#9270: GHC HEAD accepts a manual instance decl but not equivalent TH decl ------------------------------------+------------------------------------- Reporter: Feuerbach | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- If I try to build the traverse-with-class package with GHC HEAD, I get an error: {{{ Data/Generics/Traversable/Instances.hs:21:1: Illegal type constructor or class name: ‘c’ When splicing a TH declaration: instance c_0 a_1 => Data.Generics.Traversable.Core.GTraversable c_0 (Data.Maybe.Maybe a_1) where Data.Generics.Traversable.Core.gtraverse = \f_2 x_3 -> case x_3 of Data.Maybe.Nothing -> Control.Applicative.pure Data.Maybe.Nothing Data.Maybe.Just arg_4 -> (Control.Applicative.<*>) (Control.Applicative.pure Data.Maybe.Just) (f_2 arg_4) }}} This code used to compile with all the previous versions. Additionally, if I replace the TH splice with the generated instance declaration (with minimal corrections to make all names resolve), it is accepted: {{{ instance c_0 a_1 => Data.Generics.Traversable.Core.GTraversable c_0 (Maybe a_1) where gtraverse = \f_2 x_3 -> case x_3 of Nothing -> Control.Applicative.pure Nothing Just arg_4 -> (Control.Applicative.<*>) (Control.Applicative.pure Just) (f_2 arg_4) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9270 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler