Support java collections & streams in for-do loops by AugustNagro · Pull Request #15379 · scala/scala3

AugustNagro · 2022-06-05T15:43:12Z

Support java collections & streams in for-do loops by defining foreach extension methods in scala.Predef

Alternative to #15368

…h extension methods in scala.Predef

AugustNagro · 2022-06-05T17:02:11Z

I can reproduce the failure of tests/neg/i12640.scala on current main; it doesn't seem related to this PR

TheElectronWill · 2022-06-05T17:07:21Z

Yes this test seems to be flaky, see #15371 (comment)

dwijnand · 2022-06-07T17:40:08Z

Isn't this fixed with import scala.jdk.CollectionConverters.* and .asScala?

AugustNagro · 2022-06-07T17:54:15Z

It is certainly a hassle and hard for beginners.

Java collections already work fine with for-yield, but refactor to for-do and it suddenly requires an import and .asScala call.

som-snytt · 2022-06-07T19:22:39Z

scala> for x <- List.of(1,2,3) do println(x)
-- [E008] Not Found Error: ---------------------------------------------------------------------------------------------
1 |for x <- List.of(1,2,3) do println(x)
  |         ^^^^^^^^^^^^^^
  |         value foreach is not a member of java.util.List[Int] - did you mean java.util.List[Int].forEach?
1 error found

scala>

should be

scala> for x <- List.of(1,2,3) do println(x)
-- [E008] Not Found Error: ---------------------------------------------------------------------------------------------
1 |for x <- List.of(1,2,3) do println(x)
  |         ^^^^^^^^^^^^^^
  |         value foreach is not a member of java.util.List[Int] - did you mean import scala.jdk.CollectionConverters.*; List.of(1,2,3).asScala?
1 error found

scala>

dwijnand · 2022-06-08T08:05:39Z

It is certainly a hassle and hard for beginners.

Java collections already work fine with for-yield, but refactor to for-do and it suddenly requires an import and .asScala call.

Work fine without, how?

Welcome to Scala 3.2.0-RC1-bin-20220606-cec9aa3-NIGHTLY-git-cec9aa3 (11.0.15, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.

scala> for x <- java.util.List.of(1, 2, 3) yield x
-- [E008] Not Found Error: -----------------------------------------------------
1 |for x <- java.util.List.of(1, 2, 3) yield x
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |         value map is not a member of java.util.List[Int]
1 error found

scala> for x <- java.util.List.of(1, 2, 3); y <- List(x) yield y
-- [E008] Not Found Error: -----------------------------------------------------
1 |for x <- java.util.List.of(1, 2, 3); y <- List(x) yield y
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |value flatMap is not a member of java.util.List[Int], but could be made available as an extension method.
  |
  |One of the following imports might fix the problem:
  |
  |  import collection.convert.ImplicitConversions.list asScalaBuffer
  |  import collection.convert.ImplicitConversionsToScala.list asScalaBuffer
  |  import collection.convert.ImplicitConversions.collection AsScalaIterable
  |  import collection.convert.ImplicitConversionsToScala.collection AsScalaIterable
  |  import collection.convert.ImplicitConversions.iterable AsScalaIterable
  |  import collection.convert.ImplicitConversionsToScala.iterable AsScalaIterable
  |
1 error found

TheElectronWill · 2022-06-08T09:26:46Z

It does work with Stream because it provides map and flatMap.

scala> for x <- java.util.List.of(1).stream yield x
val res0: java.util.stream.Stream[Int] = java.util.stream.ReferencePipeline$3@6ac9b66b

scala> for x <- java.util.List.of(1).stream; y <- java.util.List.of(2).stream yield y
val res1: java.util.stream.Stream[Int] = java.util.stream.ReferencePipeline$7@129b0ed

AugustNagro · 2022-06-10T04:57:09Z

That's a good point @dwijnand

bishabosha · 2022-06-10T08:12:04Z

I guess that extensions could be added for map and flatMap by converting to a Scala collection and then back to a Java collection.

bishabosha · 2022-06-10T08:15:30Z

+    def foreach(f: java.util.function.Consumer[T]): Unit =
+      it.forEach(f)
+
+  extension [T](s: java.util.stream.Stream[T])


Should we also support IntStream and the other primitive streams?

It we could support BaseStream (javadoc), which is the common interface above Stream, IntStream and all, it would be even better :)
edit: actually I got carried away: BaseStream doesn't provide any meaningful operation on its elements...

I think it would be good to also support IntStream and the other primitive streams.

sjrd · 2022-06-10T08:41:50Z

I really don't think this has its place in Predef, of all things! Predef is for:

things that are essential to the language,
things for which requiring an import would make them totally useless (that's ???, and this one got in after a very long discussion of its merits)
some historical mistakes (manifests, printf).

There is no precedent for putting things in Predef that make using the JDK more convenient. There would need to be a very strong motivation to do so. Avoiding one import to use Java Streams in for do is far from making the cut, as far as I'm concerned.

AugustNagro · 2022-06-10T22:02:42Z

What if we make a type class like

trait ForLoopSyntax[F[_]]:
  def foreach[A](fa: F[A])(f: A => Unit): Unit
  def map[A, B](fa: F[A])(f: A => B): F[B]
  def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B]

And if the compiler can't resolve the desugared foreach, map, or flatMap call, then it tries to summon ForLoopSyntax.

The standard library could define some common givens, like for CompletableFuture and j.u.List.

TheElectronWill · 2022-06-11T08:36:58Z

Isn't that a bit "overkill"?
The extensions methods could just be moved somewhere else, like in the package scala.jdk. With an import, you'll be able to for-do and for-yield on java collections, without converting the collections with asScala.

odersky

I also think it's probably better to move this to some module that can be explicitly imported. Putting things in Predef is a big step. For instance, what should happen on the JS platform?

odersky · 2022-06-15T07:53:47Z

+    def foreach(f: java.util.function.Consumer[T]): Unit =
+      it.forEach(f)
+
+  extension [T](s: java.util.stream.Stream[T])


I think it would be good to also support IntStream and the other primitive streams.

odersky · 2022-06-15T07:54:37Z

+      it.forEach(f)
+
+  extension [T](s: java.util.stream.Stream[T])
+    def foreach(f: java.util.function.Consumer[T]): Unit =


If it is in Predef then foreach needs to be inline, since stdLibPatches/Predef.scala has no runtime equivalent. Or else we need a PR against 2.13 Predef instead.

support java collections & streams in for-do loops by defining foreac…

971c8bd

…h extension methods in scala.Predef

AugustNagro mentioned this pull request Jun 5, 2022

allow forEach as an alternative to foreach #15368

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bishabosha reviewed Jun 10, 2022

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odersky requested changes Jun 15, 2022

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dwijnand closed this Apr 24, 2023

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TheElectronWill commented Jun 5, 2022 •

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