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Today's Topics:
1. Re: Question about example in 'using do notation with
Writer' section of LYH (Olumide)
2. Need help understanding the tell function in the Monad
Writer example in LYAH (Olumide)
----------------------------------------------------------------------
Message: 1
Date: Fri, 3 Feb 2017 11:20:16 +0000
From: Olumide <50...@web.de>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Question about example in 'using do
notation with Writer' section of LYH
Message-ID: <43a69956-d89e-492c-1a0d-c4659f7ee...@web.de>
Content-Type: text/plain; charset=utf-8; format=flowed
On 27/01/2017 08:34, Theodore Lief Gannon wrote:
> Fully expanding your program might help. One of the great things about
> Haskell is equational reasoning: if two things have been declared equal,
> you can substitute one for the other.
>
> First, let's desugar that do notation to the equivalent bind chain:
>
> multWithLog =
> logNumber 3 >>= \a ->
> logNumber 5 >>= \b ->
> return (a*b)
>
>
> Evaluate the logNumber and return calls to normal form from their
> definitions, also considering the monoid definitions of (++) and mempty
> for lists:
>
> multWithLog =
> Writer (3, ["Got number: 3"]) >>= \a ->
> Writer (5, ["Got number: 5"]) >>= \b ->
> Writer (a*b, [])
>
>
> Now, refer to the definition of (>>=) for Writer (as shown in LYAH):
>
> (Writer (x, v)) >>= f = let (Writer (y, v')) = f x in Writer (y, v
> `mappend` v')
>
Thank you for your explanation. The substitutions helped a lot even
though I wasn't able to follow the equational reasoning. The expansion
gave me enough starting information to figure out what's going on by
starting from the innermost bind.
BTW, I thought the definition of (>>=) was supposed to come from
Control.Monad.Writer.
Regards,
- Olumide
------------------------------
Message: 2
Date: Fri, 3 Feb 2017 11:31:32 +0000
From: Olumide <50...@web.de>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: [Haskell-beginners] Need help understanding the tell function
in the Monad Writer example in LYAH
Message-ID: <7195359e-cfb5-a3de-d473-76450f2d0...@web.de>
Content-Type: text/plain; charset=utf-8; format=flowed
Hello List,
Would someone kindly explain how the tell function interacts with the
Writer Monad as show below:
(Example taken from chapter 14 of LYAH
http://learnyouahaskell.com/for-a-few-monads-more#reader )
multWithLog :: Writer [String] Int
multWithLog = do
a <- logNumber 3
b <- logNumber 5
tell ["Gonna multiply these two"]
return (a*b)
Result:
ghci> runWriter multWithLog
(15,["Got number: 3","Got number: 5","Gonna multiply these two"])
I know that tell function binds to an argument that is discarded but I
don't know how its argument "Gonna multiply these two" is concatenated
with the other the Writer created by logNumber 3 and logNumber 5.
Also, I don't understand the paragraph following the example:
"It's important that return (a*b) is the last line, because the result
of the last line in a do expression is the result of the whole do
expression. Had we put tell as the last line, () would have been the
result of this do expression. We'd lose the result of the
multiplication. However, the log would be the same."
Regards,
- Olumide
------------------------------
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