#12918: Make DefaultSignatures more particular about their types on the RHS of a context -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.0.2-rc1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #12784 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): OK, I've made some progress on this, but I'm once again stuck. I'm using `tcUnifyTy` once again to check if two type signatures coincide according to the criteria listed above. But there's a case it doesn't catch: {{{#!hs class Foo a where bar :: forall b. a -> b default bar :: a -> Int bar = ... }}} Here, `tcUnifyTy` happily unifies `b` with `Int`, yielding a false positive. So I need to find some way to prevent `b` from unifying with concrete types like `Int`. (Does skolemizing it accomplish this?) But we'd also want this to be accepted: {{{#!hs class Foo a where bar :: forall b. a -> b default bar :: forall c. a -> c bar = ... }}} Since `a -> c` is just a simple alpha-renaming of `a -> b`. So how can we tell `b` to unify with `c` but not with `Int`? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12918#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler