Bug #18883: parse.y: trailing comma cannot coexist with star

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
• Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN

[qnighy (Masaki Hara)]

The following code is a syntax error:

*x, y, = 1, 2 # syntax error, unexpected '='

although the following:

x, y = 1, 2  # OK
x, y, = 1, 2 # OK
*x, y = 1, 2 # OK
# *x, y, = 1, 2

In my understanding, the trailing comma does nothing unless the lhs becomes otherwise a single assignment.

Therefore it is natural to allow the trailing comma whether * exists or not.

[zverok (Victor Shepelev)]

As far as I understand, trailing comma is an equivalent of *_ (unpack the rest and drop it):

x, = [1, 2, 3]
# is the same as
x, *_ = [1, 2, 3]
# put 1 in x, ignore the rest, it is the shortest way to do it

So the fact that it is prohibited syntactically is in line with the fact that *x, y, *z = ... are also prohibited: Ruby can't handle two unpacks in one assignment.

If this would be allowed, what would be the semantics of it?..

*x, y, = 1, 2, 3
*x, y, *z = 1, 2, 3

What it should put in x and y?..

[jeremyevans0 (Jeremy Evans)]

zverok (Victor Shepelev) wrote in #note-1:

As far as I understand, trailing comma is an equivalent of *_ (unpack the rest and drop it):

x, = [1, 2, 3]
# is the same as
x, *_ = [1, 2, 3]
# put 1 in x, ignore the rest, it is the shortest way to do it

Close. *_ will set a local variable, and allocate an extra array. x, = [1, 2, 3] is actually the same as x, * = [1, 2, 3] (same VM instructions). However, this doesn't fully explain the issue, because x = (single assignment) and x, = (multiple assignment) are significantly different. It would be better to compare with:

x, a = [1, 2, 3]
x, a, = [1, 2, 3]
x, a, * = [1, 2, 3]

All of these result in the same VM instructions, indicating that a trailing comma shouldn't make a difference.

That being said, I don't think this is a bug, but I wouldn't be opposed to modifying the parser to support this as a feature, assuming it is doable.

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Rejected

After giving this some more thought, I am fairly sure this is not a bug, and I'm rejecting this. @zverok (Victor Shepelev) is correct, a trailing comma is an implicit splat, and you cannot have multiple splats in a multiple assignment, so a syntax error is the correct behavior in this case.