I’ve been thinking about how coroutines work in Prolog and whether they follow the same rules for determinism as regular predicates. There’s this interesting question about what counts as a solution versus what counts as an answer, and I’m not sure if there’s actually a difference between these two concepts.
For example, if I write something like:
?- when(ground(X), fail).
Does this query actually have a solution? The execution gets delayed because of the constraint, so it doesn’t immediately succeed or fail. This makes me wonder about the whole concept of determinism when dealing with delayed goals.
I’m trying to understand if coroutines change how we think about deterministic versus non-deterministic behavior in Prolog. When a goal is suspended and waiting for some condition to be met, how do we classify its deterministic properties? Does the delayed nature affect whether we consider it to have found a solution or not?
Can someone explain how determinism applies to coroutines and suspended goals in Prolog?
yeah, it’s a real head-scratcher! with coroutines, the answer just sits there waiting. your when(ground(X), fail) query won’t resolve yet - it’s waiting to see what x becomes. it’s deterministic, but we won’t know until those conditions actually fire.
Here’s what’s happening: determinism in Prolog with coroutines works on a different timeline than normal goal evaluation. When you’ve got suspended goals like when(ground(X), fail), the deterministic classification becomes conditional instead of immediate. I’ve worked with constraint logic programming quite a bit, and suspended goals exist in what I call a “potential state” - they haven’t produced a solution yet, but they also haven’t definitively failed. The determinism analysis has to consider all possible future states where the suspension condition might get satisfied. In your case, once X becomes ground, the goal will deterministically fail. So your overall query is deterministically false, even though it’s not immediately obvious. Prolog systems usually treat suspended goals as having unknown determinism until the constraints resolve. This creates an interesting distinction - your program’s logical meaning stays deterministic, but the operational behavior looks non-deterministic because of the delay in evaluation. The coroutine mechanism basically defers the determinism check rather than changing your predicates’ fundamental logical properties.
Coroutines don’t break determinism - they just delay when it’s evaluated. I’ve seen this with freeze/2 and dif/2 constraints where goals get suspended until certain conditions are met. The suspended goal keeps the same deterministic properties it’ll have when it wakes up, but the system can’t figure this out until the constraint fires. In your when(ground(X), fail) example, the goal still has its deterministic failure property even while suspended. The coroutine mechanism creates a dependency graph where determinism depends on variable bindings. What’s cool is that Prolog engines keep the logical semantics while putting off the operational evaluation. Suspended goals pile up in the constraint store and show their deterministic behavior once the triggering conditions are met. Your program stays correct even though execution involves delays. The determinism is there logically but gets revealed bit by bit through constraint resolution.