#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): You should ask users not me! But looking at {{{ Extend :: ExtensionType ext a -> Test ext a }}} a user might say that `ExtentionType ext a` ''does'' use `a` as the last arg of a saturated type-family application, namely `ExtensionType ext a`! An what is "the last type variable"? Guessing wildly, what about {{{ Illegal use of type variable 'a' in the first argument of `Extend`. Such uses must be of form `ty a` where `ty` is a data type or saturated type family application. }}} I'm not sure if that's better... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13731#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler