#12652: Type checker no longer accepting code using function composition and rank-n types -------------------------------------+------------------------------------- Reporter: ezyang | Owner: simonpj Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler (Type | Version: 8.1 checker) | 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: | -------------------------------------+------------------------------------- Changes (by bgamari): * owner: => simonpj @@ -4,1 +4,1 @@ - {{{ + {{{#!hs New description: The following program (reduced from Cabal code that uses HasCallStack) typechecks in GHC 8.0, but not on HEAD: {{{#!hs {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ImplicitParams #-} module Foo where type T a = (?x :: Int) => a type M a = T (IO a) f :: T (T a -> a) f x = x g :: Int -> M () g = undefined h :: Int -> M () -- h x = f (g x) -- works on HEAD h = f . g -- fails on HEAD, works on GHC 8.0 }}} It's possible that this is just fall out from the recent impredicativity changes but I just wanted to make sure that this was on purpose. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12652#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler