prolog: built-in or other method to use function-like return values

I'm trying to le开发者_JAVA技巧t Prolog instantiate a variable to the opposite key of a public/private key pair, if that is even possible.

So I basically have:

publicKey(k_c).
privateKey(k_c-1).

keyPair(k_c,k_c-1).

Given the nature of Prolog I don't think you could manage something like

inverseOf(X) :-
(keyPair(X,Y), return(Y),!);(keyPair(Y,X),return(Y),!).

Why I want/need this?

I have a rule

init(Init_1, Init_2,Init_3)

and I want to check for something like

Init_3 == inverseOf(Init_2).

Any ideas?

Define

inverseOf(X, Y):- keyPair(X,Y), !.
inverseOf(X, Y):- keyPair(Y,X), !.

and then use

inverseOf(Init_2, Init_3).

With something like this defined:

keypair(pub1, priv1).
keypair(pub2, priv2).
keypair(pub3, priv3).

keymatch(A, B) :- keypair(A, B).
keymatch(A, B) :- keypair(B, A).

You could ask things like:

keymatch(pub1, X).
keymatch(priv2, X).
keymatch(priv3, pub3).

An aside...

Prolog does not have functions; you can't say something like Y = f(X) in Prolog (actually you can, but it doesn't do what you'd expect if you're coming from a C/Java/Python/etc background.

To implement the functionality of Y = f(X) in Prolog, you would do something like this

f(X, Y) :- Y is X + 1.

and invoke it like this

f(3, Y).

Y would evaluate to the value 4.