#13752: Odd pattern synonym type errors -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1-rc2 (Type checker) | Keywords: | Operating System: Unknown/Multiple PatternSynonyms | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Suppose we have {{{#!hs newtype Arrange = Arrange {getArrange :: [Int] -> [Int]} }}} If I write {{{#!hs pattern Heh :: (c ~ ((~) Int)) => (forall a. c a => [a] -> [a]) -> Arrange pattern Heh f <- Arrange f }}} I get {{{ • Couldn't match type ‘[Int] -> [Int]’ with ‘forall a. Int ~ a => [a] -> [a]’ Expected type: forall a. c a => [a] -> [a] Actual type: [Int] -> [Int] • In the declaration for pattern synonym ‘Heh’ }}} It surprises me that these don't match. I can hack around that a bit: {{{#!hs newtype Help = Help {getHelp :: forall a. Int ~ a => [a] -> [a]} pattern Heh :: (c ~ ((~) Int)) => (forall a. c a => [a] -> [a]) -> Arrange pattern Heh f <- ((\x -> Help (getArrange x)) -> Help f) }}} Now this compiles, and I can use it in a variety of ways, but not quite all. If I write {{{#!hs pattern Heh' :: ([Int] -> [Int]) -> Arrange pattern Heh' f <- Heh f }}} I get {{{ Haha.hs:78:23: error: • Couldn't match expected type ‘[Int] -> [Int]’ with actual type ‘forall a. Int ~ a => [a] -> [a]’ • In the declaration for pattern synonym ‘Heh'’ | 78 | pattern Heh' f <- Heh f | ^ }}} This strikes me as perhaps even more surprising. I can hack around that too, of course, but yuck! {{{#!hs pattern Heh' :: ([Int] -> [Int]) -> Arrange pattern Heh' f <- ((\(Heh g) -> g @ Int) -> f) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13752 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler