[ruby-core:95180] [Ruby master Feature#16182] Should `expr in a, b, c` be allowed or not?

[keystonelemur@gmail.com]

Issue #16182 has been updated by baweaver (Brandon Weaver).


I wonder if it would make sense to reverse this to be left-to-right (LTR) rather than right-to-left (RTL) to make it easier to parse.

I cannot think of another RTL syntax in Ruby at the moment, including the current `for ... in` statement:

```
for item in collection
```

A full example might be:

```
for a, b in { a: 1, b: 2 }
  p a, b
end
:a
1
:b
2
=> {:a=>1, :b=>2}
```

Of course this does not currently work with keyword arguments:

```
[2] pry(main)> for a: 1, b: 2 in [{ a: 1 }, { b: 2 }]
SyntaxError: unexpected ':', expecting '.' or &. or :: or '['
for a: 1, b: 2 in [{ a: 1 }, { b: 2...
     ^
[2] pry(main)> for a:, b: in [{ a: 1 }, { b: 2 }]
SyntaxError: unexpected tSYMBEG, expecting do or '{' or '('
for a:, b: in [{ a: 1 }, { b: 2 }]
```

What if we leveraged some of the current logic for parsing a `for ... in` statement to make single-line pattern matching into a LTR syntax? This may be a solution for the parsing difficulties, as well as build on the intuition of Ruby developers expecting LTR syntaxes naturally.

----------------------------------------
Feature #16182: Should `expr in a, b, c` be allowed or not?
https://bugs.ruby-lang.org/issues/16182#change-81813

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
In #15865, a new syntax `<expr> in <pattern>` was introduced.  By using this, we can write:

```
json = { foo: 1, bar: 2}

if json in { foo:, bar: }
  p [foo, bar] #=> [1, 2]
end
```

However, we cannot write:

```
p(json in { foo:, bar: }) #=> expected: true, actual: syntax error
```

This is because `<expr> in <pattern>` is an expression but not an argument.  For example, `foo(json in a, b, c)` is ambiguous: it is considered `foo((json in a), b, c)` and `foo((json in a, b, c))`.

What should we do?

1. Do nothing; we admit that it is a spec
2. Revert the feature
3. Disallow a pattern like `a, b, c` or `a:, b:, c:` in this one-line pattern matching syntax; we ask a user to write `json in [a, b, c]` or `json in {a:, b:, c:}`



-- 
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