[Haskell-beginners] Trying to grasp Monads - IO and delayed actions
Sylvain Henry
hsyl20 at gmail.com
Sun Apr 6 09:20:24 UTC 2014
[IO b] is a list of functions that perform side-effects. You can remove
elements of the list, add new ones, change the order of the elements, etc.
All of these operations are pure because you do not execute side-effecting
functions: you just manipulate a list. When you use "sequence" on it and
get a "IO [b]", you build a single function that calls every function of
the previous list in sequence (think ";" in imperative programming
languages) and returns their results in a list. As long as you don't call
it, no side-effect occurs.
The IO monad is used to ensure that side-effecting functions are performed
in a deterministic order and are not mixed up with pure code. See my
introduction in [1], maybe it can help you understand the motivation and
the code in GHC.Base where RealWorld is used explicitly.
Cheers
Sylvain
[1]
http://www.sylvain-henry.info/home/data/uploads/talks/shenry-2013-02-05-haskell-intro.pdf(from
slide 20)
2014-04-06 9:41 GMT+02:00 John M. Dlugosz <ngnr63q02 at sneakemail.com>:
> A spiral approach to learning: you understand, then you learn more, and
> then you are more confused than ever.
>
> I recall that a function in the IO Monad just combines actions to make a
> big action list, and doesn't actually do the side-effect-laden "work" until
> it is later triggered, normally because the program is defined to evaluate
> main.
>
> Depending on the time of day and which example I pick, I can sometimes
> follow what's "really happening" in the definition of >>=. But here are
> some specific questions:
>
> In <http://www.haskell.org/haskellwiki/Monads_as_computation>
>
> What is a for-each loop really? It's something which performs some action
>> based on each
>> element of a list. So we might imagine a function with the type:
>>
>> forM :: (Monad m) => [a] -> (a -> m b) -> m [b]
>>
>> (as an added bonus, we'll have it collect the results of each iteration).
>>
>> We can write this with sequence and map:
>>
>> forM xs f = sequence (map f xs)
>>
>> we apply the function to each element of the list to construct the action
>> for that
>> iteration, and then sequence the actions together into a single
>> computation.
>>
>>
> So map by itself produces a [m b]. Why does it need to be turned into m
> [b]? What does the 'sequence' accomplish, other than restructuring the
> results that already exist?
>
> The reason I asked about (#⋯#) is because I wanted to see what IO was
> really doing, to see what the difference was between using >>= initially
> and then somehow "cranking" it later.
>
> <http://hackage.haskell.org/package/base-4.6.0.1/docs/src/
> GHC-Base.html#%3E%3E%3D> lists
>
>> instance Monad IO where
>> {-# INLINE return #-}
>> {-# INLINE (>>) #-}
>> {-# INLINE (>>=) #-}
>> m >> k = m >>= \ _ -> k
>> return = returnIO
>> (>>=) = bindIO
>> fail s = failIO s
>>
>> returnIO :: a -> IO a
>> returnIO x = IO $ \ s -> (# s, x #)
>>
>> bindIO :: IO a -> (a -> IO b) -> IO b
>> bindIO (IO m) k = IO $ \ s -> case m s of (# new_s, a #) -> unIO (k a)
>> new_s
>>
>> thenIO :: IO a -> IO b -> IO b
>> thenIO (IO m) k = IO $ \ s -> case m s of (# new_s, _ #) -> unIO k new_s
>>
>> unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
>> unIO (IO a) = a
>>
>
> where bindIO is the function of interest. In a chain of commands, getLine
> might be the 'k' argument to bindIO. Somewhere there's a real machine
> function called to do the reading from the file, right?
>
>
>
>
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