Loop with StateT: Why this loop doesn't loop

Question

I don't understand why this code does loop only once then exit ? in Ghci I can answer only to the first loop then it seems the variable cont is set to false and I don't have the prompt to answer.

The result is:

*Main> testLoop1 td10
test
Do you want to continue? (y/N)
y
we continue
test
Do you want to continue? (y/N)
We stop

---

code:

type TDeckSTIO    = StateT TableDecks IO

continue = do
putStrLn "Do you want to continue? (y/N)"
c <- getChar
return $ c == 'y'


loop1 :: TDeckSTIO () 
loop1 = do 
    liftIO $ putStrLn "test"
    cont<- liftIO continue 
    if cont
    then do 
        liftIO $ putStrLn "we continue"
        liftIO $ testLoop1 td

    else liftIO $ putStrLn "We stop"

testLoop1 td =  runStateT (loop1 ) td   >> return ()

Answer 1

The problem is that when you type y and hit enter, that's actually typing two characters: 'y' itself, and the newline character that gets sent by pressing the return key. The first time round, the loop sees the 'y', but the next time round, it sees the '\n', and since '\n' isn't 'y', it exits.

You can either do hSetBuffering stdin NoBuffering before you enter your loop (you'll need to import System.IO), which will let you process characters without waiting for a newline, or specifically process lines at a time:

continue = do
  putStrLn "Do you want to continue? (y/N)"
  s <- getLine
  return $ s == "y"

By the way, instead of writing liftIO $ testLoop1 td, you can just stay in the same state monad: you can replace it with loop1 and it'll work exactly the same.

Also, testLoop1 is better written as:

testLoop1 = evalStateT loop1

evalStateT is like runStateT, but doesn't include the final state, so you don't have to explicitly discard the value with >> return ().