[Haskell-beginners] Adapting code from an imperative loop

[Bob Ippolito]

An example of a function of (a -> a -> a) would be (+), which works if your
values are numbers (which mirrors your Python-like pseudocode).

On Friday, June 19, 2015, Matt Williams <matt.williams45.mw at gmail.com>
wrote:

> 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 <vlatko.basic at gmail.com
> <javascript:_e(%7B%7D,'cvml','vlatko.basic at gmail.com');>> 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 <matt.williams45.mw at gmail.com>
>> <javascript:_e(%7B%7D,'cvml','matt.williams45.mw at gmail.com');>
>> To: The Haskell-Beginners Mailing List - Discussion of primarily
>> beginner-level topics related to Haskell <beginners at haskell.org>
>> <javascript:_e(%7B%7D,'cvml','beginners at haskell.org');>
>> 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 <
>> <javascript:_e(%7B%7D,'cvml','bob at redivi.com');>bob at redivi.com
>> <javascript:_e(%7B%7D,'cvml','bob at redivi.com');>> wrote:
>>
>>> On Friday, June 19, 2015, Matt Williams <matt.williams45.mw at gmail.com
>>> <javascript:_e(%7B%7D,'cvml','matt.williams45.mw at gmail.com');>> 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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>>
>>
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