io_interpreter.hs

data IOOp
  = EndProgram
  | ReadLine (String -> IOOp)
  | WriteLine String IOOp

runOp :: IOOp -> IO ()
runOp EndProgram = return ()
runOp (ReadLine f) = do
  line <- getLine
  let nextOp = f line
  runOp nextOp
runOp (WriteLine line nextOp) = do
  putStrLn line
  runOp nextOp

main :: IO ()
main = 
  if checkProgram testProgram
    then do
         putStrLn "test passed"
         runOp testProgram
    else putStrLn "test failed"

testProgram :: IOOp
testProgram =
  WriteLine
  "enter a line>"
  $ ReadLine (\l ->
               WriteLine l $
               WriteLine l $
               EndProgram)

checkProgram :: IOOp -> Bool
checkProgram p = case p of
  (WriteLine "enter a line>" x) -> case x of
    (ReadLine f) -> case f "test string" of
      (WriteLine "test string" y) -> case y of
        (WriteLine "test string" EndProgram) -> True
        _ -> False
      _ -> False
    _ -> False
  _ -> False