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Predicate nonground/2
Availability:built-in
[semidet]nonground(+Term, -Var)
True when Var is a variable in Term. Fails if Term is ground (see ground/1). This predicate is intended for coroutining to trigger a wakeup if Term becomes ground, e.g., using when/2. The current implementation always returns the first variable in depth-first left-right search. Ideally it should return a random member of the set of variables (see term_variables/2) to realise logarithmic complexity for the ground trigger. Compatible with ECLiPSe and hProlog.
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LogicalCaptain said (2021-01-14T21:09:29):0 upvotes 0 0 downvotes
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Grabbing an unbound variable from the term that appears as argument 1:

Standard usage

?- nonground(f(_),K).
true.

?- nonground(f(X,_,_),K).
X = K.

Failure if first argument is ground

?- nonground(f(x),K).
false.

Unusual usage

?- nonground(f(X),X).
true.

Unusual usage leading to sharing of argument 1

?- nonground(f(X,Y,_),Y).
X = Y.

This violates the mode. Weird things happen

?- nonground(f(X,_,_),foo).
F = foo.