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Today's Topics:
1. Join'ing a State Monad, example from LYH (Olumide)
2. Re: Join'ing a State Monad, example from LYH (Francesco Ariis)
----------------------------------------------------------------------
Message: 1
Date: Sat, 10 Feb 2018 14:48:07 +0000
From: Olumide <50...@web.de>
To: beginners@haskell.org
Subject: [Haskell-beginners] Join'ing a State Monad, example from LYH
Message-ID: <edf89a88-b331-b42d-572b-e6b128d67...@web.de>
Content-Type: text/plain; charset=utf-8; format=flowed
Dear List,
I've been stumped for a few months on the following example, from
chapter 13 of LYH
http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions
runState (join (State $ \s -> (push 10,1:2:s))) [0,0,0]
I find the following implementation of join in the text is hard to
understand or apply
join :: (Monad m) => m (m a) -> m a
join mm = do
m <- mm
m
In contrast, I find the following definition(?) on Haskell Wikibooks
https://en.wikibooks.org/wiki/Haskell/Category_theory#Monads
join :: Monad m => m (m a) -> m a
join x = x >>= id
easier to understand, and although I can apply it to the following
Writer Monad example, in the same section of LYH,
runWriter $ join (Writer (Writer (1,"aaa"),"bbb"))
I cannot apply it to the State Monad example.
Regards,
- Olumide
------------------------------
Message: 2
Date: Sat, 10 Feb 2018 16:25:40 +0100
From: Francesco Ariis <fa...@ariis.it>
To: beginners@haskell.org
Subject: Re: [Haskell-beginners] Join'ing a State Monad, example from
LYH
Message-ID: <20180210152540.llwsh53dgobkw...@x60s.casa>
Content-Type: text/plain; charset=us-ascii
On Sat, Feb 10, 2018 at 02:48:07PM +0000, Olumide wrote:
> I find the following implementation of join in the text is hard to
> understand or apply
>
> join :: (Monad m) => m (m a) -> m a
> join mm = do
> m <- mm
> m
Hello Olumide,
remember that:
join :: (Monad m) => m (m a) -> m a
join mm = do m <- mm
m
is the same as:
join :: (Monad m) => m (m a) -> m a
join mm = mm >>= \m ->
m
In general remember that when you have a "plain" value, the last line
of a monadic expression is often:
return someSimpleVal
So:
monadicexpr = do x <- [4]
return x -- can't just write `x`
When you have a monad inside a monad, you can just "peel" the outer
layer and live happily thereafter:
monadicexpr = do x <- [[4]]
x -- result will be: [4], no need to use return
-- because [4] (and not 4) is still a
-- list monad
As for State, remember that State is:
data State s a = State $ s -> (a, s) -- almost
So a function that from a state s, calculates a new state s' and returns
a value of type `a`.
When we use the bind operator in a do block, it's like we're extracting
that value of type `a`
monadicexpr = do x <- someState
return x -- again we need to wrap this value
-- before returning it, this state being
--
-- \s -> (x, s)
--
-- i.e. we do nothing to the parameter state
-- and place `x` as a result.
--
Same trick there, if `x` is actually a State-inside-State (e.g. of
type `State s (State s a)`), there is no need for wrapping anymore.
Does this make sense?
-F
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