FP Test3 2025

Home Preparation For Test 3 - 2025

(Theme: Vending Machine)

Data types and examples

Let's have a data types representing the state of a vending machine:

• a type Stock for the available ingredients (name and quantity),
• a type Drink with the drink’s name, price, and list of required ingredients,
• a type Machine that combines both stock and available drinks.

type Stock = [(String, Int)]
data Drink = Drink { name :: String , price :: Int , ingredients :: [String] } deriving Show
data Machine = Machine {stock :: Stock, drinks :: [Drink] } deriving Show

stockExample :: Stock
stockExample = [ ("water", 10) , ("coffee", 4) , ("milk", 2) , ("tea", 3) , ("sugar", 5) , ("cocoa", 1) ]

drinksList :: [Drink]
drinksList = [ 
    Drink "Coffee" 30 ["water","coffee"],
    Drink "StrongCoffee" 40 ["water","coffee","coffee"],
    Drink "Tea" 25 ["water", "water","tea"],
    Drink "Cappuccino" 45 ["water","coffee","milk"],
    Drink "Latte" 50 ["water","coffee","milk","sugar"],
    Drink "Chocolate" 50 ["water","cocoa","milk"],
    Drink "Cocoa" 60 ["milk","cocoa","milk"]
    ]

machineExample :: Machine
machineExample = Machine stockExample drinksList

Tasks

Create functions that:

• Implement a function findDrink that finds a drink by name from a list.

If the drink does not exist, raise an error "No such drink".

findDrink :: [Drink] -> String -> Drink

ghci> findDrink drinksList "Tea"  
Drink {name = "Tea", price = 25, ingredients = ["water","water","tea"]}

• Implement a function inStock that checks whether a given ingredient is currently available in stock.

inStock :: Stock -> String -> Bool

ghci> inStock stockExample "milk"
True
ghci> inStock stockExample "juice"
False

• Implement a function useIngredient that decreases the quantity of a given ingredient by one unit.

Ingredients with zero quantity should be removed from the stock.

useIngredient :: Stock -> String -> Stock

ghci> useIngredient  stockExample "milk"
[("water",10),("coffee",4),("milk",1),("tea",3),("sugar",5),("cocoa",1)]
ghci> useIngredient  stockExample "cocoa"
[("water",10),("coffee",4),("milk",2),("tea",3),("sugar",5)]

• Implement a function canServeDrink that checks whether all ingredients required for a drink are available.

canServeDrink :: Stock -> Drink -> Bool

ghci> canServeDrink stockExample (findDrink drinksList "Cocoa")
True

• Implement a function serveDrink that updates the stock after preparing a drink.

If some ingredient is missing, raise an error "Can not serve this drink".

serveDrink :: Stock -> Drink -> Stock

ghci> serveDrink stockExample (findDrink drinksList "Cocoa")   
[("water",10),("coffee",4),("tea",3),("sugar",5)]

• Implement a function processOrders that processes a list of orders (drink names) in sequence. It returns:
  • Just a new machine with the updated stock if all drinks can be served, or
  • Nothing if any of the orders cannot be fulfilled.

processOrders :: Machine -> [String] -> Maybe Machine

ghci> processOrders machineExample ["Coffee","Tea","Coffee","Chocolate"]
Just (Machine {stock = [("water",5),("coffee",2),("milk",1),("tea",2),("sugar",5)], drinks = [ {- skiping list of drinks -} ]})

• Implement a function processPayment that processes orders similarly but returns the total price of all successfully served drinks,

or Nothing if any order fails.

processPayment :: Machine -> [String] -> Maybe Int

ghci> processPayment  machineExample ["Coffee","Tea","Coffee","Chocolate"]
Just 135

• Implement a function bestValue that that finds the best possible subsequence of orders maximizing the total profit.

Hint: You can generate all possible order sequences,

Use processPayment to evaluate each sequence.

Return the sequence that yields the highest total payment.

If no sequence can be completed successfully, return an empty list.

processPayment :: Machine -> [String] -> Maybe Int

ghci> processPayment  machineExample ["Coffee","Tea","Coffee","Chocolate"]
Just 135