#13731: DeriveFunctor and friends don't understand type families -------------------------------------+------------------------------------- Reporter: spacekitteh | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Replying to [comment:7 simonpj]:
Yes, it's saturated in the field, but the use of `fmap` requires us to decompose the type application, we it must be decomposable. It's only decomposable if `ExtensionType` has arity 1.
Thank you, this is the part that I wasn't understanding. So would you be happy with this error message? {{{ • Can't make a derived instance of ‘Functor (Test ListExtension)’: Constructor ‘Extend’ must use the last type variable only as the last argument of a data type or a saturated type family }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler