Hi Devs!

I encountered a curious restriction with type signatures (tyvar bindings) in GADT pattern matches.

GHC won't let me directly capture the refined type structure of GADT constructors like this:


{-# Language GADTs, ScopedTypeVariables #-}

data Foo a where
  Bar :: Foo [a]

foo :: Foo a -> ()
foo b@(Bar :: Foo [a]) = quux b
  where quux :: Foo [b] -> ()
        quux Bar = ()


I get:


test.hs:7:8: error:

    • Couldn't match type ‘a1’ with ‘[a]’

      ‘a1’ is a rigid type variable bound by

        the type signature for:

          foo :: forall a1. Foo a1 -> ()

        at test.hs:6:1-18

      Expected type: Foo a1

        Actual type: Foo [a]



To me it appears that the type refinement established by the GADT pattern match is not accounted for.

Of course I can write:

foo :: Foo a -> ()
foo b@Bar | (c :: Foo [a]) <- b = quux c
  where quux :: Foo [b] -> ()
        quux Bar = ()

but it feels like a complicated way to do it...

My question is whether this is generally seen as the way to go or whether ScopedTypeVariables coming from a GADT pattern match should be able to capture the refined type. To me the latter seems more useful.

Just wanted to feel the waters before writing a ticket about this.

Cheers and thanks,

    Gabor