http://myuon-myon.hatenablog.com/entry/2013/06/09/135407

operational.hs

{-# LANGUAGE GADTs #-}
import Control.Monad
import Control.Monad.Operational.Simple

data Simple a where
  End :: Simple a  
  Put :: String -> Simple ()
  Get :: Int -> Simple String

put :: String -> Program Simple ()
put = singleton . Put

end :: Program Simple a
end = singleton End

get :: Int -> Program Simple String
get = singleton . Get

run :: Program Simple a -> IO a
run = interpret step
  where
    step :: Simple x -> IO x
    step (Put a) = putStrLn a
    step End = putStrLn "End" >> undefined
    step (Get n) = replicateM n getChar

quiz :: Program Simple String
quiz = do
  put "Q.This program is written in _______.(7 letters)"
  ch <- get 7
  put $ "Your answer:" ++ ch
  if ch == "Haskell"
    then put "Congratulations!"
    else put "Wooooops!! You may have misspelled, haha."
  end
  
main = run quiz

output.txt

Q.This program is written in _______.(7 letters)
conglalalla
Your answer:conglal
Wooooops!! You may have misspelled, haha.
End
Operational.hs: Prelude.undefined


Q.This program is written in _______.(7 letters)
Haskell
Your answer:Haskell
Congratulations!
End
Operational.hs: Prelude.undefined