This predicate collects all the variables inside Term into the list VarList. Every variable occurs only once in VarList, even if it occurs several times in Term. The order of the variables in the list is not specified.
As usual, attributed variables are also considered variables.
This predicate terminates even with cyclic terms.
Success: term_variables(atom, []). term_variables(Term, Vs). % gives Vs = [Term] term_variables(f(a,B,c), Vs). % gives Vs = [B] term_variables([X,Y,Z], Vs). % gives Vs = [Z,Y,X] term_variables([X,Y,X], Vs). % gives Vs = [Y,X] term_variables(s(X{a}), Vs). % gives Vs = [X{a}] Fail: term_variables(f(a,B,c), []).