#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 RyanGlScott): Iceland_jack, there are two problems with using typeclasses like these to "solve" this problem: 1. They're based on explicit `Dict` values. As a result, using them requires lots of pattern matching and type variable scoping hullabaloo, which gets annoying pretty quickly. 2. What does it mean to automatically generate instances for `(:=>)`? Do we only generate instances that correspond to actual instance contexts? If so, then the following would typecheck, since there is an `instance () :=> Eq ()`: {{{#!hs ins :: () :- Eq () }}} But the following would //not// typecheck! {{{#!hs ins :: Eq () :- Eq () }}} Because there is no `instance Eq () => Eq ()` anywhere (and thus no `instance Eq () :=> Eq ()`), only `instance Eq ()`. But the implication `Eq () => Eq ()` should obviously hold. Does that mean we should also generate an `instance Eq () :=> Eq ()` automatically? But what about `(Eq (), Ord ()) => Eq ()`? `(Eq (), Ord (), Show ()) => Eq ()`? There is an extremely high number of contexts in which `Eq ()` holds, and generating every possible one of them would result in a combinatorial explosion. While those classes are neat tricks that can get you some of the functionality of `QuantifiedConstraints`/`ImplicationConstraints`, they don't go all the way. I really feel that some compiler smarts would make these features manageable. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/2893#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler