Haskell: insert or append value inside associative list

Question

I have an associative list with dynamic size of type Map k [v] like this

myList = fromList [(10,[11,12]),(20,[21,22]),(12,[10,11])]

in which I want to insert values depending on the key like so

-- insert value 13 in tuple (10,[11,12]) ==> (10,[11,12,13]) 

I'm getting more familiar with manipulation list but I'm really stack her. When I use insert like this :

--insert value 11 where key equals to 12
insert 12 [11] (fromList [(10,[11,12]),(11,[10,12]),(12,[10])]) == fromList [(10,[11,12]),(11,[10,12]),(12,[11])]

It does not produce [10,11] but rather replace [10] by [11].

And if my list is empty. myList = fromList Map.emptyand I want to insert a tuple (10,11) to my list. I want to expect this behavior as a result

myList = insert 10 11 Map.empty ==> [(10,[11]),(11,[10]))]

Any indications, ideas or hints are welcomed.

Answer 1

You can try using insertWith instead:

Prelude Data.Map> let myList = fromList [(10,[11,12]),(11,[10,12]),(12,[10])]
Prelude Data.Map> insertWith (++) 12 [11] myList
fromList [(10,[11,12]),(11,[10,12]),(12,[11,10])]

As for your second requirement, you can try this:

import Prelude hiding (null)
import Data.Map

insertEmpty :: Int -> Int -> Map Int [Int] -> Map Int [Int]
insertEmpty x y hmap

    -- If map is empty, return the paired tuples
    | null hmap = pairs

    -- If the map is the same as pairs, return it unchanged
    | hmap == pairs = hmap

    -- Otherwise insert into map
    | otherwise = insertWith (++) x [y] hmap

    where pairs = fromList [(x, [y]), (y, [x])]

Which works as follows:

*Main> let myList = fromList [(11,[12]),(12,[11])]
*Main> insertEmpty 11 12 myList
fromList [(11,[12]),(12,[11])]
*Main> let myList = fromList []
*Main> insertEmpty 11 12 myList
fromList [(11,[12]),(12,[11])]
*Main> let myList = fromList [(10,[11,12]),(11,[10,12]),(12,[10])]
*Main> insertEmpty 11 12 myList
fromList [(10,[11,12]),(11,[12,10,12]),(12,[10])]