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 <bob@redivi.com> wrote:
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 <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.
-bob _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners