Hi Edward,
> -----Original message----- > From: Edward Kmett <[hidden email]> > Sent: 6 Aug 2013, 14:26 > > On Tue, Aug 6, 2013 at 2:09 PM, Milan Straka <[hidden email]> wrote: > > > 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: > > > 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. > > 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 > > 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 _______________________________________________ Libraries mailing list [hidden email] http://www.haskell.org/mailman/listinfo/libraries |
On Tue, Aug 6, 2013 at 4:19 PM, Milan Straka <[hidden email]> wrote:
Ick. I hadn't walked through the expansion of those by hands and had admittedly just hoped they worked out of the box. That means the similar combinators we have for them in lens probably also leak. =(
This indicates to me we may want to bite the bullet and move foldlM and foldrM into the Foldable class. As hideous as that is, the unmitigable space leak strikes me as worse.
Definitely. I'll talk to Shachaf and craft a suitable proposal to fix up traverse_. -Edward
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In reply to this post by Ryan Newton
Sounds neat! Is nested CPR analysis on a branch? Not yet.. on my laptop only so far. I’ve lost context on this traverseWithKey_ thread. If you or Milan need my help, could you start again, stating the problem with
a standalone test case? I gather from Milan that may there isn’t a problem, or at least not a problem that GHC can reasonably solve. If there’s nothing to follow up, no need to reply. Thanks Simon From: Ryan Newton [mailto:[hidden email]]
On Sat, Aug 3, 2013 at 5:31 PM, Simon Peyton-Jones <[hidden email]> wrote: > Fixing this involves *nested* CPR analysis, which I am working on at the moment. Sounds neat! Is nested CPR analysis on a branch?
Milan pasted the relevant code (thanks) for traverseWithKey_ vs. foldWithKey which I reattach at the bottom of this email. On Sun, Aug 4, 2013 at 7:48 AM, Sjoerd Visscher <[hidden email]> wrote:
That sounds good to me! And straightforward. Any objections? Milan, why does it bother you that there is no specified order? I'm perfectly happy as long as its deterministic on a particular machine. (I have never been sure whether pure code in Haskell must be deterministic across multiple machines...
numCapabilities was a counter example for a long time.) Aren't we already used to using Data.Map.toList and Data.Set.toList where order is not specified? Further, other languages (like Scheme) have maps/folds that do not specify order of side effects. -Ryan P.S.: Relevant code: traverseWithKey_ :: Applicative t => (k -> a -> t b) -> Map k a -> t ()
and we call them like this: _______________________________________________ Libraries mailing list [hidden email] http://www.haskell.org/mailman/listinfo/libraries |
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