How to achive foldl operation for Map data

Questions
4

Here is one solution that satisfies the input you provided. I am not quite sure I understood your requirements correctly so if you were looking for something else, let us know:

module Main where

import Daml.Script
import DA.Assert
import DA.Next.Map (Map)
import qualified DA.Next.Map as Map

data Version = Version with
    value : Text
  deriving (Eq, Show)

processMap : Map Text [Version] -> Map Text [Version]
processMap mapList = 
  Map.fromList 
    (map (\(mapKey, versionList) -> (mapKey, processList versionList))
         (Map.toList mapList))

processList : [Version] -> [Version]
processList versionList =
  concatMap f versionList
  where
    f version
      | version.value == "1000" = [version, Version "2000"]
      | otherwise = [version]

test = script do
  -- Original example
  processMap (Map.fromList [("amount", [Version "1000"])])
          === Map.fromList [("amount", [Version "1000", Version "2000"])]

  -- 2000 is inserted directly after 1000 not at the very end
  processMap (Map.fromList [("amount", [Version "1000", Version "3000"])])
          === Map.fromList [("amount", [Version "1000", Version "2000", Version "3000"])]

  -- This applies the same to all keys not just amount
  processMap (Map.fromList [("notamount", [Version "1000", Version "3000"])])
          === Map.fromList [("notamount", [Version "1000", Version "2000", Version "3000"])]