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I wrote a function that inserts or updates a Map key value depending on whether the key is already in the map. If it is, then the value gets added to the already existing map's value.

import Data.Map as Map

updateMap :: (Ord k, Num v) => k -> v -> Map k v -> Map k v
updateMap k v map = if member k map then Map.adjust (+ v) k map
                    else Map.insert k v map 

Tests

*Main> updateMap 1 1 $ Map.singleton 1 100
fromList [(1,101)]
*Main> updateMap 2 1 $ Map.singleton 1 100
fromList [(1,100),(2,1)]
*Main> updateMap 1 33 $ Map.singleton 1 100
fromList [(1,133)]

Please critique.

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updateMapPF :: (Ord k, Num a) => k -> a -> Map k a -> Map k a
updateMapPF = Map.insertWith (+)

λ> updateMapPF 1 1 $ Map.singleton 1 100
fromList [(1,101)]
λ> updateMapPF 2 1 $ Map.singleton 1 100
fromList [(1,100),(2,1)]
λ> updateMapPF 1 33 $ Map.singleton 1 100
fromList [(1,133)]

Process:

Notice the use of Map.insert

Notice that you are mapping (+) over the possibility of having or not having a value

Notice that Data.Map has a function for applying functions to values, called Data.Map.insertWith. It's extremely common for collections to have a helper function for "insert data with a helper function / default value"

Notice the shared structure of the types of your function and insertWith

insertWith :: Ord k => (a -> a -> a) -> k -> a -> Map k a -> Map k a

updateMap  :: (Ord k, Num v) =>         k -> v -> Map k v -> Map k v

Realize that applying (+) to insertWith will make the types identical

λ> :t insertWith

insertWith
  :: Ord k => (a -> a -> a) -> k -> a -> Map k a -> Map k a

λ> :t insertWith (+)

insertWith (+) :: (Ord k, Num a) => k -> a -> Map k a -> Map k a

Validate assumptions with specific examples, more rigor is available with the use of QuickCheck.

λ> updateMapPF 1 1 $ Map.singleton 1 100
fromList [(1,101)]

λ> updateMapPF 2 1 $ Map.singleton 1 100
fromList [(1,100),(2,1)]

λ> updateMapPF 1 33 $ Map.singleton 1 100
fromList [(1,133)]

This is a practically golden opportunity to demonstrate how eta reduction can simplify code and kill off redundant logic.

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