Pure way for tracking state of variables in Prolog

state.pl

% get_value(Value, History)
% History tracks values set for variable
get_value(Val, List) :-
	nonvar(List), % just so that fails when nothing added yet.
	nextto(t(Val), Var, List),
	var(Var),
	!.

available_spot(X, History) :-
	once((member(X, History), var(X))).

add_value(Val, History) :-
  available_spot(X, History),
  t(Val) = X.

Example outputs:

?- get_value(X, History).
false.

?- add_value(abc, History).
History = [t(abc)|_2856].

?- add_value(abc, History), get_value(Current, History).
History = [t(abc), _5110|_5112],
Current = abc.

?- add_value(3, History), get_value(FirstWas, History), add_value('my-symbol', History), get_value(SecondWas, History), add_value(2, History), add_value(AVar, History), get_value(LatestIs, History), LatestIs = xyz.
History = [t(3), t('my-symbol'), t(2), t(xyz), _1946|_1948],
FirstWas = 3,
SecondWas = 'my-symbol',
AVar = LatestIs, LatestIs = xyz.

Code is released to the public domain