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Today's Topics:
1. Re: Adapting code from an imperative loop (Vlatko Basic)
2. Re: Adapting code from an imperative loop (Matt Williams)
----------------------------------------------------------------------
Message: 1
Date: Fri, 19 Jun 2015 10:04:30 +0200
From: Vlatko Basic <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Adapting code from an imperative loop
Message-ID: <[email protected]>
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Message: 2
Date: Fri, 19 Jun 2015 09:52:42 +0000
From: Matt Williams <[email protected]>
To: [email protected], The Haskell-Beginners Mailing List -
Discussion of primarily beginner-level topics related to Haskell
<[email protected]>
Subject: Re: [Haskell-beginners] Adapting code from an imperative loop
Message-ID:
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Dear All,
Thanks for your help with this.
I have got the simple version working but now need to use Map.fromListWith,
and am having syntax problems.
I found a related question on Stack overflow (here: http://
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
stackoverflow.com
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
/questions/15514486/
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
haskell-converting-a-list-
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
of-
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
a-b-key-
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
value-pairs-
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
with-
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
possibly-
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
repeated-key
<http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b-key-value-pairs-with-possibly-repeated-key>
).
However, I'm having problems understanding the additional function. The
signature should be :
(a -> a -> a) -> [(k, a)] -> Map
And so I assume I need to supply a function whose signature is:
(a -> a -> a)
Is that correct?
Thanks,
Matt
On Fri, 19 Jun 2015 09:04 Vlatko Basic <[email protected]> wrote:
> To learn, I'd suggest you implement it first with the recursion (a tip:
> use a "loop" function in where clause), and than with fold. Those are
> important features to understand in Haskell. Try to use the "higher-level"
> functions as little as possible until you grasp the basics (like fold
> syntax).
>
> If you just need any solution, fromListWith is fine.
>
> br,
> vlatko
>
>
>
> -------- Original Message --------
> Subject: Re: [Haskell-beginners] Adapting code from an imperative loop
> From: Matt Williams <[email protected]>
> <[email protected]>
> To: The Haskell-Beginners Mailing List - Discussion of primarily
> beginner-level topics related to Haskell <[email protected]>
> <[email protected]>
> Date: 19/06/15 09:05
>
>
> I tried doing it as a fold (the pattern of accumulating a value, where
> the accumulated value was the resulting Map), but couldn't manage the
> syntax.
>
> Have now got it partially working with fromListWith.
>
> Thanks a lot,
> Matt
>
> On Fri, 19 Jun 2015 07:18 Bob Ippolito <[email protected]> wrote:
>
>> On Friday, June 19, 2015, Matt Williams <[email protected]>
>> wrote:
>>
>>> Dear All,
>>>
>>> I have been wrestling with this for a while now.
>>>
>>> I have a list of data items, and want to be able to access them, in a
>>> Hash Map, via a short summary of their characteristics.
>>>
>>> In an imperative language this might look like:
>>>
>>> myMap = new map()
>>> for elem in myElems:
>>> key = makeKey(elem)
>>> myMap[key] = myMap[key] + elem
>>>
>>> I have been trying to do this in Haskell, but am stuck on how to hand
>>> the Map back to itself each time.
>>>
>>> I have a function :: elem -> [p,q] to make the key, but the Map.insert
>>> function has the following signature:
>>>
>>> insert :: (Hashable
>>> <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-Hashable.html#t:Hashable>
>>> k, Ord
>>> <https://hackage.haskell.org/packages/archive/base/4.2.0.2/doc/html/Data-Ord.html#t:Ord>
>>> k)
>>> => k -> a -> HashMap
>>> <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#t:HashMap>
>>> k
>>> a -> HashMap
>>> <https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#t:HashMap>
>>> k
>>> a
>>>
>>> My thought was that I needed to go through the list of the elems, and at
>>> each point add them to the Hash Map, handing the updated Map onto the next
>>> step - but this is what I cannot write.
>>>
>>
>> This is typically done with fromListWith or a combination of foldl' and
>> insertWith or alter.
>>
>> -bob
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