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NthPortal merged 1 commit into from
Jul 2, 2020
Merged

Increase laziness of LazyList.cons.apply #9095

NthPortal merged 1 commit into from
Jul 2, 2020

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@NthPortal NthPortal commented Jul 1, 2020 •
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Change LazyList.cons.apply so that the tail computation
is wrapped as a state computation for a new LazyList,
thus deferring its execution as long as possible.

Follow-up to #8985

Before (Scala 2.13.3-):

scala 2.13.3> LazyList.cons(1, { println("hey"); LazyList() })
val res6: scala.collection.immutable.LazyList[Int] = LazyList(<not computed>)

scala 2.13.3> res6.head
hey
val res7: Int = 1

After (Scala 2.13.4+):

scala 2.13.4> LazyList.cons(1, { println("hey"); LazyList() })
val res6: scala.collection.immutable.LazyList[Int] = LazyList(<not computed>)

scala 2.13.4> res6.head
val res7: Int = 1

@NthPortal NthPortal added the library:collections PRs involving changes to the standard collection library label Jul 1, 2020
@NthPortal NthPortal requested review from SethTisue and julienrf July 1, 2020 03:06
@scala-jenkins scala-jenkins added this to the 2.13.4 milestone Jul 1, 2020
@SethTisue SethTisue added the release-notes worth highlighting in next release notes label Jul 1, 2020
@NthPortal NthPortal mentioned this pull request Jul 1, 2020
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NthPortal commented Jul 1, 2020
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test/junit/scala/collection/immutable/LazyListLazinessTest.scala
assertLazyAllSkipping(op, 4)
}

private def genericCons_unapply_properlyLazy(unapply: LazyList[Int] => Option[(Int, LazyList[Int])]): Unit = {
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I noticed there were no tests for these while I was in the area, so I added tests

@NthPortal NthPortal merged commit 9208bc4 into scala:2.13.x Jul 2, 2020
@NthPortal NthPortal deleted the topic/ll-again branch July 2, 2020 10:23
ijuma added a commit to ijuma/scala-library-compat that referenced this pull request Jul 5, 2020
@ijuma @NthPortal
Make LazyList.cons.apply lazier
c034ca2 
Port of scala/scala#9095.

Co-authored-by: NthPortal <[email protected]>
ijuma added a commit to ijuma/scala-library-compat that referenced this pull request Jul 5, 2020
@ijuma @NthPortal
Make LazyList.cons.apply lazier
187ebc8 
Port of scala/scala#9095.

Co-authored-by: NthPortal <[email protected]>
ijuma added a commit to ijuma/scala-library-compat that referenced this pull request Jul 5, 2020
@ijuma @NthPortal
Make LazyList.cons.apply lazier
5aa6003 
Port of scala/scala#9095.

Co-authored-by: NthPortal <[email protected]>
ijuma added a commit to ijuma/scala-library-compat that referenced this pull request Jul 5, 2020
@ijuma @NthPortal
Make LazyList.cons.apply lazier
c90f543 
Port of scala/scala#9095.

Co-authored-by: NthPortal <[email protected]>
ijuma added a commit to ijuma/scala-library-compat that referenced this pull request Aug 24, 2020
@ijuma @NthPortal
Make LazyList.cons.apply lazier
76ab5b6 
Port of scala/scala#9095.

Co-authored-by: NthPortal <[email protected]>
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SethTisue commented Nov 17, 2020 •
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@NthPortal would you mind improving the PR title and description to be at least a little more informative for end users? since we're linking to this from the 2.13.4 release notes

perhaps you can borrow some verbiage from the discussion on #8985

@NthPortal NthPortal changed the title Make LazyList.cons.apply lazier Increase laziness of LazyList.cons.apply Nov 20, 2020
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NthPortal commented Nov 20, 2020

@SethTisue is that okay?

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SethTisue commented Nov 20, 2020

@NthPortal better — if you want to improve it further, I would suggest adding an example code snippet that is affected by the change, and describe how it is affected. even though I've looked at this stuff multiple times before, when I see LazyList.cons.apply my reaction is still "huh? what's that?" and I have to go look it up. an example would really help, I think.

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som-snytt commented Nov 20, 2020

"If you're as lazy as LazyList, you probably don't need to know more."

Sometimes it helps to be usefully vague.

Do I need to learn this?

  • No.

hamzaremmal pushed a commit to hamzaremmal/scala3 that referenced this pull request May 2, 2025
@NthPortal
Merge pull request scala/scala#9095 from NthPortal/topic/ll-again
2c5fe0e 
Make LazyList.cons.apply lazier
hamzaremmal pushed a commit to scala/scala3 that referenced this pull request May 7, 2025
@NthPortal
Merge pull request scala/scala#9095 from NthPortal/topic/ll-again
3720b79 
Make LazyList.cons.apply lazier
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@SethTisue SethTisue SethTisue approved these changes

@julienrf julienrf julienrf approved these changes

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