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:
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:
for example, to call
?- 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]].