term_variables(?Term, -VarList)

Term

Prolog term.

VarList

List or variable.

Description

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 corresponds to the order in which they are first encountered during a depth-first, left-to-right traversal of Term.

As usual, attributed variables are also considered variables.

This predicate can handle cyclic terms.

Compatibility Note: In ECLiPSe releases prior to 7.0, the list was constructed in reverse order. Use eclipse_6:term_variables/2 if you require this behaviour. The new behaviour is compatible with ISO-Prolog.

Modes and Determinism

• term_variables(?, -) is det

Exceptions

(5) type error

VarList instantiated but not to a list.

Examples

    term_variables(atom, Vs).	      % gives Vs = []
    term_variables(Term, Vs).         % gives Vs = [Term]
    term_variables(f(a,Y,c), Vs).     % gives Vs = [Y]
    term_variables(f(X,g(Y),c), Vs).  % gives Vs = [X,Y]
    term_variables(X+3*Y, Vs).        % gives Vs = [X,Y]
    term_variables([X,Y,Z], Vs).      % gives Vs = [X,Y,Z]
    term_variables([X,Y,X], Vs).      % gives Vs = [X,Y]
    term_variables(s(X{a}), Vs).      % gives Vs = [X{a}]

See Also