Not_found error when combining constraints with findall_solutions

[nguermond]

When I run the following

main :- declare_constraint (U = 2) [U],
        findall_solutions (U = 1) Solns.

I get the error Fatal error: exception Not_found with no explanation.
This feels like an uncaught exception somewhere.
I'm using elpi 2.0.6.

[gares]

This is an oversight. Findall creates a new runtime, but does not copy there any constraint.
There is only a check that any solution found does not come with constraints, so I cannot blindly copy the constraints over.

[gares]

Maybe we should have 2 APIs:

• findall_solutions that disregards constraints (and does not raise Not_found either, of course), to be backward compatible
• findall_solutions_constraints that copies all constraints and accepts solutions with constraints. In this case it may be reasonable to also return these constraints, eg Sols = [ pr Q [declare_constraint C L, ...] , ....]

[nguermond]

I was able to find a workaround, so no longer have a use case for this, but in principle it would make sense to have findall_solutions_constraints. In the short term it would help to have a more useful error message and a remark in the documentation for findall_solutions.