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

[Cody Goodman]

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)])]

Here is my failed attempt:

{-# LANGUAGE ScopedTypeVariables, BangPatterns #-}
import qualified Data.Map as M
import Data.List (foldl')
import Data.Maybe
import Debug.Trace

data Info = Info { i :: !Int, healthTopic :: !String, state :: !String }
deriving Show

m :: M.Map String (M.Map String Integer)
m = M.fromList [("MA", M.fromList[("Pregnancy",1)]),("TX",
M.fromList[("Alcohol",2)])]

constantValsPerState = foldl'

                        (\accum currentRow -> do
                            M.insert (state currentRow) "xxx" accum
                        )

                        M.empty

                        [ Info 0 "Alcohol" "TX"
                        , Info 1 "Alcohol" "TX"
                        , Info 2 "Pregnancy" "MA"
                        ]

-- λ> constantValsPerState
-- fromList [("MA","xxx"),("TX","xxx")]

-- how can I get something like:
-- λ> numTopicsPerState
-- fromList [("MA", fromList[("Pregnancy",1)]),("TX",
fromList[("Alcohol",2)])]

-- so we need to give the modified stae of the new map isntead of M.empty
-- λ> m
-- fromList [("MA",fromList [("Pregnancy",1)]),("TX",fromList
[("Alcohol",2)])]
-- λ> M.insertWith (\new old -> new) "MA" M.empty m
-- fromList [("MA",fromList []),("TX",fromList [("Alcohol",2)])]

numTopicsPerState = foldl'
                        (\(accum :: M.Map String (M.Map String Integer))
currentRow -> do
                            M.insertWithKey
                              (\k new old -> M.insert
                                              (healthTopic currentRow)
                                              ((fromMaybe 0 $ M.lookup
(healthTopic currentRow) old) + 1)
                                              new
                              )
                              (state currentRow)
                              (fromMaybe (M.empty) (M.lookup (state
currentRow) accum))
                              accum
                        )

                        M.empty
                        [ Info 0 "Alcohol" "TX"
                        , Info 1 "Alcohol" "TX"
                        , Info 2 "Pregnancy" "MA"
                        ]

-- WRONG OUTPUT
-- λ> numTopicsPerState
-- fromList [("MA",fromList []),("TX",fromList [("Alcohol",1)])]


-- TODO
-- turn
-- fromList [("Alcohol",fromList []),("MA",fromList
[("Pregnancy",1)]),("TX",fromList [("Alcohol",2)])]
-- into
-- fromList [("MA",fromList [("Pregnancy",1)]),("TX",fromList
[("Alcohol",3)])]
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