#12203: Allow constructors on LHS of (implicit) bidirectional pattern synonym -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: | Keywords: 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 ezyang): Actually, it turns out the example above is not a great one because no type refinement happens with pattern synonym (with good reason!) Here's my actual use case: {{{ newtype D k a = D a data Stream a = Cons a (Stream a) newtype StreamK k a = StreamK { unStreamK :: Stream a } {- -- Pattern synonyms are to give this "virtual interface" data StreamK (k :: Clock) a = Cons a (StreamK (D k a)) newtype Stream a = Stream { unStream :: forall k. StreamK k a } -} unStream :: Stream a -> (forall k. StreamK k a) unStream t = StreamK t pattern Stream :: (forall k. StreamK k a) -> Stream a pattern Stream t <- (unStream -> t) where Stream (StreamK t) = t pattern ConsK :: a -> D k (StreamK k a) -> StreamK k a pattern ConsK x xs <- StreamK (Cons x ((UnsafeD . StreamK) -> xs)) where ConsK x (UnsafeD (StreamK xs)) = StreamK (Cons x xs) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12203#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler