#16140: Cannot create type synonym for quantified constraint without ImpredicativeTypes -------------------------------------+------------------------------------- Reporter: Ashley | Owner: (none) Yakeley | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 Keywords: | 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: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE KindSignatures, RankNTypes, ConstraintKinds, QuantifiedConstraints, FlexibleInstances, UndecidableInstances #-} module Bug where type F1 (f :: * -> *) = forall a. Eq (f a) class (Functor f, F1 f) => C f instance (Functor f, F1 f) => C f type F2 f = (Functor f, F1 f) }}} {{{ Bug.hs:7:1: error: • Illegal polymorphic type: F1 f GHC doesn't yet support impredicative polymorphism • In the type synonym declaration for ‘F2’ | 7 | type F2 f = (Functor f, F1 f) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} (GHC accepts the program with ImpredicativeTypes.) `(Functor f, F1 f)` is allowed as a superclass constraint, and as an instance constraint, but a type synonym cannot be made for it. Not sure if this really counts as a bug ("just switch on ImpredicativeTypes"), but I think it's worth discussing. I prefer to keep ImpredicativeTypes switched off, but if something can be a constraint, shouldn't I be able to create a type synonym of it? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16140 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler