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term_variables(?Term, ?VarList)

Succeeds if VarList is the list of all variables in Term.
?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 is not specified. As usual, metaterms are also considered variables.

Fail Conditions

None.

Resatisfiable

No.

Exceptions

(5) type error
VarList instantiated but not to a list.

Examples

Success:
    term_variables(atom, []).
    term_variables(Term, L).       % gives L = [Term]
    term_variables(f(a,B,c), L).   % gives L = [B]
    term_variables([X,Y,Z], L).    % gives L = [Z,Y,X]
    term_variables([X,Y,X], L).    % gives L = [Y,X]
    term_variables(s(X{a}), L).    % gives L = [X{a}]

Fail:
    term_variables(f(a,B,c), []).

See Also

nonground / 1, nonground / 2, nonground / 3, nonvar / 1, var / 1