On Wed, Dec 28, 2016 at 8:07 AM, Olaf Klinke wrote:

I'd say the cleanest way to go is to split your Info data structure into an intermediate key and a value part like so:

type Key = String data Value = Value {count :: !Int, healthTopics :: ![String]}

keyvalue :: Info -> (Key,Value) keyvalue (Info _ h k) = (k,Value 1 [h])

and give Value a Monoid instance:

instance Monoid Value where mempty = Value 0 [] mappend (Value c s) (Value c' s') = Value (c+c') (s++s')

(Or you could use (Sum Int) instead of Int and have type Value = (Sum Int,[String]). Then the Monoid instance is derived for you.)

A caution on this alternative: the first component of the tuple won't be strict enough and you'll leak space. I think the proposed solution is better. Note that you *don't* necessarily need the strictness annotation on healthTopics – this shifts around when the list append gets run but won't change space much. -Jan-Willem Maessen

Now you can transform a list of Infos into a Map using a generic function:

makeMap :: [Info] -> M.Map Key Value makeMap = M.fromListWith mappend . map keyvalue

Semantically, it's pretty identical to ALeX Kazik's suggestion of using foldl' and alter. Internally, fromListWith uses a strict fold, so strictness should be the same as using foldl'.

Olaf _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.