#11357: Regression when deriving Generic1 on poly-kinded data family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: goldfire Type: bug | Status: merge Priority: highest | Milestone: 8.0.1 Component: Compiler | Version: 7.11 (CodeGen) | Keywords: Generics, Resolution: | TypeInType, TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | deriving/should_compile/T11357 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Well, that patch seemed to fix that problem, but it introduced another one. Vanilla datatypes and data family instances are still inconsistent w.r.t. which type variables are considered "instantiated" in a `Generic1` instance. For instance, this is rejected: {{{ λ> data Proxy k (a :: k) = ProxyCon deriving Generic1 <interactive>:32:43: error: • Can't make a derived instance of ‘Generic1 (Proxy *)’: Proxy must not be instantiated; try deriving `Proxy k a' instead • In the data declaration for ‘Proxy’ }}} And rightfully so, since the visible kind binder `k` is instantiated to `*`. But now it's possible to have an equivalent instance for a data family that squeaks past this check! {{{ λ> data family ProxyFam (a :: y) (b :: z) λ> data instance ProxyFam k (a :: k) = ProxyFamCon deriving Generic1 ==================== Derived instances ==================== Derived instances: instance GHC.Generics.Generic1 (Ghci13.ProxyFam *) where ... }}} I need to investigate further to see why this is the case. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11357#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler