On Wed, Dec 28, 2016 at 8:07 AM, Olaf Klinke <olf@aatal-apotheke.de> 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
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