StdLib Extensions

[odersky]

This is a mantle issue to collect areas where the standard library should be retro-fitted to work better with Scala 3. Please add more requests in comments, or, if you have access rights, edit this PR description.

• derived in classes like Ordering. See https://contributors.scala-lang.org/t/derived-ordering-for-simple-enum/5243/10
• CanEqual given instances in classes like Option
• CanEqual requirements in methods like contains
• Inlined override of foreach in Range so that we finally get fast C-style for loops

[som-snytt]

inline tap and pipe extensions, with tap result this.type. (Arguably, anyone wanted it has already written their own, shout out to Ichoran.)

[bjornregnell]

Beginner students often struggle with versatile patch on Iterable (IndexedSeq) and it would be nice if there are simpler versions for the simpler use cases such as insert and delete, or perhaps insterOne, insertAll etc. that could delegate to patch.

[MateuszKubuszok]

If we are allowed to modify the code (to break bincompat, but keep sourcecompat, kinda), then:

• replace each implicit class in scala.Predef with extension
• make implicitly inline as well
• update conversions in jdk to use given Conversion instead of implicit def.

[odersky]

We will definitely not break backwards binary compatibility at this point.

[odersky]

inline tap and pipe extensions, with tap result this.type. (Arguably, anyone wanted it has already written their own, shout out to Ichoran.)

Why is inline important? And where should these extensions go? We already have tap and pipe in the library. It's somewhat annoying to add new ones elsewhere since we believe that the old signatures are broken. Looks like a mess, not sure how to address it.

[MateuszKubuszok]

If we are not breaking any compatibility (understandable) then there is very little that can be fixed this way: mostly adding some implicit evidence in such a way that evidence would be erased and not become a part of binary signature (didn't know it was even possible? is it?). Maybe OOTB witness here and there. What other changes are allowed?

[olhotak]

Option.fromNullable

[bishabosha]

Why is inline important? And where should these extensions go? We already have tap and pipe in the library. It's somewhat annoying to add new ones elsewhere since we believe that the old signatures are broken. Looks like a mess, not sure how to address it.

possibly replace the signature in the scala2-stdlib-tasty? otherwise the trick like with stdLibPatches.Predef

[JD557]

Inlined override of foreach in Range so that we finally get fast C-style for loops

I'm not sure if this viable now in Scala 3, but I think it might be viable to also consider changing foreach[U](f: T => U) to foreach(f: T => Unit), which would help with specialization in cases like range.

For example, consider the following very simple example:

  trait Collection[T] {
    def foreachOld[U](f: T => U): Unit
    def foreachNew(f: T => Unit): Unit
  }

  final case class BoxedInt(x: Int) extends Collection[Int] {
    def foreachOld[U](f: Int => U): Unit = f(x)
    def foreachNew(f: Int => Unit): Unit = f(x)
  }

Here's how the BoxedInt bytecode looks like:

  public <U> void foreachOld(scala.Function1<java.lang.Object, U>);
    Code:
       0: aload_1
       1: aload_0
       2: invokevirtual #75                 // Method x:()I
       5: invokestatic  #110                // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
       8: invokeinterface #129,  2          // InterfaceMethod scala/Function1.apply:(Ljava/lang/Object;)Ljava/lang/Object;
      13: pop
      14: return

  public void foreachNew(scala.Function1<java.lang.Object, scala.runtime.BoxedUnit>);
    Code:
       0: aload_1
       1: aload_0
       2: invokevirtual #75                 // Method x:()I
       5: invokeinterface #135,  2          // InterfaceMethod scala/Function1.apply$mcVI$sp:(I)V
      10: return

I believe this is the reason Range uses final override def foreach[@specialized(Unit) U](f: Int => U): Unit but, AFAIK, Scala 3 doesn't use @specialized anymore, so the optimization is lost.

Note that it's also possible to just do something like:

def foreach[U](f: T => U): Unit
@scala.annotation.targetName("foreachUnboxed")
def foreach(f: T => Unit): Unit

Which I think would not break binary compatibility.

Range does have other methods with manual specialization (like indexOf and lastIndexOf). From my tests in apache/pekko#1247 I believe that removing the generic parameter from such methods can also lead to a huge performance boost in some situations, but I imagine that the variance there can also be helpful.

[soronpo]

• replace each implicit class in scala.Predef with extension

Since extension methods are not as flexible as implicit class methods, I'm fully against this. We first need to try and solve https://contributors.scala-lang.org/t/relaxed-extension-methods-sip-54-are-not-relaxed-enough/6585

[OndrejSpanel]

Why is inline important?

My reasons:

• nicer, more natural callstacks when debugging or analyzing exceptions
• it is possible to use return from inside of inline functions

And if you mentioned fast C-style loops, I guess if anyone uses pipe / tap in inner performance critical loops, I guess there might be some performance concerns as well, but I did not meet such situation yet (or if I did, avoiding pipe is usually much simpler than avoiding for).