#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 simonpj): Oh my mistake. I thought `instance Functor (Test ListExtension)` involved a type family; but it doesn't. Neither `Test` nor `ListExtension` is a type family. My mistake. Then indeed the error message makes more sense. But not total sense. If you reduce the arity of `ExtensionType1 the `deriving` clause works fine (as it should) {{{ type family ExtensionType ext :: * -> * type instance ExtensionType ListExtension = [] }}} So it's not that it must be a data type; it can be a saturated type-family application. But I now think this is a non-bug; `deriving` is working right, and reducing the arity of `ExtensionType` is the right solution. But -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler