Fibonacci calculator in Haskell

Question

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.

• Code-Guru: What is your purpose behind implementing a naive fibonacci function? Are you familiar with its the limitations? Are you familiar with more efficient fibonacci algorithms?
• Code-Guru: The Haskell wiki has an article with many different Fibonacci implementations: wiki.haskell.org/The_Fibonacci_sequence

Answer 1

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