This is just a short notice that using

   foldl' (+) 0 [0..100000::Int]

is over 10 times slower than using

  flip execState 0 $
    forLoop (0 :: Int) (< n) (+1) $ \i -> do
      x <- get
      put $! x + i

with `loopFor` as on https://github.com/nh2/loop.

Even using an IORef is twice as fast as the pure foldl' (but still 5
times slower than strict State).

The benchmark is at
http://htmlpreview.github.io/?https://github.com/nh2/loop/blob/master/results/bench-foldl-and-iorefs-are-slow.html.

(All benchmarks are linked from https://github.com/nh2/loop.)

You can see there that the problem is gone when using Vector.foldl', but
only for Int - for Word32 it persists.

It seems that manual looping is beneficial even when dealing with prime
examples of pure code that GHC ought to optimize well.

Niklas
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