#2893: Implement "Quantified contexts" proposal -------------------------------------+------------------------------------- Reporter: porges | Owner: Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 6.10.1 Resolution: | Keywords: proposal 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): This functionality does my head in so bear with me. In the definition of [https://www.reddit.com/r/haskell/comments/2ag8h2/recovering_preconditions_fr... Fmt], constraints of `(p a, p b, p (a, b))` are assumed for `:&` {{{#!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 }}} `(p a, p b)` often implies `p (a, b)`, would this be captured by `QuantifiedConstraints`? {{{#!hs (:&) :: (p a, p b, forall xx yy. (p xx, p yy) => p (xx, yy)) => Fmt p r a -> Fmt p r b -> Fmt p r (a,b) }}} Does this even give us any additional power? Is this more or less powerful than the `:=>` class from [https://hackage.haskell.org/package/constraints constraints]? {{{#!hs class b :=> h | h -> b where ins :: b :- h instance (Eq a, Eq b) :=> Eq (a, b) where ins = Sub Dict instance (Ord a, Ord b) :=> Ord (a, b) where ins = Sub Dict instance (Show a, Show b) :=> Show (a, b) where ins = Sub Dict instance (Read a, Read b) :=> Read (a, b) where ins = Sub Dict instance (Bounded a, Bounded b) :=> Bounded (a, b) where ins = Sub Dict instance (Monoid a, Monoid b) :=> Monoid (a, b) where ins = Sub Dict ... }}} What if we assume that GHC generates instances of `:=>`? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/2893#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler