#7019: Wrong error message when using forall in constraints -------------------------------------+------------------------------------- Reporter: sjoerd_visscher | Owner: simonpj Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.4.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | typecheck/should_fail/T7019, T7019a Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): This compiles using [https://hackage.haskell.org/package/constraints-0.8/docs/Data-Constraint- Forall.html Data.Constraint.Forall] for the record: {{{#!hs import Data.Constraint import Data.Constraint.Forall newtype Free c a = Free { runFree :: forall r. c r => (a -> r) -> r } deriving Functor class c (Free c a) => C c a instance c (Free c a) => C c a instance Applicative (Free c) where pure :: a -> Free c a pure a = Free ($ a) (<*>) :: Free c (a -> b) -> (Free c a -> Free c b) Free f <*> Free g = Free (\bfr -> f (\ab -> g (bfr . ab))) instance Forall (C c) => Monad (Free c) where return :: a -> Free c a return a = Free ($ a) (>>=) :: forall a b. Free c a -> (a -> Free c b) -> Free c b Free f >>= g = f g \\ inst @(C c) @b }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7019#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler