Restructure and document 'Hello, Monads!' a bit.

hs/HelloMonads.hs

{-
 - Hello, Monads!
 -
 - Exploring what Monads mean (in Haskell).
 -}
import Prelude hiding (Maybe(..), print, putStr, putStrLn, getLine, getContent)
-- define Maybe here so we can implement our own monad
type InputState = String
type OutputState = String
-- * Model IO as a function of the previous state to a new state and a result.
--
-- If you look closely (i.e. execute `:i IO` in ghci), you'll notice
-- that this is a simplification of the real IO type.
-- Of course it is missing file handles, actual output and other
-- gimmicks, but it gives an idea of what IO does (and why).
data WeirdIO a = WeirdIO ((InputState, OutputState) -> ((InputState, OutputState), a))
executeWeirdIO :: InputState -> WeirdIO a -> ((InputState, OutputState), a)
    return x = WeirdIO $ \state -> (state, x)
takeUntil :: (a -> Bool) -> [a] -> [a]
takeUntil p [] = []
takeUntil p (x:xs) = if (p x) then [x] else x : takeUntil p xs
dropUntil p [] = []
dropUntil p (x:xs) = if (p x) then xs else dropUntil p xs
print x = WeirdIO $ \(i, o) -> ((i, o ++ show x ++ "\n"), ())
print :: Show a => a -> WeirdIO ()
print x = putStrLn $ show x
putStr, putStrLn :: String -> WeirdIO ()
putStr s = WeirdIO $ \(i, o) -> ((i, o ++ s), ())
putStrLn s = putStr $ s ++ "\n"
getChar :: WeirdIO Char
getChar = WeirdIO $ \(c:cs, o) -> ((cs, o), c)
getLine, getContent :: WeirdIO String
getLine = WeirdIO $ \(i, o) -> (((dropUntil (== '\n') i), o), takeWhile (/= '\n') i)
getContent = WeirdIO $ \(i, o) -> (("", o), i)
takeUntil :: (a -> Bool) -> [a] -> [a]
takeUntil p [] = []
takeUntil p (x:xs) = if (p x) then [x] else x : takeUntil p xs
dropUntil p [] = []
dropUntil p (x:xs) = if (p x) then xs else dropUntil p xs