#9953: Pattern synonyms don't work with GADTs -------------------------------------+------------------------------------- Reporter: simonpj | Owner: cactus Type: bug | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler (Type | Version: 7.10.1-rc1 checker) | Keywords: Resolution: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by cactus): I have no idea what the proposal is anymore... which I find a bit worrying, given I am supposed to implement it. Right now, on `master` as of `6f818e08`, this is the behaviour you get: {{{ {-# LANGUAGE GADTs, KindSignatures, PatternSynonyms #-} module ShouldCompile where data X b where Y :: [a] -> X (Maybe a) pattern C1 :: [a] -> X (Maybe a) pattern C1 x <- Y x pattern C2 x <- Y x pattern C3 :: (t ~ Maybe a) => [a] -> X t pattern C3 x <- Y x }}} with this setup, you get {{{ λ» :i C1 pattern C1 :: [a] -> X (Maybe a) -- Defined at T9953-3.hs:8:9 λ» :i C2 pattern C2 :: t ~ Maybe a => [a] -> X t -- Defined at T9953-3.hs:10:9 λ» :i C3 pattern C3 :: t ~ Maybe a => [a] -> X t -- Defined at T9953-3.hs:13:9 }}} Isn't this exactly the behaviour specified by Simon's latest comment? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9953#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler