Trailing commas in function bodies and constructors

[poslegm]

Hello! Scala compiler allows trailing comma in the lambda body, function body and constructors. Is it bug or correct behavior? Looks a little weird.

In a SIP-27 I've read:

Following Dr. Martin Odersky’s suggestion, the proposal is that trailing commas are only supported in comma-separated elements that are enclosed by parentheses, square brackets or curly braces (), ], and }, respectively).

Is function bodies comma-separated elements?

reproduction steps

List(1, 2, 3).map {
  a => a + 1,
}

class A() {
  println("a"),
}

def b() = {
  println("b"),
}

new A()
b()

problem

Code compiles

expectation

Code doesn't compile

Related scalameta/scalafmt#1410 (comment)

[som-snytt]

I just noticed this, too, and also consulted the docs and commit. The implementation is just in the scanner: COMMA, NL, RBRACE will ignore the COMMA. That is as opposed to emitting an IGNORABLE_COMMA which the parser could selectively ignore.

There was a recent fix for type T, which is tricky to observe in Scala 2, but Scala 3 still accepts a top-level type T, at EOF. (PR forthcoming.)

[som-snytt]

Quick acknowledgement of @dwijnand test case:

def f: (
  Int,
  String,
)

which I think will happen never, even in the universe where the monkeys type out the complete works of Shakespeare AND Borges's story about Pierre Menard AND Menard's complete works.

[neko-kai]

@som-snytt ur wrong that's my code

[som-snytt]

Edit: I guess the use case is: returning TupleN, just not sure yet what N is.

Re-edit: doti proposes (,) syntax for EmptyTuple which is not the Unit value. So for N < 2 this is a real question.

[OlegYch]

i claim that code as well

[Jasper-M]

It was my code

[SethTisue]

I'm Spartacus!

[som-snytt]

Now I'm not sure if I'm in the universe where the monkeys type comments responding to what the monkeys typed in the other universe.