Why is there no mapM for repa arrays?

Question

Background

I am using repa more as a "management" tool. I pass around reactive-bananas AddHandlers in an Array: Array D DIM2 (AddHandler Bool).

Currently I am using this kludge:

mapMArray :: (Monad m, R.Source r a, R.Shape sh)  => (a -> m b) -> Array r sh a -> m (Array D sh b)
mapMArray f a = do
    l <- mapM f . R.toList $ a
    return $ R.fromFunction sh (\i -> l !! R.toIndex sh i)
  where sh = R.extent a

So I can do something like this:

makeNetworkDesc :: Frameworks t => Array D DIM2 (AddHandler Bool) -> Moment t ()
makeNetworkDesc events = do

    -- inputs
    aes <- mapMArray fromAddHandler events

    -- outputs
    _ <- mapMArray (reactimate . (print <$>)) aes

Question

Is there a reason why this is not included in repa?

Chockfull

Answer 1

Basically for the same reason there's nothing like parMapM in parallel: mapM and mapM_ (or monadic actions in general) destroy parallelism. Here's a simple example:

next :: State Int Int
next = modify (+1) >> get

Now, a hypothetical repaMapM needs to sequence all steps in the State monad, if one would use repaMapM (const next). Since this clearly defies parallelism (and can also lead to low performance), it isn't part of repa. After all, high performance and parallelism is right there in repa's description (emphasis mine):

Repa provides high performance, regular, multi-dimensional, shape polymorphic parallel arrays.

Janinejanis

Hmm ... so as long as I don't need the parallelism, I should be fine. Otoh I seem to misuse repa. A cursory search on hackage provided no other matrix libraries that have stencil operations. Is there an alternative to repa in that regard? – Chockfull

If the three year old answer still hold, then no. There's accelerate, but that's a whole other beast. All in all it depends on what you want to do. The code you provided in your answer can be easily accomplished with Data.Vector(*), but in this case you need to write stencils yourself (also Data.Vector doesn't support multi-dimensional indices, but that's something you can easily fix with a wrapper and additional functions). – Janinejanis

Yeah ... I was hoping to get around that ;) – Chockfull

Couple years later and we now do have a library called massiv that has all of the desired features. It has sequential mapM for any Monad, it has mapIO that can be parallelized and it has much better implementation of stencils that work for arbitrary dimension. Overall it is more performant than Repa, easier to use than accelerate (and just as fast with llvm backend) – Harriott