What is the correct way to perform constant-space nested loops in Haskell?

Question

There are two obvious, \"idiomatic\" ways to perform nested loops in Haskell: using the list monad or using forM_ to replace traditional fors. I\'ve se

Answer 1

In my experience forM_ [0..n-1] can perform well, but unfortunately it's not reliable. Just adding an INLINE pragma to test_a and using -O2 makes it run much faster (4s to 1s for me), but manually inlining it (copy paste) slows it down again.

A more reliable function is is for from statistics which is implemented as

-- | Simple for loop.  Counts from /start/ to /end/-1.
for :: Monad m => Int -> Int -> (Int -> m ()) -> m ()
for n0 !n f = loop n0
  where
    loop i | i == n    = return ()
           | otherwise = f i >> loop (i+1)
{-# INLINE for #-}

Using it looks similar to forM_ with lists:

test_d :: MV.IOVector Int -> IO ()
test_d mv =
  for 0 times $ \_ ->
    for 0 side $ \i ->
      for 0 side $ \j ->
        MV.unsafeWrite mv (i*side + j) 1

but performs reliably well (0.85s for me) without any risk of allocating a list.