#14733: Won't use (forall xx. f xx) with -XQuantifiedConstraints -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: | QuantifiedContexts wipT2893 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 simonpj): Currently this is by-design, but we could change the design. These forall-constraints currently behave just like a local version of a top-level instance declaration, and those are all for classes. You can't say {{{ instance ... => f Int }}} although it'd make sense to do so. So similarly you currently can't do that with the local-foralld constraints. * If we retain the restriction to class constraints only, we should reject the type signature with a civilised error message. * How bad is the restriction? You can always say {{{ class f a => C f a proof :: (forall xx. C f xx) => D (f a) }}} Would that do? Or what do your use-cases look like? It'd be a moderate pain to generalise the facility, mainly because `InstEnv` (in which we look up instances) has class constraints as a deeply-baked-in assumption. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14733#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler