#8560: undeducable Typeable error with data kinds when deriving Data for GADT in GHC version 7.7.20131122 -------------------------------------+------------------------------------- Reporter: carter | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 (Type checker) | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by thomie): * component: Compiler => Compiler (Type checker) Comment: With HEAD the error is now: {{{ $ ghc-7.9.20141115 test.hs ... Could not deduce (Data a) arising from a use of ‘k’ from the context (Typeable (Shape n a)) bound by the instance declaration at test.hs:33:1-34 or from (n ~ 'S r) bound by a pattern with constructor :* :: forall a (r :: Nat). a -> Shape r a -> Shape ('S r) a, in an equation for ‘gfoldl’ at test.hs:33:1-34 Possible fix: add (Data a) to the context of the data constructor ‘:*’ or the type signature for gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Shape n a -> c (Shape n a) or the instance declaration ... }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8560#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler