#13992: Error message, room for improvement (polykinds) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: PolyKinds Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: Old description:

Something like

{{{#!hs -- • No instance for (Show (Compose Proxy Proxy a)) -- arising from the 'deriving' clause of a data type declaration -- Possible fix: -- use a standalone 'deriving instance' declaration, -- so you can specify the instance context yourself -- • When deriving the instance for (Show (FlipProxy a))

{-# Language DerivingStrategies, GeneralizedNewtypeDeriving, PolyKinds, KindSignatures #-}

import Data.Functor.Compose import Data.Proxy import Data.Kind

newtype FlipProxy a = FlipProxy_ (Compose Proxy Proxy a) deriving newtype Show }}}

where the solution is to constraint the kind of `a :: Type`, it would be nice if GHC could reference kind variables.

New description: Something like {{{#!hs -- • No instance for (Show (Compose Proxy Proxy a)) -- arising from the 'deriving' clause of a data type declaration -- Possible fix: -- use a standalone 'deriving instance' declaration, -- so you can specify the instance context yourself -- • When deriving the instance for (Show (FlipProxy a)) {-# Language DerivingStrategies, GeneralizedNewtypeDeriving, PolyKinds, KindSignatures #-} import Data.Functor.Compose import Data.Proxy import Data.Kind newtype FlipProxy a = FlipProxy_ (Compose Proxy Proxy a) deriving newtype Show }}} where the solution is to constrain the kind of `a` to `Type`, it would be nice if GHC could reference kind variables. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13992#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler