Port Capture-checked Scala 2 collections #23769

natsukagami · 2025-08-16T19:12:08Z

Changes:

New final class collection.immutable.LazyListIterable[A], implements Iterable and IterableOps. LazyList and Stream are not capture-checked. Note added to LazyList.
New trait collection.StrictSeqOps[A, CC[_] <: Pure, C], which extends SeqOps (which can now be impure) and requires the implementation to be pure. Seq and other data structures extend this.
Similarly, collection.StrictIndexedSeqOps, collection.StrictMapOps, collection.StrictFactory.
- Note that collection.SetOps is pure by default. Maybe we want a StrictSetOps as well?
- Sorted versions of Ops (SortedSeq, SortedSet, SortedMap) are also pure by default.

Suspicious changes:

Might be suspicious: MapOps.keySet changes implementation to be always strict. This means it is less efficient, as it always copy the keys into a new list.
- StrictMapOps however overrides this behavior back to a lazy KeySet.
- Why? keySet returns a Set[K], which requires strictness. We cannot lie to the compiler about this case, as we can take a keySet out of an impure MapView, which may capture map/filter function bodies.
- In reality this should only affect code taking keysets of a MapView, or directly using MapOps (should be rare).

Not capture-checked (yet?)

Generated functions and tuples files. I tried turning CC on for functions and it hung the compiler. Maybe not a problem.
collection.convert.impl a.k.a Steppers, due to some purity problems (maybe because we don't compile steppers and rely on Scala 2, for specialization?).
LazyList and Streams.

…straint

Iterator and IterableWrappers are capturing, the rest stays pure-only

Specifically for Scala.js, which the compiler hasn't yet known that Array is pure.

hamzaremmal · 2025-08-16T19:31:40Z

library/src/scala/caps/Pure.scala

@@ -7,5 +7,5 @@ import language.experimental.captureChecking
 *  sense that their values retain no capabilities including capabilities needed
 *  to perform effects. This has formal meaning only under capture checking.
 */
-trait Pure:
+trait Pure extends Any:


I thought I left a comment about this somewhere but I'm glad it is a universal trait now.

bracevac · 2025-08-16T19:37:51Z

library/src/scala/collection/concurrent/TrieMap.scala

@@ -711,7 +713,7 @@ final class TrieMap[K, V] private (r: AnyRef, rtupd: AtomicReferenceFieldUpdater

  def this() = this(Hashing.default, Equiv.universal)

-  override def mapFactory: MapFactory[TrieMap] = TrieMap
+  override def mapFactory: StrictMapFactory[TrieMap] = TrieMap


It might be a good idea if we created a changelog?

bracevac · 2025-08-16T19:40:01Z

library/src/scala/collection/convert/AsScalaConverters.scala

@@ -91,7 +94,7 @@ trait AsScalaConverters {
   */
  def asScala[A](c: ju.Collection[A]): Iterable[A] = c match {
    case null                            => null
-    case wrapper: IterableWrapper[A @uc] => wrapper.underlying
+    case wrapper: IterableWrapper[A @uc] => wrapper.underlying.unsafeAssumePure // TODO: Remove when pattern matching recognizes this is safe


Do we have an issue ticket for those?

library/src/scala/collection/immutable/WrappedString.scala

Also, provide a LazyKeySet implementation when the Map is strict.

natsukagami · 2025-08-17T00:35:09Z

CC test is failing because scala2-library-cc doesn't have PartialFunctions capture-checked. @hamzaremmal can we move the test to use the unified lib instead?

We can also just disable it for now, since it only test scala2-library-cc which we will soon remove after this.

hamzaremmal · 2025-08-17T09:15:10Z

CC test is failing because scala2-library-cc doesn't have PartialFunctions capture-checked. @hamzaremmal can we move the test to use the unified lib instead?

We can also just disable it for now, since it only test scala2-library-cc which we will soon remove after this.

I will see what I can do this morning.

The compiler can properly figure out the return types based on the Pure upper bound on CC

natsukagami added 30 commits August 16, 2025 21:10

Extend TODO with Scala 2

ac2ffaa

Add capture checking to scala.Array

40f875f

Copy generator implementations from scala 2.13.17

0e37625

Add Scala 2.13 import to generated files to match current source

00dc9d3

Rename cardinal to integer

6372132

Make opType documentation and code clearer

88ee08c

Typo and style changes

9070166

Add documentation and simplify some implicit classes

0c31c4a

Simplify some generation logic of GenerateFunctionConverters

fe0c8ca

Style changes in GenerateLibraryNTemplates

f00abf8

Drop self type for FunctionN

2152a4d

Add capture checking to PartialFunction

7aca413

Add captures to ArrayOps

ce268f3

Add captures to BitSet

300ee39

Add capture checking to BufferedIterator

426b0c7

Add capture checking to BuildFrom

498f156

First pass thru IterableOnce

c235ee2

First pass to Iterable

2e2393a

Iterators are compiling

20a38bb

Add this capture to Iterator methods

83c3d01

Add capture checking to View

117b9ab

DO NOT MERGE remove warnings when building stdlib

3ac001a

Split IterableFactory to a strict version, with a pure collection con…

ba6167f

…straint

Capture-check Seq and IndexedSeq

db84885

Capture check LazyZipOps, LinearSeq, Searching

b3f868a

Capture-check stepper

9993dd6

Capture-check StrictOptimizedOps for Seq and Iterable

71a45e3

Capture check StringOps and StringParsers

919a59d

Capture-check WithFilter

48044bc

Capture-check Set and SortedSet

a864d71

natsukagami added 15 commits August 16, 2025 21:11

Capture-check concurrent files

7d83add

Capture-check converters

6121ae2

Iterator and IterableWrappers are capturing, the rest stays pure-only

Capture-check collection/generic

6214347

Capture-check AsJavaConverters

468e05c

Capture-check collection package

ebd6887

Fix partial function test

8f2fd14

Fix wrong casting to Nothing

c8dd681

Map's KeySet has to be pure, so we make it strict

9fd6bb1

Removal of extra captures

f5f5b94

For some reason Pure has to extend Any

07547f4

Add a couple missing hats

cb2fd3b

Remove generators

ab8cde1

Undo build changes

64c3cd0

Remove TODO

7d99e9f

Emit StrictFactory for arrays and strings

b94bf42

Specifically for Scala.js, which the compiler hasn't yet known that Array is pure.

natsukagami requested a review from bracevac August 16, 2025 19:13

Fix new errors stemming from pattern matching working ;)

6540914

hamzaremmal reviewed Aug 16, 2025

View reviewed changes

bracevac approved these changes Aug 16, 2025

View reviewed changes

natsukagami added 2 commits August 16, 2025 22:27

Remove the TODO on WrappedString

703cceb

Properly _not_ capture *this* inside the KeySet implementation

783ea88

Also, provide a LazyKeySet implementation when the Map is strict.

hamzaremmal assigned natsukagami and hamzaremmal Aug 17, 2025

This was referenced Aug 17, 2025

Port scala/scala#10437 to scala2-library-cc #23383

Closed

Capture check concurrent under the new scheme #19014

Open

Drop strict overrides from StrictMapOps and StrictSeqOps

98e7b80

The compiler can properly figure out the return types based on the Pure upper bound on CC

natsukagami force-pushed the port-scala2-cc branch from d67f99c to 98e7b80 Compare August 18, 2025 15:58

bracevac approved these changes Aug 18, 2025

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Port Capture-checked Scala 2 collections #23769

Port Capture-checked Scala 2 collections #23769

natsukagami commented Aug 16, 2025 •

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natsukagami commented Aug 17, 2025 •

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Port Capture-checked Scala 2 collections #23769

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Port Capture-checked Scala 2 collections #23769

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Changes:

Suspicious changes:

Not capture-checked (yet?)

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hamzaremmal Aug 16, 2025

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bracevac Aug 16, 2025

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natsukagami commented Aug 17, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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natsukagami commented Aug 16, 2025 •

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natsukagami commented Aug 17, 2025 •

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