Re: [Haskell-beginners] Adapting code from an imperative loop

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 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... stackoverflow.com http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... /questions/15514486/ http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... haskell-converting-a-list- http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... of- http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... a-b-key- http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... value-pairs- http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... with- http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... possibly- http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... repeated-key http://stackoverflow.com/questions/15514486/haskell-converting-a-list-of-a-b... ).

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 javascript:_e(%7B%7D,'cvml','vlatko.basic@gmail.com');> wrote:

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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 javascript:_e(%7B%7D,'cvml','matt.williams45.mw@gmail.com'); To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell javascript:_e(%7B%7D,'cvml','beginners@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@redivi.com');bob@redivi.com javascript:_e(%7B%7D,'cvml','bob@redivi.com');> wrote:

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On Friday, June 19, 2015, Matt Williams javascript:_e(%7B%7D,'cvml','matt.williams45.mw@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#... k, Ord https://hackage.haskell.org/packages/archive/base/4.2.0.2/doc/html/Data-Ord.... k) => k -> a -> HashMap https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#t... k a -> HashMap https://hackage.haskell.org/package/hashmap-1.0.0.2/docs/Data-HashMap.html#t... 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 _______________________________________________ Beginners mailing list Beginners@haskell.org javascript:_e(%7B%7D,'cvml','Beginners@haskell.org'); http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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