[ruby-core:119916] [Ruby master Bug#20785] Should `a in b, and c` `a in b, or c` `a in b, rescue c` be syntax ok?

13 Nov 2024

      Issue #20785 has been updated by Eregon (Benoit Daloze).

I think @Dan0042 has a point.
For instance if I'm using some longer variables names and the array/tuple has many elements it would be natural to wrap it into two lines, but IIUC this change would break it:
```ruby
if tuple in element_type, element_size, dimensions, pointer, offset, buffer_length, stride
  ...
end
# might be wrapped as:
if tuple in element_type, element_size, dimensions, pointer, offset,
    buffer_length, stride
  ...
end
```

In the OP description, I think all cases can be very easily made clear and unambiguous by appending `*`.
For example:
```ruby
tap do
  a in b, * and c
  a in b, * or c
  a in b, * rescue c
end

tap do
  a in b, *
end

tap do
  a in b, *
rescue
end
```

From that I *think* considering trailing comma for `in` pattern matching as SyntaxError is good, as it encourages clarity and removes syntactical ambiguity (at least from a human perception).

----------------------------------------
Bug #20785: Should `a in b, and c` `a in b, or c` `a in b, rescue c` be syntax ok?
https://bugs.ruby-lang.org/issues/20785#change-110637

* Author: tompng (tomoya ishida)
* Status: Assigned
* Assignee: matz (Yukihiro Matsumoto)
* ruby -v: ruby 3.4.0dev (2024-10-04T03:22:53Z master 939ec9f080) +YJIT +MN +PRISM [arm64-darwin22]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
This code is accepted in parse.y but rejected in prism
~~~ruby
tap do
  a in b, and c
  a in b, or c
  a in b, rescue c
end

# parsed as
tap do
  (a in b,;) and c
  (a in b,;) or c
  a in b,;
rescue c
end
~~~

I think these should be rejected like prism (parse.y accepts)
~~~ruby
a in b, and c

a in b,
and c

tap do
  a in b, rescue c
end
~~~

I think these should be accepted like parse.y (prism rejects)
~~~ruby
tap do
  a in b,
end

tap do
  a in b,
rescue
end
~~~

-- 
https://bugs.ruby-lang.org/