A couple different Fibonacci's

fib.hs

import Data.Function.Memoize

-- SLOW
fib :: Int -> Integer
fib 0 = 0
fib 1 = 1
fib n = fib (n - 1) + fib (n - 2)

-- Memoized / FAST
fib' :: Int -> Integer
fib' = memoize h
    where h 0 = 0
          h 1 = 1
          h n = fib' (n - 1) + fib' (n - 2)

fib'' :: Int -> Integer
fib'' = (map h [0..] !!)
    where h 0 = 0
          h 1 = 1
          h n = fib'' (n - 1) + fib'' (n - 2)

main :: IO ()
main = putStrLn . show $ fib' 30000