Re: Proposal: Non-allocating way to iterate over a Data.Map: traverseWithKey_

Hi Edward,

-----Original message----- From: Edward Kmett Sent: 6 Aug 2013, 14:26

On Tue, Aug 6, 2013 at 2:09 PM, Milan Straka wrote:

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Hi Edward,I am not suggesting we should change the behaviour of existing functions and traverseWithKey_ should definitely use the same order as traverseWithKey. Changing semantics without changing type signatures is really suspicious and usually plainly wrong.

I wholeheartedly agree. =) I was just basing that on the code Ryan posted:

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traverseWithKey_ f = go where go Tip = pure () go (Bin _ k v l r) = f k v *> go l *> go r

... which visits the key/value pairs out of order unlike, say:

go (Bin _ k v l r = go l *> f k v *> go r

Oh, yes, we will definitely use the definition you suggest.

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Nevertheless, I was wondering whether we should have a monadic fold (foldrM and foldlM) which would process the elements in a given order (ascending and descending, analogously to foldr and foldl). From one point of view, we can implement foldrM and foldlM using foldr and foldl,

Sure, foldrM is typically implemented in terms of foldl and foldlM is typically implemented in terms of foldr.

Do the usual definitions like that leak on a Map?

It is difficult to say whether it is a 'leak'. These methods (they are the same as Foldable.foldrM and Foldable.foldlM) heap-allocate space linear in the size of the map (to create the closures). When implemented directly, they do not heap-allocate.

foldrM :: (Foldable t, Monad m) => (a -> b -> m b) -> b -> t a -> m bfoldrM f z0 xs = foldl f' return xs z0 where f' k x z = f x z >>= k

foldlM :: (Foldable t, Monad m) => (a -> b -> m a) -> a -> t b -> m afoldlM f z0 xs = foldr f' return xs z0 where f' x k z = f z x >>= k

nevertheless using linear heap space complexity compared to constant

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heap space complexity we can achieve with specialized implementations. This is the same situation as traverseWithKey_ -- we can implement it using traverseWithKey, but the heap space complexity increases.

traverseWithKey_ would normally be implemented with an appropriate newtype and foldMapWithKey, rather than traverseWithKey. Does that also leak?

That does not leak, as Shachaf Ben-Kiki pointed out. That is one of the reasons why this discussion is so long :) BTW, Foldable.traverse_ also heap-allocates space linear in the size of the map, because it is defined as traverse_ :: (Foldable t, Applicative f) => (a -> f b) -> t a -> f () traverse_ f = foldr ((*>) . f) (pure ()) Maybe it would be better to define it using foldMap + the appropriate newtype? Then it would not heap-allocate, at least for Data.Map. Cheers, Milan