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Bug #18883

open

parse.y: trailing comma cannot coexist with star

Added by qnighy (Masaki Hara) about 1 month ago. Updated 24 days ago.

Status:
Open
Priority:
Normal
Assignee:
-
Target version:
-
ruby -v:
ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
[ruby-core:109077]

Description

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.

Updated by zverok (Victor Shepelev) about 1 month ago

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?..

Updated by jeremyevans0 (Jeremy Evans) 24 days ago

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.

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