#12563: Bad error message around lack of impredicativity -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): This appears to be fixed, as of GHC 8.2.2: {{{ $ /opt/ghc/8.2.2/bin/ghci Bug.hs -XRankNTypes GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Bug.hs:4:15: error: • Cannot instantiate unification variable ‘p0’ with a type involving foralls: (forall a. f0 a) -> f0 r0 GHC doesn't yet support impredicative polymorphism • In the first argument of ‘foo’, namely ‘g’ In the expression: foo g In the expression: \ g -> foo g • Relevant bindings include g :: p0 (bound at Bug.hs:4:6) x :: p0 -> f0 r0 (bound at Bug.hs:4:1) | 4 | x = \g -> foo g | ^ }}} I'll add a regression test. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12563#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler