#14579: GeneralizedNewtypeDeriving produces ambiguously-kinded code -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: deriving Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4264 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): What change to `typeToHsType` do we want? The problem is to specify the invisible kind arguments to occurrences of poly-kinded type-constructor applications. Eg {{{ instance Eq a => Eq (Glurp a) where (==) = coerce @(Wat 'Proxy -> Wat 'Proxy -> Bool) @(Glurp a -> Glurp a -> Bool) (==) }}} The problem is the invisible kind arguments to the poly-kinded `'Proxy` occurrences. If we had visible kind application we could use that, but we don't. Solution: add a kind signature, as in the Description: {{{ instance Eq a => Eq (Glurp a) where (==) = coerce @(Wat ('Proxy :: Proxy a) -> Wat ('Proxy :: Proxy a) -> Bool) @(Glurp a -> Glurp a -> Bool) (==) }}} When do we need a kind signature? * (a) When we have an application of a type constructor with invisible arguments * (b) And the invisible arguments are not fixed by the kinds of the visible arguments. Probably just using (a) is enough to reduce the noise of redundant kind signatures to reasonable levels. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14579#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler