[Haskell-cafe] Multi groupby with foldl' and Map.insertWithKey

Olaf Klinke olf at aatal-apotheke.de
Wed Dec 28 13:07:44 UTC 2016

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.)

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'. 


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