Arrow Notation / GADT panic (GHC Trac #5777)

[Ben Moseley]

The following code seems to trigger a panic (under 7.03, 7.2 and 7.4):

{-# LANGUAGE Arrows, GADTs #-}
import Control.Arrow

data Value a where BoolVal :: Value Bool

class ArrowInit f where
    arrif :: f b -> ()

instance ArrowInit Value where
    arrif = proc BoolVal -> returnA -< () -- this panics
    -- arrif = arr (\BoolVal -> ()) -- this works


I've filed this as: http://hackage.haskell.org/trac/ghc/ticket/5777

--Ben