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:

...

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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Le ven. 19 juin 2015 à 19:03, Michael Orlitzky a écrit :

loop :: [Int] -> (Map Int Int) -> (Map Int Int) loop elems initial_map = foldr update_map initial_map elems where update_map :: Int -> (Map Int Int) -> (Map Int Int) update_map x m = let k = key_from_elem x in case (M.lookup k m) of Nothing -> insert k x m Just v -> insert k (v + x) m

While this code will do the right thing, it won't do it efficiently, at all ! First (and huge) problem is using foldr : starting from the end of the list is the wrong move here if you want to consume your Int stream efficiently (which you probably do and which is vital if the stream is big). So you should be using foldl' and the strict version of Map (Data.Map.Strict). Also, though less important, looking up a key then updating the value with insert is wasteful : you will be doing the search twice, instead of using one of the nice combinators of Data.Map which find and update a value in one pass like "insertWith (+) k x m" In other words : import qualified Data.Map.Strict as M countingMap :: [(key, Int)] -> Map key Int countingMap kvs = foldl' update M.empty kvs where update (k,v) m = M.insertWith (+) k v m Since this is a very common need, there's even a combinator to do this : countingMap :: [(key, Int)] -> Map key Int countingMap kvs = M.fromListWith (+) kvs At which point you may even use directly this code and forgo creating a function for it (except if you're using it several times or as a minor step in a big pipeline where you would like to label each step clearly). -- Jedaï