#10997: Pattern synonym causes Iface error. -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- From the mailing list.. Hello, We have a pattern synonym as follows type family Showable (a :: k) :: Constraint where Showable (a :: *) = (Show a) Showable a = () pattern Just' :: () => (Showable a) => a -> (Maybe a) pattern Just' a <- (extractJust -> (True, a)) where Just' a = Just a When we try to use the pattern in a different package, the error was [1 of 1] Compiling Bar ( Bar.hs, .stack- work/dist/x86_64-linux/Cabal-1.22.4.0/build/Bar.o ) /tmp/test/p2/.stack- work/install/x86_64-linux/lts-3.5/7.10.2/lib/x86_64-linux- ghc-7.10.2/p1-0.1.0.0-I5t4il6dN7vIqsT1XgYsM3/Foo.hi Declaration for Just' Pattern synonym Just': Iface type variable out of scope: k Cannot continue after interface file error The error only occurred when Showable was polykinded and we used synonym in a different package . Using the synonym in the same package works fine. This problem did not happen with the following definition of (non polykinded ) Showable type family Showable a :: Constraint where Showable a = (Show a) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10997 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler