#8346: Rank 1 type signature still requires RankNTypes ----------------------------------------------+---------------------------- Reporter: tinctorius | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects valid program | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: #2605 ----------------------------------------------+---------------------------- Comment (by tinctorius): Now, about a fix for this. It can be done quickly, and it can be done elegantly. * The quick fix: when you see `A -> (forall B. C)`, think `forall B. (A -> C)`. This specific bug should vanish. However, the problem is bigger: {{{ {-# LANGUAGE ExplicitForAll #-} newtype Wrap a r = Wrap (a -> r) tuple :: forall a. Wrap a (forall b. b -> (a, b)) tuple = Wrap $ \x y -> (x, y) }}} * The elegant fix: when you see `C T1 T2 .. (forall X. TK .. TN)`, and `C` is covariant in its `K`-th parameter, think `forall X. C T1 T2 .. TK .. TN`. This is of course a lot harder. Perhaps the new roles feature can be leveraged to carry this information around. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8346#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler