#14860: QuantifiedConstraints: Can't quantify constraint involving type family -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints wipT2893 Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): A skolem type variable is just like a type constructor. It is equal to itself, nothing else. A type function application stands for what it reduces to. It's not a rigid thing you can match against when matching in instances or type- family definitions. We don't allow {{{ type instance F (G a) = [a] }}} Why not? Because the call `F (G Int)` would match the type instance -- but could also reduce to something that wouldn't match it any more, which would be terrible. In contrast, skolem type variables have no such difficulty. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14860#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler