warning: [PATH]s.pl:5:
Goal (directive) failed: user:orthogonal((1,0,0), (0,0,1))
You have used (==)/2
in place of (=:=)/2
which evaluates its arguments as arithmetic expressions.
You can use (X,Y,Z)
, but it isn't a triple as in e.g. Haskell. To see this:
?- write_canonical((1,2,3)).
','(1,','(2,3))
?- (1,2,3) = (X,Y).
X = 1, Y = (2,3).
Expressions in Prolog simply represent syntactic term trees. To evaluate an expression you need to X is Y
which evaluates Y as an arithmetic expression and unifies the result with X. Alternatively you can use X =:= Y
which evaluates both X and Y as arithmetic expressions, then unifies the results.
Cheers!
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