#15351: QuantifiedConstraints ignore FunctionalDependencies -------------------------------------+------------------------------------- Reporter: aaronvargo | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 (Type checker) | Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code fails to compile: {{{#!hs {-# LANGUAGE QuantifiedConstraints #-} {-# LANGUAGE FunctionalDependencies #-} class C a b | a -> b where foo :: a -> b bar :: (forall a. C (f a) Int) => f a -> String bar = show . foo }}} {{{ • Could not deduce (Show a0) arising from a use of ‘show’ ... The type variable ‘a0’ is ambiguous }}} Yet it ought to work, since this is perfectly fine with top-level instances: {{{#!hs instance C [a] Int where ... baz :: [a] -> String baz = show . foo }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15351 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler