#8565: New GeneralisedNewtypeDeriving needs help with higher rank types -------------------------------------------------+------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: typecheck/should_compile/T8565 | Unknown Blocking: | Blocked By: | Related Tickets: -------------------------------------------------+------------------------- Comment (by nomeata): Unfortunately, it seems that no such instances, with `forall` on the left, existed so far, so writing it by hand yields `Malformed instance head`. I tried to trick GHC to accept a forall on the left hand side of the instance heads (using `ConstraintTypes` and `KindSignatures`) {{{ class C a where op :: a type Foo (a:: * -> Constraint) = forall b. a b instance Foo C => C Int }}} but that only yielded {{{ Malformed predicate ‛Foo C’ In the context: (Foo C) While checking an instance declaration In the instance declaration for ‛C Int’ }}} Do we really want instances for `Coercible` where the instance head is not valid GHC code? (I don’t even dare to call it Haskell code ;-)) Or can the instance be somehow phrased differently? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8565#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler