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@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@gmail.com <javascript:_e(%7B%7D,'cvml','vlatko.basic@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@gmail.com> <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 <beginners@haskell.org> <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:
On Friday, June 19, 2015, Matt Williams <matt.williams45.mw@gmail.com <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#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.
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