Project Euler Problems 1-3

even_fibonacci_sum.hs

-- Note: fib_inf generates an infinite list
fib_inf :: Int -> Int -> [Int]
fib_inf a b = a : fib_inf b (a+b)

-- Which is ok, since Haskell uses lazy evaluation
fib_to n = takeWhile (< n) (fib_inf 0 1)

is_even x = x `mod` 2 == 0
even_fib_sum = sum . (filter is_even) . fib_to

-- Boilerplate to print our result
main :: IO ()
main = do
	print (even_fib_sum 4000000)

largest_prime_factor.hs

-- Note: need to have the primes package installed (didn't want to write my own primes implementation xD)
import Data.Numbers.Primes

largest_prime_factor :: Int -> Int
largest_prime_factor n = lpf n 1 primes
	where lpf x m (p:ps)
		| x < p  = m
		| x >= p = case x `mod` p of
			0         -> lpf (x `quot` p) p ps
			otherwise -> lpf x m ps

main :: IO ()
main = do
	print (largest_prime_factor 600851475143)

sum_of_multiples.hs

sum_mult_35_below n = sum . (filter divby_3_or_5) $ [ 1 .. n ]
	where divby_3_or_5 x = (x `mod` 3 == 0) || (x `mod` 5 == 0)

main :: IO ()
main = do
	print (sum_mult_35_below 1000)