[Haskell-beginners] Adapting code from an imperative loop

[Matt Williams]

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.

Any help much appreciated.

Thanks,
Matt
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