In-class-2

-- Get the total sum of the items in the list
sumList :: [Int] -> Int
    -- Note: [Int] means a list of ints
-- This will resemble recurssion
sumList [] = 0 
sumList (x : xs) = x + sumList xs
    -- x is the first element of the list
    -- xs is the rest of the list
    -- Usage: ghci> sumList [1,2,3,...]

-- Get the double of the sum of the ints in an array
doubleSum :: [Int] -> Int
doubleSum [] = 0
doubleSum (x : xs) = sumList (x : xs) * 2

-- Get the sum of the even numbers in an int array
sumEvenNumbers :: [Int] -> Int
sumEvenNumbers [] = 0
sumEvenNumbers (x : xs) = sumList (filter (even) (x : xs))
    -- Note the use of the filter function and the 'even' keyword
    -- filter (> 5) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
        -- Output: [6, 7, 8, 9, 10]

-- Get the double of the sum of even numbers in an int array
sumDoubleEvenNumbers :: [Int] -> Int
sumDoubleEvenNumbers [] = 0
sumDoubleEvenNumbers (x : xs) = sumEvenNumbers (x : xs) * 2

main :: IO ()
main = do
  let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
  print (sumList numbers)               -- Output: 55
  print (doubleSum numbers)             -- Output: 110
  print (sumEvenNumbers numbers)        -- Output: 30
  print (sumDoubleEvenNumbers numbers)  -- Output: 60