Lazy foldM for "Iterables" #1414

peterneyens · 2016-10-19T09:19:15Z

If I am understanding correctly, now that FlatMap has tailRecM we can revisit #1030.

I adapted an old commit by @ceedubs for a PR to add MonadRec by @TomasMikula.

Resolves typelevel#1030, though it may be desirable change the default implementation to be lazy in the future. This can be done with `FreeT` but as of now I'm not sure how to do it without.

codecov-io · 2016-10-19T10:26:03Z

Current coverage is 91.67% (diff: 100%)

Merging #1414 into master will decrease coverage by <.01%

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  Lines          3608       3617     +9   
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- Misses          300        301     +1   
  Partials          0          0

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TomasMikula · 2016-10-19T15:20:40Z

Some structures, such as Vector or a balanced tree, allocate new memory in tail, but usually not in Iterator.next. Thus I believe that an implementation that uses an iterator instead of repeatedly calling tail can be significantly more efficient in some cases.

kailuowang · 2016-10-21T15:05:21Z

core/src/main/scala/cats/Foldable.scala

@@ -372,4 +378,12 @@ object Foldable {
      Eval.defer(if (it.hasNext) f(it.next, loop()) else lb)
    loop()
  }
+
+  def iteratorFoldM[M[_], A, B](it: Iterator[A], z: B)(f: (B, A) => M[B])(implicit M: Monad[M]): M[B] = {


How about we also mention here in the doc the incentive of this special implementation for Iteratables?

non · 2016-10-21T19:34:56Z

The code seems good to me. 👍 Thanks!

TomasMikula · 2016-10-21T19:39:28Z

core/src/main/scala/cats/Foldable.scala

+  def iteratorFoldM[M[_], A, B](it: Iterator[A], z: B)(f: (B, A) => M[B])(implicit M: Monad[M]): M[B] = {
+    val go: ((B, Iterator[A])) => M[Either[(B, Iterator[A]), B]] = { case (b, it) =>
+      if (it.hasNext) M.map(f(b, it.next))(b1 => Left((b1, it)))
+      else M.pure(Right(b))


Since Iterator is mutable, I would keep it private to the method (otherwise we lose referential transparency). I would keep the signature as you had before, and only use Iterator inside.

Sorry if this was not clear from my previous comment.

TomasMikula · 2016-10-21T19:44:24Z

Not sure what I did wrong that in this conversation it shows my newest comment as outdated. I will repeat here:

Since Iterator is mutable, I would keep it private to the method (otherwise we lose referential transparency). I would keep the signature as you had before, and only use Iterator inside.

Sorry if this was not clear from my previous comment.

non · 2016-10-21T19:45:16Z

@TomasMikula in the past we've exposed other methods using Iterator to make it easier for third-parties to build their instances (e.g. iterateRight). We could have the method take something like () => Iterator[A] if that makes it seem better.

TomasMikula · 2016-10-21T19:50:32Z

Well, in that case, ship it!

TomasMikula · 2016-10-21T19:51:37Z

Another, perhaps premature, optimization would be to avoid allocating the tuple in each step by accessing the iterator from the outer scope instead of passing it along.

TomasMikula · 2016-10-21T22:24:20Z

LGTM 👍

kailuowang · 2016-10-22T00:34:11Z

👍 thanks very much!

johnynek · 2016-10-25T18:12:05Z

Yeah, I'm concerned everything except List will be slower here.
On Wed, Oct 19, 2016 at 05:20 Tomas Mikula [email protected] wrote:

Some structures, such as Vector or a balanced tree, allocate new memory
in tail, but usually not in Iterator.next. Thus I believe that an
implementation that uses an iterator instead of repeatedly calling tail
can be significantly more efficient in some cases.

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ceedubs and others added 2 commits October 19, 2016 11:15

Add foldMRec

2cc1122

Resolves typelevel#1030, though it may be desirable change the default implementation to be lazy in the future. This can be done with `FreeT` but as of now I'm not sure how to do it without.

Lazy foldM for "Iterables" after FlatMap.tailRec.

6bc8f58

stew added the in progress label Oct 19, 2016

Use Iterator instead of Iterable for laxy foldM

52c3b7f

kailuowang reviewed Oct 21, 2016

View reviewed changes

TomasMikula reviewed Oct 21, 2016

View reviewed changes

Document and optimize Foldable.iteratorFoldM

6f3da09

peterneyens force-pushed the lazy-foldM branch from 381aa3d to 6f3da09 Compare October 21, 2016 22:24

kailuowang merged commit f90245d into typelevel:master Oct 22, 2016

peterneyens removed the in progress label Oct 22, 2016

peterneyens deleted the lazy-foldM branch October 22, 2016 00:45

peterneyens mentioned this pull request Jun 3, 2017

Foldable instance for List breaks substitution #1716

Closed

kailuowang mentioned this pull request Jun 23, 2017

foldLeftM without Free. #1117

Merged

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Lazy foldM for "Iterables" #1414

Lazy foldM for "Iterables" #1414

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Lazy foldM for "Iterables" #1414

Lazy foldM for "Iterables" #1414

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peterneyens commented Oct 19, 2016

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