foolingAround.hs

by6 :: Int -> [Int]
by6 n = [(6*n)-1,(6*n)+1]

genPrimeCandidates :: Int -> [Int]
genPrimeCandidates n = (by6 n) ++ genPrimeCandidates (n+1)

primeCandidates :: [Int]
primeCandidates = [2,3] ++ (genPrimeCandidates 1)

primeCandidatesBelow :: Int -> [Int]
primeCandidatesBelow n = takeWhile (<n) primeCandidates

divisibleBy :: Int -> Int -> Bool
divisibleBy x y = (rem x y) == 0

closestIntegerSqrt = floor . sqrt .fromIntegral

isPrime :: Int -> Bool
isPrime x =
  let t = closestIntegerSqrt x
      xNotDivisibleBy = not . (divisibleBy x)
  in all xNotDivisibleBy (primeCandidatesBelow t)

primes :: [Int]
primes = filter isPrime primeCandidates

fibo :: [Int]
fibo =
  let fibo' :: Int -> Int -> [Int]
      fibo' a b = a:(fibo' b (a+b))
  in (fibo' 0 1)

nthFibo = last . (flip take fibo)