#5927: A type-level "implies" constraint on Constraints -------------------------------------+------------------------------------- Reporter: illissius | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): Would this `Show` instance for [https://www.reddit.com/r/haskell/comments/2ag8h2/recovering_preconditions_fr... Fmt] work with `ImplicationConstraints` (`(=>) :: Constraints -> Constraints -> Constraints`): {{{#!hs data Fmt :: (* -> Constraint) -> (* -> *) -> * -> * where Int :: p Int => Fmt p r Int (:&) :: (p a, p b, p (a,b)) => Fmt p r a -> Fmt p r b -> Fmt p r (a,b) Lift :: p (r a) => r a -> Fmt p r a instance (p => Show) => (Show (Fmt ctx r a) where show t = case t of Int -> "Int" l :& r -> show l ++ "&" ++ show r Lift r -> show r }}} A question on `QuantifiedConstraints` on this same example in ticket:2893#comment18. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5927#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler