Why Prolog can't see the obvious answer?-CodePudding

f(9) = f(A), X = A, X = T * T

Why prolog cant see A = 9, X = 9, T = 3/-3, am I making a syntax error here? If so what is the correct form of this query? I am getting false, but there is obviously an easy answer to this equation.

CodePudding user response：

in one sense you are making an interesting error because you are magically expecting prolog to be able to deduce a solution from a constraint (X=T*T) (NOTE: that's a good thing, that's a very prolog expectation). there are two things not working here

to get prolog to understand you want to evaluate a math expression you need to use "is" instead of equal
you need to give prolog a hint about what values might work for T

here is something in the neighborhood of what you were trying that will work

f(9) = f(A), X = A, between(-100,100,T), X is T * T

CodePudding user response：

Besides the good solution already proposed by @madeofmistake, another alternative is to use the library for constraint logic programming over finite domains (clpfd). After loading this library, you can use the operator #= to evaluate arithmetic constraints.

?- use_module(library(clpfd)).
true.

?- f(9) = f(A), X = A, X #= T * T.
A = X, X = 9,
T in -3 \/ 3.