#14579: GeneralizedNewtypeDeriving produces ambiguously-kinded code -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: GHC rejects | Test Case: valid program | deriving/should_compile/T14579 Blocked By: 12045 | Blocking: Related Tickets: | Differential Rev(s): Phab:D4264 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: closed => new * resolution: fixed => * blockedby: => 12045 * milestone: 8.4.1 => 8.8.1 Comment: I wonder if the hack we implemented to fix this issue originally could be made substantially simpler with visible kind applications (which should [https://phabricator.haskell.org/D5229 be available soon-ish]). With VKAs, we could instead generate the following code: {{{#!hs {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE TypeInType #-} module Bug where import Data.Kind import Data.Proxy newtype Wat (x :: Proxy (a :: Type)) = MkWat (Maybe a) deriving Eq newtype Glurp a = MkGlurp (Wat ('Proxy :: Proxy a)) instance Eq a => Eq (Glurp a) where (==) = coerce @(Wat ('Proxy @a) -> Wat ('Proxy @a) -> Bool) @(Glurp a -> Glurp a -> Bool) (==) }}} I'll reopen this ticket to keep track of this idea. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14579#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler