#14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: #3485 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC rejects the following instance, complaining about the use of a type synonym family in the instance head: {{{#!hs type family Fam a class Multi a b | a -> b instance Multi (Maybe a) (Fam a) }}} However, this seems too strict to me, since the second parameter of `Multi` is determined by a functional dependency. If I tweak the instance slightly to lift the second parameter out of the instance head into an equality constraint, GHC accepts the instance: {{{#!hs instance (b ~ Fam a) => Multi (Maybe a) b }}} It seems to me that, due to the functional dependency, these two instances are completely equivalent. Therefore, GHC should accept the first instance. This is similar to an old ticket, #3485, but that did not involve fundeps, so GHC’s error was correct. I’m pretty sure the addition of functional dependencies changes things, making the two actually equivalent. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14046 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler