#9420: Impredicative type instantiation without -XImpredicativeTypes -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.9 checker) | Keywords: Resolution: fixed | ImpredicativeTypes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #11319 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => fixed * related: => #11319 Comment: This has been fixed since GHC 8.0.1, since GHC now does properly detect the impredicative instantiation: {{{ GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug2.hs, interpreted ) Bug2.hs:14:13: error: • Cannot instantiate unification variable ‘a0’ with a type involving foralls: (forall a. a -> a) -> b0 GHC doesn't yet support impredicative polymorphism • In the second argument of ‘(.)’, namely ‘rank2’ In the expression: foo . rank2 In the expression: foo . rank2 $ quux | 14 | bar = foo . rank2 $ quux | ^^^^^ Bug2.hs:14:21: error: • Cannot instantiate unification variable ‘a0’ with a type involving foralls: (forall a. a -> a) -> Int GHC doesn't yet support impredicative polymorphism • In the second argument of ‘($)’, namely ‘quux’ In the expression: foo . rank2 $ quux In an equation for ‘bar’: bar = foo . rank2 $ quux | 14 | bar = foo . rank2 $ quux | ^^^^ }}} (See also #11319 for the plan to allow such impredicative instantiations via `TypeApplications`.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9420#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler