#11732: Deriving Generic1 interacts poorly with TypeInType -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: TypeInType, | Operating System: Unknown/Multiple Generics | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- From @RyanGlScott, comment:9:ticket:11357: 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 ... }}} [Ryan needs] to investigate further to see why this is the case. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11732 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler