eta-fibbo

fibbtcoinner 0 sum presum  = sum
fibbtcoinner n sum presum = fibbtcoinner  (n-1) (sum + presum) sum

fibbtco n = fibbtcoinner n 1 0

fibbnaive  0 = 1
fibbnaive  1 = 1
fibbnaive  n = fibbnaive ( n-1) + fibbnaive ( n - 2)

main = do
        putStrLn $ show $ fibbtco <$> arr
        putStrLn $ show $ fibbnaive <$> arr
        where
            arr = [0..10]

{- eta with -O0, -O1, -O2  produces
[1,1,2,4,8,16,32,64,128,256,512]
[1,1,2,3,5,8,13,21,34,55,89]
-}

{- GHC 7.10.3 with -O2 produces 
[1,1,2,3,5,8,13,21,34,55,89]
[1,1,2,3,5,8,13,21,34,55,89]
-}

{- interesting eta with -O0 (only!) and BangPatterns 
fibbtcoinner 0 sum presum  = sum
fibbtcoinner n !sum !presum = fibbtcoinner  (n-1) (sum + presum) sum

produces correct result
-}