#14734: QuantifiedConstraints conflated with impredicative polymorphism? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: invalid | Keywords: | QuantifiedConstraints 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): The difficulty with impredicativity is that, during inference, a unification variable may be unified with a polytype. Thus {{{ f :: forall a. [a] -> [a] xs :: [forall b. b->b] ...(f xs).. }}} Here f really has a type argument, and that type is polymorphic. We really have {{{ ...(f @(forall b b->b) xs)... }}} The hard bit is infeering the implicit, invisible type arguments. But here we are the level of types. With explicit arguments there should be nothing wrong with allowing polytypes. (It's a different matter for the implicit kind arguments, of course.) So... may it'd be OK. I'm just not sure what a principled story is. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14734#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler