All right, after a bit of dinner and some time to mess about, here's another attempt to check my understanding: here is a simplified version of the lisp-time example:
module Main where import System.Time
pure_fn :: Integer -> String pure_fn n = calendarTimeToString (toUTCTime (TOD n 0))
wicked_fn :: IO String wicked_fn = getClockTime >>= return . pure_fn . toI where toI (TOD n _) = n
make_wicked :: String -> IO String make_wicked str = return str
-- use of pure_fn -- main = putStrLn (pure_fn 1230000000)
-- use of wicked_fn -- main = wicked_fn >>= putStrLn
-- use of make_wicked main = (make_wicked (pure_fn 1234567890)) >>= putStrLn
If I use the first of the three "main" alternatives, I'm calling a pure function directly: it takes an integer, 123..., and produces a string. If I pass the same integer to the pure function, I'll get the same value, every time. This string is passed to putStrLn, an IO action, in order that I may gaze upon it, but the string itself is not thereby stuck in the IO monad. If I use the second of the three "main" alternatives, I'm calling an IO action: wicked_fn, which returns the current time formatted as UTC. In principle, every time I call wicked_fn, I could get a different answer. Because it's an IO action, I can't just pass it to putStrLn in the same way I passed in the previous pure_fn value, but instead I have to use the bind operator >>=. If I use the third of the "main" alternatives, I am starting with a pure function: it's that number formatted as UTC (it happens to come to Fri Feb 13 of next year), but then I pass it through the make_wicked function, which transmogrifies it into the IO monad. Therefore, as in the above, I have to use >>= in order to get it to work; "putStrLn (make_wicked (pure_fn 123...))" doesn't work. <deep breath> OK, after all that, my original question, in terms of this example: "the IO monad is one-way" is equivalent to saying there is no haskell function that I could write that would take
(make_wicked (pure_fn 123456))
and make it into something that could be used in the same way and the same places as just plain
(pure_fn 123456)
? And, coming back to my scheme interpreter, this is at least somewhat irrelevant, because, since I am in a REPL of my own devising, I'm firmly in IO-monad-land, now and forever. Right? thanks, Uwe