Without state monad

gistfile1.txt

module Main where

import Lib
import Data.Maybe

data Product = Product String
  deriving (Show, Eq)

data ProductInfo = ProductInfo Product Int
  deriving (Show, Eq)

data VendingMachine = VendingMachine [ProductInfo] Int String
  deriving (Show)

initVendingMachine = VendingMachine [] 0 "init"

product1 = Product "coffee"

productInfo1 = ProductInfo product1 1

charge :: VendingMachine -> Int -> VendingMachine
charge (VendingMachine info currentCash state) cash =
    VendingMachine info (currentCash + cash) "Thanks!"

addProductInfo :: VendingMachine -> ProductInfo -> VendingMachine
addProductInfo (VendingMachine info currentCash state) newProduct =
        VendingMachine (info ++ [newProduct]) currentCash "Thanks!"


choose :: VendingMachine -> ProductInfo -> (VendingMachine, Either String Product)
choose vm@(VendingMachine infos currentCash state) productInfo =
  let chosen = chooseProductInfo productInfo currentCash infos in
  case chosen of
    Nothing -> (vm, Left "Product not found")
    Just p@(ProductInfo product price) ->
      let newVendingMachine = VendingMachine infos (currentCash - price) state in
      (newVendingMachine, Right product)


chooseProductInfo :: ProductInfo -> Int -> [ProductInfo] -> Maybe ProductInfo
chooseProductInfo currentProduct currentCash infos =
  listToMaybe $ filter (\p -> p == currentProduct && isEnough currentCash p) infos

isEnough :: Int -> ProductInfo -> Bool
isEnough current (ProductInfo p i) = current > i

main :: IO ()
main = do
    let charged = charge initVendingMachine 10
    let withProduct = addProductInfo charged productInfo1
    let res = choose withProduct productInfo1
    print res