CODEWITHSUNDEEP
haskelliostdoutmonads
Nacht
Nacht

Reputation: 11366

Custom while loop in Haskell prints a statement twice

I'm new to Haskell and have been trying to better understand the IO monad (after playing with pure functions for a while).

I'm following a tutorial on the IO monad

One of the exercises is making a while function. They don't show an example so you can't check your answer.

Here is mine: 

while :: IO Bool -> IO ()
while action = do p <- action
                  if p then putStrLn "You win!" >> return ()
                       else putStrLn "Nope. Try again!" >> while action


main = do putStrLn "Come and guess the letter!"
          while (constAskForC)
          where constAskForC = do c <- getChar
                                  return $ c == 'c'

Now, my issue is that if you enter a wrong character (pretty much a character that is not 'c'), then string "Nope. Try again!" gets printed twice to StdOut. Why is this? Here's the program running:

Come and guess the letter!


"Nope. Try again!"

"Nope. Try again!"
d
"Nope. Try again!"
"Nope. Try again!"

"Nope. Try again!"

"Nope. Try again!"
a
"Nope. Try again!"
"Nope. Try again!"
d
"Nope. Try again!"
"Nope. Try again!"
f
"Nope. Try again!"
"Nope. Try again!"
a
"Nope. Try again!"
"Nope. Try again!"
s
"Nope. Try again!"
"Nope. Try again!"

If you just hit enter (enter no character) then it only gets printed once. Can anyone explain to me what am I doing wrong?

Thanks.

Upvotes: 1

Views: 727

Answers (3)

kosmikus
kosmikus

Reputation: 19637

The problem here is an interaction of the behaviour of the getChar command with the normal buffering behaviour of a compiled program, which is to use line buffering.

The getChar command consumes only a single character. In particular, hitting return creates a newline character on its own.

However, with line buffering, no input will actually arrive before you enter a complete line. So if you interleave a character and the Return key, two characters will be generated in one go, leading to the strange output.

You can fix this by adding the line

import System.IO

in the beginning and then adding the statement

hSetBuffering stdin NoBuffering

to the main program. Alternatively, use GHCi which uses NoBuffering by default.

Upvotes: 4

bennofs
bennofs

Reputation: 11963

The problem is in the constAskForC function. You use getChar, but that will only read one character. So you read c, and after you read c, you will get the end-of-line character (\n). There is really no way to get single characters, but you can get a whole line and only take the first character:

main = do putStrLn "Come and guess the letter!"
          while (constAskForC)
  where constAskForC = do c <- getLine
                          return $ case c of
                            [] ->     False -- There was no input              
                            'c':[] -> True -- The first letter was a c, and nothing else was entered
                            _      -> False -- Otherwise, the result is False

Another minor note on your code: putStrLn "You win!" >> return () is the same as putStrLn "You win!"

Upvotes: 4

shang
shang

Reputation: 24802

When you type a character and press enter that's actually two characters (the printable character plus a linefeed character). You probably want to use getLine instead.

Upvotes: 2

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