#9792: map/coerce rule never seems to fire -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: ekmett Type: bug | Status: new Priority: normal | Milestone: 7.10.1 Component: Core | Version: 7.9 Libraries | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: performance bug | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by goldfire): Depends on the meaning of "guarantee". You're right in that `Coercible` is lifted and can therefore, in theory, contain ⊥. However, as I understand it, we do some work to avoid ever creating ⊥ here. This is why we track stack depths (or some such) in the solver, right? Another way to say this is that we've designed the solver to avoid creating a ⊥ `Coercible`. So, if by "guarantee", we mean "If GHC is a bug-free implementation of its specification, then there are no ⊥ `Coercible`s", I think we're close. I believe the situation here is very close to that of `~`, which is a lifted type but is supposedly never ⊥. We're not quite there because of deferred type errors, which surely can create ⊥ `Coercible`s. But I'm personally OK with wonky semantics in this case, if the wonky semantics means real-world speedups in the vastly more common no-deferred-type-errors case. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9792#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler