#12163: Incorrect error message when deriving Functor for a GADT -------------------------------------+------------------------------------- Reporter: benjamin.hodgson | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: Operating System: MacOS X | Architecture: x86_64 Type of failure: Incorrect | (amd64) warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by benjamin.hodgson: @@ -20,1 +20,2 @@ - deriving declaration works though. + deriving declaration works though. I'm guessing GHC is turning my + declaration into `Mk :: (a ~ Int) => a -> b -> T a b`? New description: {{{#!hs {-# LANGUAGE DeriveFunctor #-} {-# LANGUAGE GADTs #-} data T a b where Mk :: Int -> b -> T Int b deriving (Functor) }}} GHC says: {{{ Can't make a derived instance of ‘Functor (T a)’: Constructor ‘Mk’ has existentials or constraints in its type Possible fix: use a standalone deriving declaration instead In the data declaration for ‘T’ }}} This is not true - Mk does not existentially quantify anything, nor does it have a constraint in its type. The suggested fix of using a standalone deriving declaration works though. I'm guessing GHC is turning my declaration into `Mk :: (a ~ Int) => a -> b -> T a b`? Tested on 7.10.3 running in OSX Mavericks. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12163#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler