#13061: Incorrect constraints given single flexible undecidable instance. -------------------------------------+------------------------------------- Reporter: JCarr | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Constraint, Resolution: | UndecidableInstances, | FlexibleInstances Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): Interesting. This happens because when you bind a variable with `let`/`where`, GHC simplifies the type. In your case, it simplifies `A a => a -> a` to `Eq a => a -> a`, choosing the `Eq a => A a` instance. This is a bit bogus, though, because `Eq a => a -> a` is not actually more general than `A a => a -> a` in the presence of other instances. I'm not sure what to do here, really. I think this may just be an oddity of `FlexibleInstances`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13061#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler