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Today's Topics:
1. Re: Count with Writer asynchronously (Michael Snoyman)
2. Re: Count with Writer asynchronously (Baa)
----------------------------------------------------------------------
Message: 1
Date: Tue, 25 Jul 2017 13:41:35 +0300
From: Michael Snoyman <mich...@snoyman.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Count with Writer asynchronously
Message-ID:
<CAKT9ecNwHUTL=qwn2dknrrgemqdqtxl7is-6hx-cocq7x3s...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
You're asking very good questions, about a topic I considered bringing up
in my previous response but didn't. I guess I will now :)
The important thing to realize about monad transformers (and monads in
general) is that _they're nothing special_. Under the surface, WriterT is
simply:
* Modifying your functions to have an extra value returned
* Providing convenience functions like `tell` and `liftIO` to work with
that extra value
I would strongly advise getting used to writing code in both the
transformer and non-transformer style to convince yourself that they are
the same thing. To make this concrete, look at the WriterT definition:
newtype WriterT w m a = WriterT { runWriterT :: m (a, w) }
If you take away the newtype wrapping business, you can see that it's just
saying `WriterT w m a = m (a, w)`. And therefore, if we go back to your
types, we can replace:
WriterT (Sum a) IO ()
with
IO ((), Sum a)
In fact, that's exactly what `runWriterT` is doing. I would recommend
trying to rewrite your code to not even bother with the `WriterT` at all
and see if you can get the same output.
So your intuition is correct: there's no output parameter like in C. You
could simulate that in Haskell by passing in a mutable variable (like an
IORef), but that's not idiomatic. Passing back pairs of values is common in
Haskell.
Transformers can be useful for avoiding a lot of boilerplate code. They can
also introduce a lot of complexity in some cases. Figuring out when is the
right time to use them and when not is subjective and nuanced. Knowing the
skill is great; being able to avoid using the skill is also important :)
On Tue, Jul 25, 2017 at 1:29 PM, Baa <aqua...@gmail.com> wrote:
> Hello, Michael! This answers to my question completely, thank you so
> much!
>
> But looking at this code, I thought: how is it right to wrap/unwrap
> write monad? In languages like C or Python we can pass input/output
> argument (`int*` in C or `[0]` in Python) and use it as some
> accumulator. But here writer monad is not using as accumulator,
> accumulating (summation) happens in `mconcat`, right? It's using only
> as output value, i.e. place to "yield" result. I mean `w` is 0, each
> call of `runIt` sets there 1, after all calls we calculate sum of this
> 1's. And instead of `censor (+1) w` I can do `tell 1` only.
>
> It means that `runIt` can return not `IO ()` but `IO Int` and results
> of all `runIt`'s asynchnronously gotten values can be accumulated with
> `mconcat` without using of writer monad. Am I right, writer monad here
> is not accumulator but only output value (like output arguments in
> C/C++/IDL/etc)? How is this a typical solution in Haskell - to use
> writer monad with wrap/unwrap multiple times, only to save output
> value?
>
>
>
> On Tue, 25 Jul 2017 12:31:56 +0300
> Michael Snoyman <mich...@snoyman.com> wrote:
>
> > Firstly, a direct answer to your question: use mconcat.
> >
> > main :: IO ()
> > main = do
> > let l = [1,2,3,4]
> > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > z <- sequence
> > (map (A.async . runWriterT . runIt w) l)
> > >>= mapM A.wait
> > print $ snd $ mconcat z
> >
> > Under the surface, WriterT is using mappend to combine the `Sum`
> > values anyway, so it's natural is `mconcat` (the version of mappend
> > that applies to list) to get the same result. Now some possible
> > improvements.
> >
> > You're not actually using the return value from the `runIt` call,
> > just the writer value. There's a function called `execWriter` for
> > this:
> >
> > z <- sequence
> > (map (A.async . execWriterT . runIt w) l)
> > >>= mapM A.wait
> > print $ mconcat z
> >
> > Next, the combination of map and sequence can be written as traverse:
> >
> > z <- traverse (A.async . execWriterT . runIt w) l
> > >>= mapM A.wait
> >
> > But the async library is cool enough that it provides a function
> > called mapConcurrently that deals with the async/wait dance for you:
> >
> > main :: IO ()
> > main = do
> > let l = [1,2,3,4]
> > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > z <- A.mapConcurrently (execWriterT . runIt w) l
> > print $ mconcat z
> >
> > One final note: usage of `print` like this in a concurrent context
> > can run into interleaved output if you have the wrong buffer mode
> > turned out, leading to output like this:
> >
> > 2
> > 3
> > 41
> >
> > This is especially common when using runghc or ghci. You can either
> > change the buffering mode or use a different output function like
> > sayShow (from the say package, which I wrote):
> >
> > module Main where
> >
> > import qualified Control.Concurrent.Async as A
> > import Control.Monad.Trans.Writer
> > import Data.Monoid
> > import Say
> >
> > runIt :: (Show a, Num a)
> > => WriterT (Sum a) IO ()
> > -> a
> > -> WriterT (Sum a) IO ()
> > runIt w x = do
> > censor (+1) w -- emulates conditional count of something
> > sayShow x
> >
> > main :: IO ()
> > main = do
> > let l = [1,2,3,4]
> > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > z <- A.mapConcurrently (execWriterT . runIt w) l
> > sayShow $ mconcat z
> >
> >
> > On Tue, Jul 25, 2017 at 11:36 AM, Baa <aqua...@gmail.com> wrote:
> >
> > > Hello, Dear List!
> > >
> > > There is package `async`
> > > (https://hackage.haskell.org/package/async-2.1.1.1/docs/
> > > Control-Concurrent-Async.html).
> > >
> > > Before, I had:
> > >
> > > import qualified Control.Concurent.Async as A
> > > ...
> > > runIt :: ... -> IO ()
> > > ...
> > > sequence [A.async $ runIt ...] >>= mapM_ A.wait
> > >
> > > But now I want to count something inside `runIt`. I will use
> > > `Writer` monad for it (sure, it can be `State` also, not in
> > > principle to me). To do it synchronously, I done:
> > >
> > > module Main where
> > >
> > > import Control.Monad.Trans.Writer
> > > import Control.Monad.IO.Class
> > > import Data.Monoid
> > >
> > > runIt :: (Show a, Num a) => WriterT (Sum a) IO () -> a ->
> > > WriterT (Sum a) IO ()
> > > runIt w x = do
> > > censor (+1) w -- emulates conditional count of something
> > > liftIO $ print x
> > >
> > > main = do
> > > let l = [1,2,3,4]
> > > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > > z <- runWriterT $ sequence [runIt w i | i <- l]
> > > print $ snd z
> > >
> > > but now my `runIt` changes it's signature:
> > >
> > > runIt :: Num a => WriterT (Sum a) IO () -> ... -> WriterT (Sum
> > > a) IO () ...
> > > sequence [A.async $ runIt ...] >>= mapM_ A.wait
> > > ^^^^^^^^^^^
> > > ` ERROR is here!
> > >
> > > I get the error because `async`::IO () -> IO (A.Async ()) but I'm
> > > trying to pass it `WriterT (Sum a) IO ()`!
> > >
> > > To fix it I added `runWriterT` there:
> > >
> > > res <- sequence [A.async $ runWriterT (runIt ...) ...] >>= mapM
> > > A.wait
> > >
> > > but now I will get list of counters, not one (res::[((), Sum Int)])!
> > >
> > > How to solve this problem: to run several actions asyncronously and
> > > to count something
> > > inside the action with `Writer` monad?
> > >
> > >
> > > ===
> > > Best regards, Paul
> > > _______________________________________________
> > > Beginners mailing list
> > > Beginners@haskell.org
> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > >
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 2
Date: Tue, 25 Jul 2017 14:00:10 +0300
From: Baa <aqua...@gmail.com>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Count with Writer asynchronously
Message-ID: <20170725140010.77fd5da2@Pavel>
Content-Type: text/plain; charset=US-ASCII
Yes, it's true. I see now. Thank you again for that good
explanation!
On Tue, 25 Jul 2017 13:41:35 +0300
Michael Snoyman <mich...@snoyman.com> wrote:
> You're asking very good questions, about a topic I considered
> bringing up in my previous response but didn't. I guess I will now :)
>
> The important thing to realize about monad transformers (and monads in
> general) is that _they're nothing special_. Under the surface,
> WriterT is simply:
>
> * Modifying your functions to have an extra value returned
> * Providing convenience functions like `tell` and `liftIO` to work
> with that extra value
>
> I would strongly advise getting used to writing code in both the
> transformer and non-transformer style to convince yourself that they
> are the same thing. To make this concrete, look at the WriterT
> definition:
>
> newtype WriterT w m a = WriterT { runWriterT :: m (a, w) }
>
> If you take away the newtype wrapping business, you can see that it's
> just saying `WriterT w m a = m (a, w)`. And therefore, if we go back
> to your types, we can replace:
>
> WriterT (Sum a) IO ()
>
> with
>
> IO ((), Sum a)
>
> In fact, that's exactly what `runWriterT` is doing. I would recommend
> trying to rewrite your code to not even bother with the `WriterT` at
> all and see if you can get the same output.
>
> So your intuition is correct: there's no output parameter like in C.
> You could simulate that in Haskell by passing in a mutable variable
> (like an IORef), but that's not idiomatic. Passing back pairs of
> values is common in Haskell.
>
> Transformers can be useful for avoiding a lot of boilerplate code.
> They can also introduce a lot of complexity in some cases. Figuring
> out when is the right time to use them and when not is subjective and
> nuanced. Knowing the skill is great; being able to avoid using the
> skill is also important :)
>
> On Tue, Jul 25, 2017 at 1:29 PM, Baa <aqua...@gmail.com> wrote:
>
> > Hello, Michael! This answers to my question completely, thank you so
> > much!
> >
> > But looking at this code, I thought: how is it right to wrap/unwrap
> > write monad? In languages like C or Python we can pass input/output
> > argument (`int*` in C or `[0]` in Python) and use it as some
> > accumulator. But here writer monad is not using as accumulator,
> > accumulating (summation) happens in `mconcat`, right? It's using
> > only as output value, i.e. place to "yield" result. I mean `w` is
> > 0, each call of `runIt` sets there 1, after all calls we calculate
> > sum of this 1's. And instead of `censor (+1) w` I can do `tell 1`
> > only.
> >
> > It means that `runIt` can return not `IO ()` but `IO Int` and
> > results of all `runIt`'s asynchnronously gotten values can be
> > accumulated with `mconcat` without using of writer monad. Am I
> > right, writer monad here is not accumulator but only output value
> > (like output arguments in C/C++/IDL/etc)? How is this a typical
> > solution in Haskell - to use writer monad with wrap/unwrap multiple
> > times, only to save output value?
> >
> >
> >
> > On Tue, 25 Jul 2017 12:31:56 +0300
> > Michael Snoyman <mich...@snoyman.com> wrote:
> >
> > > Firstly, a direct answer to your question: use mconcat.
> > >
> > > main :: IO ()
> > > main = do
> > > let l = [1,2,3,4]
> > > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > > z <- sequence
> > > (map (A.async . runWriterT . runIt w) l)
> > > >>= mapM A.wait
> > > print $ snd $ mconcat z
> > >
> > > Under the surface, WriterT is using mappend to combine the `Sum`
> > > values anyway, so it's natural is `mconcat` (the version of
> > > mappend that applies to list) to get the same result. Now some
> > > possible improvements.
> > >
> > > You're not actually using the return value from the `runIt` call,
> > > just the writer value. There's a function called `execWriter` for
> > > this:
> > >
> > > z <- sequence
> > > (map (A.async . execWriterT . runIt w) l)
> > > >>= mapM A.wait
> > > print $ mconcat z
> > >
> > > Next, the combination of map and sequence can be written as
> > > traverse:
> > >
> > > z <- traverse (A.async . execWriterT . runIt w) l
> > > >>= mapM A.wait
> > >
> > > But the async library is cool enough that it provides a function
> > > called mapConcurrently that deals with the async/wait dance for
> > > you:
> > >
> > > main :: IO ()
> > > main = do
> > > let l = [1,2,3,4]
> > > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > > z <- A.mapConcurrently (execWriterT . runIt w) l
> > > print $ mconcat z
> > >
> > > One final note: usage of `print` like this in a concurrent context
> > > can run into interleaved output if you have the wrong buffer mode
> > > turned out, leading to output like this:
> > >
> > > 2
> > > 3
> > > 41
> > >
> > > This is especially common when using runghc or ghci. You can
> > > either change the buffering mode or use a different output
> > > function like sayShow (from the say package, which I wrote):
> > >
> > > module Main where
> > >
> > > import qualified Control.Concurrent.Async as A
> > > import Control.Monad.Trans.Writer
> > > import Data.Monoid
> > > import Say
> > >
> > > runIt :: (Show a, Num a)
> > > => WriterT (Sum a) IO ()
> > > -> a
> > > -> WriterT (Sum a) IO ()
> > > runIt w x = do
> > > censor (+1) w -- emulates conditional count of something
> > > sayShow x
> > >
> > > main :: IO ()
> > > main = do
> > > let l = [1,2,3,4]
> > > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > > z <- A.mapConcurrently (execWriterT . runIt w) l
> > > sayShow $ mconcat z
> > >
> > >
> > > On Tue, Jul 25, 2017 at 11:36 AM, Baa <aqua...@gmail.com> wrote:
> > >
> > > > Hello, Dear List!
> > > >
> > > > There is package `async`
> > > > (https://hackage.haskell.org/package/async-2.1.1.1/docs/
> > > > Control-Concurrent-Async.html).
> > > >
> > > > Before, I had:
> > > >
> > > > import qualified Control.Concurent.Async as A
> > > > ...
> > > > runIt :: ... -> IO ()
> > > > ...
> > > > sequence [A.async $ runIt ...] >>= mapM_ A.wait
> > > >
> > > > But now I want to count something inside `runIt`. I will use
> > > > `Writer` monad for it (sure, it can be `State` also, not in
> > > > principle to me). To do it synchronously, I done:
> > > >
> > > > module Main where
> > > >
> > > > import Control.Monad.Trans.Writer
> > > > import Control.Monad.IO.Class
> > > > import Data.Monoid
> > > >
> > > > runIt :: (Show a, Num a) => WriterT (Sum a) IO () -> a ->
> > > > WriterT (Sum a) IO ()
> > > > runIt w x = do
> > > > censor (+1) w -- emulates conditional count of something
> > > > liftIO $ print x
> > > >
> > > > main = do
> > > > let l = [1,2,3,4]
> > > > w = writer ((), 0) :: WriterT (Sum Int) IO ()
> > > > z <- runWriterT $ sequence [runIt w i | i <- l]
> > > > print $ snd z
> > > >
> > > > but now my `runIt` changes it's signature:
> > > >
> > > > runIt :: Num a => WriterT (Sum a) IO () -> ... -> WriterT
> > > > (Sum a) IO () ...
> > > > sequence [A.async $ runIt ...] >>= mapM_ A.wait
> > > > ^^^^^^^^^^^
> > > > ` ERROR is here!
> > > >
> > > > I get the error because `async`::IO () -> IO (A.Async ()) but
> > > > I'm trying to pass it `WriterT (Sum a) IO ()`!
> > > >
> > > > To fix it I added `runWriterT` there:
> > > >
> > > > res <- sequence [A.async $ runWriterT (runIt ...) ...] >>=
> > > > mapM A.wait
> > > >
> > > > but now I will get list of counters, not one (res::[((), Sum
> > > > Int)])!
> > > >
> > > > How to solve this problem: to run several actions asyncronously
> > > > and to count something
> > > > inside the action with `Writer` monad?
> > > >
> > > >
> > > > ===
> > > > Best regards, Paul
> > > > _______________________________________________
> > > > Beginners mailing list
> > > > Beginners@haskell.org
> > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> > > >
> >
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> >
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