#14046: “Illegal type synonym family application in instance” is too strict in the presence of functional dependencies -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by lexi.lambda):
It affects type inference: [snip]
Yes, I’m aware of that, but my impression was that the functional dependency changes that. Multiple instances where the only thing that changes is variables constrained by a functional dependency are illegal, ''anyway''. Therefore, I’m trying to understand if there’s some difference between these two: {{{#!hs class C a b | a -> b instance C (Maybe a) [a] }}} vs {{{#!hs class C a b | a -> b instance (b ~ [a]) => C (Maybe a) b }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14046#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler