Fibonacci Stream

Fib.hs

module Fib where

import qualified Data.Vector.Unboxed as V
import qualified Data.Vector.Fusion.Stream as S
import qualified Data.Vector.Fusion.Stream.Monadic as SM
import Data.Vector.Fusion.Stream.Size
import Data.Vector.Fusion.Util
{-
euler2 n = sum $ filter even $ takeWhile (<n) $ fibs
How can I write this and fuse into what's below?
-}
fibsStep :: (Int, Int) -> S.Step (Int,Int) Int
fibsStep (a,b) = S.Yield a (b, a + b)

fibs :: S.Stream Int
fibs = SM.Stream (Id . fibsStep) (1,2) Unknown

euler2 :: Int -> Int
euler2 n = S.foldl' (+) 0 $ S.filter even $ S.takeWhile (<n) fibs
{-
Yaay!!!
This compiles to what's below, a tight loop in cpu registers:
-}
{-
euler2 :: Int -> Int
euler2 n = go 0 1 2
           where go sum a b = if a < n
                              then
                                if even a
                                then go (sum + a) b (a + b)
                                else go sum       b (a + b)
                              else sum
-}
main :: IO ()
main = do
  t <- readLn
  ns <- V.replicateM t readLn
  V.forM_ ns $ \n ->
    print $ euler2 n