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)

We have attached the sample packages which simulate the exact scenario described above.

Yours truly,
Sreenidhi Nair