Positive,

    Budget (..),

    positive,
    getPositive,

import Data.List
import Data.Pos
import Control.Monad.State

    { needs :: Positive
    , wants :: Positive
    , savings :: Positive

    { needs :: Pos
    , wants :: Pos
    , savings :: Pos

newtype Positive = Positive {unPositive :: Word} deriving (Show)
type Income = Positive
positive :: Int -> Maybe Positive
positive x
    | x > 0 = Just . Positive . fromIntegral $ x
    | otherwise = Nothing

getPositive :: Positive -> Word
getPositive = unPositive

type Income = Pos

calculate (Positive income) = Budget (Positive needs) (Positive wants) (Positive savings)

calculate x = Budget needs wants savings
  where
    income = fromIntegral . unP $ x
    (needs:wants:savings:_) = map (unsafePos "") . distribute [0.5, 0.3, 0.2] $ income
distribute :: [Double] -> Int -> [Int]
distribute xs target = flip evalState 0 . mapM (\x -> distributeState x weight target) $ xs

    needs = income `quot` 2
    wants = income - needs - savings
    savings = income `quot` 5

    weight = sum xs
distributeState :: Double -> Double -> Int -> State Double Int
distributeState basis weight target = do
    old <- get
    let r = (basis / weight * fromIntegral target) + old
    put r
    pure $ round r - round old

packages: .
source-repository-package
    type: git
    location: https://github.com/gbwey/pos.git
    tag: 659d493eb3fa7d04a9fcf5e3038e350e7808d06a

    exposed-modules:  Budget

    exposed-modules:
        Budget

    build-depends:    base ^>=4.16.4.0

    build-depends:
        base ^>=4.16.4.0,
        pos,
        mtl

import qualified MyLib (someFunc)

  MyLib.someFunc