
#15974: QuantifiedConstraints: Spurious error involving superclass constraints -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 Resolution: invalid | Keywords: | QuantifiedConstraints 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 simonpj): Great explanation lexi.lambda. I too claim this is not a but. Reason: as you say:
`forall a. B a => B (f a)` does not actually entail `forall a. A a => A (f a)`
So it's not a shortcoming in the implementation (although there are plenty of those). It's just that there is no way to construct evidence for `forall a. A a => A (f a)` from evidence for `forall a. B a => B (f a)` If you agree, would you like to close as invalid? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15974#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler