#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 goldfire): The challenge impredicativity causes is in knowing where there is an invisible parameter. (I'm pretty sure that's the challenge, at least.) So if `x :: ty` and we don't know whether or not `ty` is a `forall`-type, we have a problem. This simply doesn't happen with constraints, so we avoid the challenge. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14968#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler