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

[Anatoly Zaretsky]

On Wed, Dec 28, 2016 at 9:28 AM, Cody Goodman <codygman.consulting at gmail.com
> wrote:

> My goal is to take data like this:
>
>                         [ Info 0 "Alcohol" "TX"
>                         , Info 1 "Alcohol" "TX"
>                         , Info 2 "Pregnancy" "MA"
>                         ]
>
> and build a Map like this:
>
> [("MA",[("Pregnancy",1)]),("TX",[("Alcohol",2)])]
>

You can convert each individual record to a singleton map:

wrapRecord :: Info -> M.Map String (M.Map String Integer)
wrapRecord (Info _ state healthTopic) = M.singleton state (M.singleton
healthTopic 1)

and then union all these singletons:

numTopicsPerState = M.unionsWith (M.unionWith (+)) . map wrapRecord $ [Info
0 "Alcohol" "TX", Info 1 "Alcohol" "TX", Info 2 "Pregnancy" "MA"]
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