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Today's Topics:
1. Re: figured out use for join! (Frerich Raabe)
2. Re: figured out use for join! (Marcin Mrotek)
3. Re: figured out use for join! (Frerich Raabe)
4. question about list processing (Dennis Raddle)
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Message: 1
Date: Wed, 11 Nov 2015 23:01:29 +0100
From: Frerich Raabe <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] figured out use for join!
Message-ID: <[email protected]>
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On 2015-11-11 22:12, Dennis Raddle wrote:
> f k m = case M.lookup k m of
> Nothing -> Nothing
> Just xs -> listToMaybe xs
>
> But the case "Nothing -> Nothing" is suspicious... seems like that's always
> a clue some typeclass could simplify it. Eventually I figured out
>
> f k = join . fmap listToMaybe . M.lookup k
The 'Nothing -> Nothing' thing may also be a good hint that using the Maybe
monad would be useful. Your function can also be defined as
f k m = M.lookup k m >>= listToMaybe
--
Frerich Raabe - [email protected]
www.froglogic.com - Multi-Platform GUI Testing
------------------------------
Message: 2
Date: Thu, 12 Nov 2015 10:17:09 +0100
From: Marcin Mrotek <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] figured out use for join!
Message-ID:
<CAJcfPz=9sPwV=byfr4xpkbnaomj-mdl5yjgxgpa3y4upq9k...@mail.gmail.com>
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Hello,
> using the Maybe monad would be useful.
But he already is? Anyway, `(x >>= f) == join (fmap f x)`
Best regards,
Marcin Mrotek
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Message: 3
Date: Thu, 12 Nov 2015 10:38:22 +0100
From: Frerich Raabe <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] figured out use for join!
Message-ID: <[email protected]>
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On 2015-11-12 10:17, Marcin Mrotek wrote:
>> using the Maybe monad would be useful.
>
> But he already is?
Yes, of course, sorry for the imprecision. I meant to write 'using bind as
defined for the Maybe monad'.
--
Frerich Raabe - [email protected]
www.froglogic.com - Multi-Platform GUI Testing
------------------------------
Message: 4
Date: Thu, 12 Nov 2015 03:45:24 -0800
From: Dennis Raddle <[email protected]>
To: Haskell Beginners <[email protected]>
Subject: [Haskell-beginners] question about list processing
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<CAKxLvorMvAk4gixYNjqw4D0jz=37=y_zzjez-6btpzej59j...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
What would be an elegant way of writing this
computeHead :: (a -> [b]) -> [a] -> [b]
Where when [a] is null, it returns a null list, but when [a] contains one
or more elements, it applies the given function to the head of a and
returns that? Is there some existing typeclass operator that facilitates
this?
You can write
computeHead _ [] = []
computeHead f (x:_) = f x
But that first line seems suspicious to me... it makes me think about how
in the list Monad, an empty list falls through. But I can't quite make it
work.
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