Tidying up the example at http://www.lshift.net/blog/2013/06/30/tail-calls-vs-laziness

gistfile1.hs

import qualified Data.Vector as V
import qualified System.Random as R
sample :: R.RandomGen g => g -> Int -> [a] -> V.Vector a
sample r n l = go r vec0 (succ n) remainder where
    vec0 = V.fromList beginning
    (beginning, remainder) = splitAt n l
    go _ v _ [] = v
    go r v i (x:xs) = v `seq` go r'' updated (succ i) xs
      where
        updated | p < n = v V.// upd
                | otherwise = v
        upd = [(idx, x)]
        (p, r') = rand0    where rand0 = R.randomR (0, i-1) r
        (idx, r'') = rand1 where rand1 = R.randomR (0, n-1) r'