No. As you say the world1 value is immutable, but that's not contradictory. If between 'getLine' and 'print' something was done by a concurrent thread, then that change to the world is captured by 'print'. Well, that's technically true, but it basically amounts to saying that the 'model' of IO is itself. If I see 'print x', I don't really think of an action that performs arbitrary side effects and prints x in the process (unless some error prevents even that). Some notion of compositionality is required, which would allow me to state how 'print x' contributes to the changes of the world in a way that doesn't depend on any context. Actions in the state monad are composable in that sense, while the same doesn't apply to the IO monad when explained in terms of state passing.
Gergely -- http://www.fastmail.fm - The professional email service
"Patai Gergely"
No. As you say the world1 value is immutable, but that's not contradictory. If between 'getLine' and 'print' something was done by a concurrent thread, then that change to the world is captured by 'print'.
Well, that's technically true, but it basically amounts to saying that the 'model' of IO is itself. If I see 'print x', I don't really think of an action that performs arbitrary side effects and prints x in the process (unless some error prevents even that). Some notion of compositionality is required, which would allow me to state how 'print x' contributes to the changes of the world in a way that doesn't depend on any context. Actions in the state monad are composable in that sense, while the same doesn't apply to the IO monad when explained in terms of state passing.
Why not? I don't see any problems here. Note that the mental model of IO I presented is not explicit state passing, but a state /monad/. I'm proposing the hypothetical existence of a function like this: advanceEverything :: Universe -> Universe such that: print x = \world0 -> doTheActualPrinting x (advanceEverything world0) After all 'print' is not constrained to the effect of printing something. All sorts of things can happen while printing, including even network communication, when stdout is not a terminal. In IO everything can happen everywhere. The advanceEverything function could just as well be used by (>>=) instead of the individual IO computations. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/
participants (2)
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Ertugrul Soeylemez -
Patai Gergely