Expenses.hs

import Data.Map (Map, fromList, toList, unionWith, adjust, insert)
import Data.List (isPrefixOf)

data Area = Savings | Food | FoodSavings | Investments | Expenses | Play deriving (Show, Read, Ord, Eq, Enum)
data Entry = Expense Area Integer | Credit Integer | Plan [(Area, Integer)] | Comment  deriving (Show, Read)
type Savings = Map Area Integer
type Plan = Map Area Integer

noSavings = fromList [(area, 0) | area <- [Savings .. Play]]
defaultPlan = fromList $ (Savings, 100) : [(area, 0) | area <- [Food .. Play]]

process :: [Entry] -> Savings
process = fst . foldl update (noSavings, defaultPlan)

update :: (Savings, Plan) -> Entry -> (Savings, Plan)
update (savings, plan) entry = case entry of
    Expense area amount -> (adjust (subtract amount) area savings, plan)
    Credit amount -> (unionWith (+) (amount `splitBy` plan) savings, plan)
    Plan new | sum (map snd new) == 100 -> (savings, fromList new)
             | otherwise -> error "Plan doesn't add to 100%"

splitBy :: Integer -> Plan -> Savings
splitBy amount plan = fromList
    [(area, (percent * amount) `div` 100) | (area, percent) <- toList plan]

parse :: String -> [Entry]
parse str = map read $ filter (not . isComment) (lines str)
    where
    isComment ('#':_) = True
    isComment _ = False

main = do
    entries <- parse `fmap` readFile "Savings.txt"
    print $ process entries