On Friday, June 19, 2015, 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#... 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