Is there something like anonymous predicates in SWI Prolog?

Question

Can I define an anonymous predicate in SWI Prolog, bind it to a variable, and call it later? Something like this:

?- F = {(X, Y) :- Y is 2 * X}, call(F, 2.0, Y).

Answer 1

Alternatively, in SWI-Prolog, you can use library(yall). It can be autoloaded so you don't need to import anything:

?- F = [X, Y]>>( Y is 2*X ), call(F, 2.0, Y).
F = [X, 4.0]>>(4.0 is 2*X),
Y = 4.0.

I think that in the general case it would be better to use a new variable for the result of calling the lambda:

?- F = [X, Y]>>( Y is 2*X ), call(F, 2.0, R).
F = [X, Y]>>(Y is 2*X),
R = 4.0.