#10831: DeriveFunctor not capable of deriving when wrapping Either -------------------------------------+------------------------------------- Reporter: htebalaka | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by thomie): * os: MacOS X => Unknown/Multiple * component: Compiler => Compiler (Type checker) Old description:
GHC can derive Functor for Okay, but rejects the equivalent Bad. {{{#!hs {-# LANGUAGE DeriveFunctor #-} data Okay f g a b = Inl (f a b) | Inr (g a b) deriving (Functor) newtype Bad f g a b = Bad (Either (f a b) (g a b)) deriving (Functor) }}}
New description: GHC can derive Functor for Okay, but rejects the equivalent Bad. {{{#!hs {-# LANGUAGE DeriveFunctor #-} data Okay f g a b = Inl (f a b) | Inr (g a b) deriving (Functor) newtype Bad f g a b = Bad (Either (f a b) (g a b)) deriving (Functor) }}} {{{ Can't make a derived instance of ‘Functor (Bad f g a)’: Constructor ‘Bad’ must use the type variable only as the last argument of a data type In the data declaration for ‘Bad’ }}} -- Comment: This comment (option `c`) in `compiler/typecheck/TcDeriv.hs` says this is to be expected: {{{ cond_functorOK :: Bool -> Bool -> Condition -- OK for Functor/Foldable/Traversable class -- Currently: (a) at least one argument -- (b) don't use argument contravariantly -- (c) don't use argument in the wrong place, e.g. data T a = T (X a a) -- (d) optionally: don't use function types -- (e) no "stupid context" on data type }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10831#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler