#12652: Type checker no longer accepting code using function composition and rank-n types -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following program (reduced from Cabal code that uses HasCallStack) typechecks in GHC 8.0, but not on HEAD: {{{ {-# 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler