{-# LANGUAGE PatternSynonyms #-}
data Free  f a
  = Pure a
  | Free (f (Free f a))
  
data ListF a b = Nil | Cons a b
  deriving (Show)

type Way a b = Free (ListF a) b

pattern End end     = Pure end
pattern Path path r = Free (Cons path r)

showWay :: (Show a, Show b) => Way a b -> String
showWay (End end) = show end
showWay (Path path (End end)) = show path ++ " :| " ++ show end
showWay (Path path r) = show path ++ " :> " ++ showWay r

main :: IO ()
main = do
  putStrLn $ showWay $ Path "foo" $ Path "bar" $ End 0