On Friday, June 19, 2015, Matt Williams <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 k, Ord k) => k -> a -> HashMap k a -> 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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