[Haskell-cafe] Floating phi, round and even Fibonnaci numbers

[haskell at list.mightyreason.com]

Brian L. Troutwine wrote:
>> phi :: (Floating t) => t
>> phi = (1+sqrt(5))/2
> 
> 
>> even_fibs :: (Num t) => [t]
>> even_fibs = iterate (\x -> round(x * (phi**3))) 2

*Main> :t iterate
iterate :: forall a. (a -> a) -> a -> [a]
*Main> :t round
round :: forall a b. (RealFrac a, Integral b) => a -> b

So the 'x' in your anonymous lambda must be (a->a) and the type 'a' must be an 
integral.

You need to convert this Integral 'x' into something that can be used in the 
math (x * (phi**3)) which is where you need to insert fromIntegral:

> even_fibs :: (Integral t) => [t]
> even_fibs = iterate (\x -> round(fromIntegral x * (phi**3))) 2
> 

Which of course can be tested against

> even_fibs_2 = filter even fibs
>   where fibs = 1 : 1 : zipWith (+) (fibs) (tail fibs)

And the phi version fails at

> head $ dropWhile (uncurry (==)) $ zip even_fibs even_fibs_2

Which is
(37889062373143904,37889062373143906)