Tukukkk

I'm just starting to look into Haskell. I've written a naive Fibonacci implementation, and I've also written a more advanced one that uses tail-call recursion for efficiency.

module Fibonacci where



import System.Environment



fibonacci :: Integer -> Integer

fibonacci 0 = 0

fibonacci 1 = 1

fibonacci n

    | n < 0 = error "Cannot find a negative fibonacci number"

    | otherwise = fibonacci (n - 1) + fibonacci (n - 2)



fibonacci' :: Integer -> Integer

fibonacci' n

    | n < 0 = error "Cannot find a negative fibonacci number"

    | otherwise = fibHelper n 0 1

    where

        fibHelper :: Integer -> Integer -> Integer -> Integer

        fibHelper n a b

            | n == 0 = a

            | otherwise = fibHelper (n - 1) b (a + b)



firstNumberFrom :: [String] -> Integer

firstNumberFrom  = 10

firstNumberFrom args = read $ args !! 0



main = do

    args <- getArgs

    let num = firstNumberFrom args in

        putStrLn $ show (fibonacci' num)

I'd appreciate any reviews on correctness and idiomatic usage.

The many approaches in main and firstNumberFrom can be unified:

main = print . fibonacci' . maybe 10 read . listToMaybe =<< getArgs

The explicit recursion in fibbonacci' is captured by iterate:

fibbonacci' n = fst $ iterate ((a,b) -> (b, a+b)) (0,1) !! n