QuickSort in Haskell

quicksort.hs

-- code from https://play.haskell.org/ with my own commentary
-- import one function from one module
import Data.List (partition)

-- main is the entry point for I/O side effects and lets you prefer to have imperative programming with the "do" operator
main :: IO ()
main = do
  let unsorted = [10,9..1]
  -- $ means group the rest of the line as function ars together (like a left paren)
  putStrLn $ show $ quicksort unsorted

-- you declare a type signature once, with generic type constraints between :: and =>
quicksort :: Ord a => [a] -> [a]
-- you can declare implementations for different signatures separately, which lets us declare the recursive alogorithm's base case without an explicit conditional
quicksort []     = []
-- x is the first list item and xs gets the rest (similar to car and cdr in LISP, or first and rest in modern languages)
quicksort (x:xs) = let (lesser, greater) = partition (<= x) xs
                   in quicksort lesser ++ [x] ++ quicksort greater