#12660: singletons doesn't compile with GHC 8.0.2 snapshot -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.2 Component: Compiler | Version: Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I see what's going on, but I don't know why things are the way they are nor how to fix. The problem is, essentially, that we unflatten and then continue solving. Then, we have a fuv (flattening unification variable) that's filled in with the function it equals. So the flattener ends up bouncing between the function application and its fuv (as found in the flat-cache). The unflattening is happening in `TcInteract.solve_simple_wanteds`, as called from `solveSimpleWanteds`. But then the `go` loop in the latter goes around again. It does so in the same instance of the `TcS` monad, so that flat-cache is retained, even though unflattening has happened. And then we're in trouble. Questions: 1. Should the flat-cache be wiped every time we unflatten? 2. Should we even allow unflattening in the `TcS` monad? I always thought unflattening was about restoring nice-looking types for error messages and something that should happen just as we leave the `TcS` monad. But we shouldn't resume solving after unflattening, I think. I'm afraid I've reached the limit of what I can do with this ticket. Simon? :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12660#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler