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