#12763: Incorrect behavior with empty functional dependencies -------------------------------------+------------------------------------- Reporter: diatchki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This is a corner case, but it would appear that GHC doesn't do proper improvements for classes with "empty" functional dependencies. Consider the following example: {{{#!hs class C a | -> a where m :: a -> () instance C Int f x = m x }}} The inferred type for `f` is `C a => a -> ()`, however I was expecting it to infer `Int -> ()`. The reasoning is as follows: the use of `m` generates a `C a` constraint (`a` is a unification variable). This should have interacted with the `C Int` instance (in a rather odd vacuous sort of way) to generate a derived constraint `a ~ Int`, which should have then instantiated `a` to `Int`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12763 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler