example.hs

primes :: [Integer]
primes = 2 : 3 : filter isPrime [4..]

isPrime :: Integer -> Bool
isPrime x = all (doesNotDivide x) (takeWhile (<= lb) primes)
  where
  lb = floor (sqrt (fromIntegral x :: Double))

doesNotDivide :: Integer -> Integer -> Bool
doesNotDivide x y = (x `mod` y) /= 0

fibs = 1 : 1 : zipWith (+) fibs (tail fibs)

data BinaryTree x = Value x | Branch (BinaryTree x) (BinaryTree x)
                deriving (Show)

prettyTree n (Value v)    = putStrLn (replicate n ' ' ++ show v)
prettyTree n (Branch l r) = do
  putStrLn (replicate n ' ' ++ "<l>")
  prettyTree (n + 1) l
  putStrLn (replicate n ' ' ++ "<r>")
  prettyTree (n + 1) r