Simple fibonacci generator

fib_comp.hs

import System.Environment

fib_gen = 0 : 1 : [a + b | (a, b) <- zip fib_gen (tail fib_gen)]

main = do
  args <- getArgs
  let first_arg = if null args then "1" else head args
  let fa = read first_arg :: Int
  print $ last $ take fa fib_gen