No matching Ball

If the argument of shift/1 cannot be unified with any Ball of any reset/3 on the stack, shift/1 throws an exception:

existence_error('reset/3',Ball).

What happens if...

"Goal does not call shift/1" means Goal succeeds.

• If Goal succeeds, then reset/3 succeeds with the Continuation == 0, which is distinguishable from callable continuations, which are goals (compound terms or possibly an atom)
• If Goal fails, then reset/3 fails.
• If Goal throws, then reset/3 throws.
• If Goal calls shift/1 and the shifted term unifies with a Ball used at a reset/3 point on the call stack (if there are several, the nearest one), then execution flow continues as if reset/3 had succeeded, with
  • The unification of Ball performed (which may communicate information from the coroutine or unify a local variable with an unbound variable held by the coroutine, which can then be unified with a term to communicate information to the coroutine)
  • With Continuation instantiated to a compound term: a callable goal to transfer the execution flow back to the coroutine through another reset/3.

Passing initial arguments to Goal

If you have to pass additional arguments to an atomic Goal before calling it, simply do:

reset(call(Goal,AnotherArg),Ball,Cont)

for example, to call append([1,2],[3,4],X):

?- reset(call(append([1,2]),[3,4],X),Ball,Cont).
X = [1, 2, 3, 4],
Cont = 0.

More Goal tricks

Goal may be a complex goal

?- reset((bagof(X,member(X,[1,2,3]),Bag),memberchk(1,Bag)),Ball,Cont).
Bag = [1, 2, 3],
Cont = 0.

Backtracking over reset/3 actually works. This goal succeeds 3 times:

?- reset(member(X,[1,2,3]),Ball,Cont).
X = 1,
Cont = 0 ;
X = 2,
Cont = 0 ;
X = 3,
Cont = 0.

Two successes, and one open choicepoint which does not lead to success at the end:

?- reset(member(1,[1,2,1,3]),Ball,Count).
Count = 0 ;
Count = 0 ;
false.

You can collect the results of reset/3 normally:

?- bagof([X-Cont],reset(member(X,[1,2,3]),Ball,Cont),Bag).
Bag = [[1-0], [2-0], [3-0]].