#12473: Ambiguous type var with DefaultSignatures and FunctionalDependencies -------------------------------------+------------------------------------- Reporter: dylex | 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: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider: {{{ {-# LANGUAGE DefaultSignatures, FunctionalDependencies, ScopedTypeVariables #-} class Foo a b where foo :: a -> b -> Int class Bar a b | b -> a where get :: b -> a bar :: b -> Int default bar :: Foo a b => b -> Int bar b = foo (get b :: a) b }}} This works fine with ghc 7.10.3 (even without the explicit :: a type spec), but with 8.0.1, produces: {{{ - Could not deduce (Foo a0 b) from the context: Foo a b bound by the type signature for: bar :: Foo a b => b -> Int at t.hs:6:3-17 The type variable `a0' is ambiguous - In the ambiguity check for `bar' To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the class method: bar :: forall a b. Bar a b => b -> Int In the class declaration for `Bar' }}} Not sure if this is intentional or some good way to get around it, but I couldn't find anything related to it in the release notes or any similar looking bugs. (Real-life case: https://github.com/dylex/postgresql- typed/blob/master/Database/PostgreSQL/Typed/Dynamic.hs#L48 ) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12473 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler