#13735: RankNTypes don't work with PatternSynonyms -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: invalid | Keywords: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by dfeuer): Your `get` is partial, so matching on `LLAM` will throw an exception (instead of failing) when it doesn't match. It's possible to accomplish your purpose thus: {{{#!hs data Yeah = Yeah (forall a. a -> PLambda a) | Nah getYeah :: Lam -> Yeah getYeah l@(L (Lam _)) = Yeah (case l of L (Lam f) -> f; _ -> error "impossible") getYeah _ = Nah pattern LLam :: (forall a. a -> PLambda a) -> Lam pattern LLam x <- (getYeah -> Yeah x) }}} It's pretty ugly, though. You have to first pattern match to determine that the constructor is `Lam`, then pattern match on the same value again under the `Yeah` constructor to capture the polymorphism. If there's a way to avoid that, I don't see it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13735#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler