#15347: QuantifiedConstraints: Implication constraints with type families don't work -------------------------------------+------------------------------------- Reporter: aaronvargo | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | 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 aaronvargo): Sorry, ignore comment:1, which was stupid. This situation is different from #14860 in that the quantified variable doesn't appear in the application of the type family. As a result, it should be possible to pull the application out, replacing it with a variable, as I've described in comment:2. That is, the application `F b` can just be treated as if it's a variable `c`, allowing it to be matched. Then everything seems to work, as demonstrated by both `qux` and the example which uses fundeps. Not doing this seems to undermine the usefulness of type families as an alternative to fundeps. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15347#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler