kindsig.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE GADTs #-}

data Req = A | B

data ReqData :: Req -> * where
    ReqDataA :: ReqData A
    ReqDataB :: ReqData B

data Resp = Ra | Rb | Rc

data RespData :: Resp -> * where
    RespDataA :: RespData Ra
    RespDataB :: RespData Rb
    RespDataC :: RespData Rc

sendA :: ReqData A -> RespData Ra
sendA = undefined

unSendA :: RespData Ra -> ReqData A
unSendA = undefined

main :: IO ()
main = do
  let a = (unSendA (sendA ReqDataA)) :: ReqData A
  putStrLn "hello"