Re: [GHC] #10561: "deriving (Functor)" on a polykinded type produces ill-kinded instance

#10561: "deriving (Functor)" on a polykinded type produces ill-kinded instance -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: closed Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.11 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | deriving/should_compile/T10561 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"aadde2b90817c577336da0d4a10ea47551d60c7e/ghc" aadde2b/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="aadde2b90817c577336da0d4a10ea47551d60c7e" Deriving Functor-like classes should unify kind variables While the deriving machinery always unifies the kind of the typeclass argument with the kind of the datatype, this proves not to be sufficient to produce well kinded instances for some poly-kinded datatypes. For example: ``` newtype Compose (f :: k2 -> *) (g :: k1 -> k2) (a :: k1) = Compose (f (g a)) deriving Functor ``` would fail because only `k1` would get unified with `*`, causing the following ill kinded instance to be generated: ``` instance (Functor (f :: k2 -> *), Functor (g :: * -> k2)) => Functor (Compose f g) where ... ``` To prevent this, we need to take the subtypes and unify their kinds with `* -> *`. Fixes #10524 for good. Test Plan: ./validate Reviewers: simonpj, hvr, austin, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2097 GHC Trac Issues: #10524, #10561 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10561#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler