#10675: GHC does not check the functional dependency consistency condition correctly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Resolution: | Keywords: FunDeps 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 AntC): hmm curioser and curioser. Going back to the O.P., I see this behaviour at both GHC 7.10 and 8.0.1: * Take out the `instance C Char ...`; and `f` is happy with any type of operand, not necessarily a `Maybe`. * Keep in the `instance C Char ...`; but give `f` a signature and again it's happy: `f :: (C Bool [a] [a]) => a -> [a]` * Keep in the `instance C Char ...`; give `f` this signature, and we're back needing `Maybe`: `f :: (C Bool [a] b) => a -> b` So there is something wrong: why is the `C Char ...` instance getting tangled up with the `C Bool ...` instance? The only connection would be checking conformance with FunDeps; but that should be only for validation, not 'type improvement' for function `f`(?) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10675#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler