#11357: Regression when deriving Generic1 on poly-kinded data family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler | Version: 7.11 (CodeGen) | Keywords: Generics, Resolution: | TypeInType Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Something fishy is going on with the kind of the `ProxyFam` instance `TyCon`. I added `pprTrace`-ed the result of `tyConKind tc` right before the call to `filterOutInvisibleTypes` mentioned above, and noticed a discrepancy between datatypes and data families: {{{ $ inplace/bin/ghc-stage2 --interactive GHCi, version 8.1.20160109: http://www.haskell.org/ghc/ :? for help λ> :set -XDeriveGeneric -XTypeFamilies -XPolyKinds -fprint-explicit- foralls λ> import GHC.Generics λ> data Proxy (a :: k) = Proxy deriving Generic1 tc: Proxy tyConKind tc: forall k_a1Gj. k_a1Gj -> * tc_args: [*] filterOutInvisibleTypes tc tc_args: [] all isTyVarTy: True λ> data family ProxyFam (a :: k) λ> data instance ProxyFam (a :: k) = ProxyCon deriving Generic1 tc: R:ProxyFamka tyConKind tc: forall k_a1Ps -> k_a1Ps -> * tc_args: [*] filterOutInvisibleTypes tc tc_args: [*] all isTyVarTy: False <interactive>:5:53: error: • Can't make a derived instance of ‘Generic1 ProxyFam’: ProxyFam must not be instantiated; try deriving `ProxyFam k a' instead • In the data instance declaration for ‘ProxyFam’ }}} Notice that `tyConKind` for the datatype `Proxy` is `forall k. k -> *` (i.e., `k` is specified but not visible), but the `tyConKind` for the `ProxyFam` instance is `forall k -> k -> *` (`k` is visible)! This is definitely the reason why this bug is manifesting itself, although I'm not sure why `k` is a visible type argument in the `ProxyFam` instance. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11357#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler