Bulat Ziganshin wrote:
Hello Wolfgang,
Saturday, February 11, 2006, 3:17:12 PM, you wrote:
each and every monadic operation is a function!
WJ> What do you mean with "monadic operatation"? (>>=), (>>) and return are, of WJ> course, functions but an I/O action like getChar is *not* a function. Also a WJ> list is not a function but a value of the list monad.
type "IO a" is really "RealWorld -> (RealWorld,a)"
WJ> This representation is just there to help people understand what I/O is but WJ> actually, IO a is a type which is not implementable in ordinary Haskell and WJ> therefore cannot be a function. In addition, RealWorld -> (RealWorld,a) as WJ> an explanation of what IO a is has its limitations. If we run an I/O action, WJ> we aren't just interested in the final state but also in intermediate states.
{putStr "a"} is a function, which receives previous world state and returns updated world state where "a" is written to the terminal. it's an _essential_ part of monadic way to I/O
There is nothing in the Haskell specification that tells you how the IO type is implemented, so you can't say that putStr takes an old world state and returns a new one. I've personally done an implementation that was totally different, so making assumptions about how IO is implemented is just wrong. -- Lennart