[Haskell-cafe] background question about IO monad

[Jonathan Cast]

On 6 Feb 2008, at 11:32 PM, Uwe Hollerbach wrote:

> 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.

jcc