#14968: QuantifiedConstraints: Can't be RHS of type family instances -------------------------------------+------------------------------------- Reporter: josef | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #9269 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): The challenge is knowing whether a type `(F t)` might reduce to a polytype. Currently that cannot happen. Allowing it to happen gives all the same problems as impredicativity in general. So I think that allowing a forall on the RHS of a `type instance` is a big step. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14968#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler