#13901: Lift the "Illegal polymorphic type" restriction on type families -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11962 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): This isn't so easy. With type families, there are potentially many right- hand sides. Some may have polymorphism, and some may not. And we might not know which is chosen until we get to the constraint solver. When the constraint-generation algorithm looks at something with type `Foo`, does it add a wrapper for instantiating `forall a. Maybe a`? Or not? Impossible to know. This all smells a bit like impredicativity. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13901#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler