#!/usr/bin/runghc

module Recursion09Fibonacci where

-- This version is not efficient.
-- It re-computes values it already computed
fibRec :: Int -> Integer
fibRec 0 = 0
fibRec 1 = 1
fibRec n = fibRec (n - 1) + fibRec (n - 2)

-- The versions below are efficient:

fibTail :: Integer -> Integer -> Integer -> Integer -> Integer
fibTail n previous current i
  | i < n = fibTail n current (previous + current) (i + 1)
  | otherwise = previous

fibWrapExternal :: Integer -> Integer
fibWrapExternal n = fibTail n 0 1 0

fibWrapInternal :: Integer -> Integer
fibWrapInternal n =
  let go n previous current i =
        if i < n
          then fibTail n current (previous + current) (i + 1)
          else previous
   in go 10 0 1 0

main :: IO ()
main = do
  print (fibRec 10)
  print (fibTail 10 0 1 0)
  print (fibWrapExternal 10)
  print (fibWrapInternal 10)