[I summarize with some direct questions at the bottom of this email.]
I spent time last night trying to eliminate -dcore-lint errors from my record and variant library using the coxswain row types plugin. I made some progress, but I'm currently stuck, as discussed on this github Issue.
Here's the relevant bit:
The latest unresolved -dcore-lint error is an out-of-scope cobox co var. I'm certainly not creating it directly (there are no U(plugin:coxswain,... in the Core Lint warning), but I have to wonder if my somewhat loose use of UnivCo is violating some assumptions somewhere that's causing GHC to drop the co var binding or overlook this occurrence of it on a renaming/subst pass. I checked UnivCo for source comments looking for anything it should not be used for, but I didn't find an obvious explanation along those lines.
I haven't yet been able to effectively distill the test case.
I'm doing this all at -O0.
With `-ddump-tc-trace`, I can see the offending cobox (cobox_a67M) is present in an "implication evbinds" listing after a "solveImplication end }" delimiter, but that's the last obvious binding of it.
[G] cobox_a67J = CO Sym cobox_a654,
[G] cobox_a67M
= cobox_a67J `cast` U(plugin:coxswain,...)
cobox_a654 is introduced by a GADT pattern match.
I'm also not seeing obvious occurrences of cobox_a67M, but I think the reason is that I'm seeing several (Sym cobox) with no uniques printed (even with `-dppr-debug`). Those are probably the cobox in question, but I can't confirm.
Questions:
1) Is there a robust way to ensure that covar's uniques are always printed? (Is the pprIface reuse with a free cobox part of the issue here?)
2) Is my plugin asking for this kind of trouble by using UnivCo to cast coboxes?
3) If I spent the effort to create non-UnivCo coercions where possible, would that likely help? This is currently an "eventually" task, but I haven't seen an urgency for it yet. I could bump its priority if it might help. E.G. I'm using UnivCo to cast entire givens when all I'm doing is reducing a type family application somewhere "deep" within the given's predtype. I could, with considerable effort, instead wrap a single, localized UnivCo within a bunch of non-UnivCo "lifting" coercion constructors. Would that likely help?
3) Is there a usual suspect for this kind of situation where a cobox binding is seemingly dropped (by the typechecker) even though there's an occurrence of it?
Thank you for your time. -Nick