Issue #20931 has been updated by alanwu (Alan Wu). Status changed from Open to Rejected I'm closing this since I'm pretty sure this isn't a bug. An imperfect explanation follows. Feel free to jump in if anyone has a better explanation. To understand the precedence, note that `in` has a symbolic friend `=>`, and much like how `or` binds lower than `||`, `in` binds lower than `=>`. (Runtime behavior of `=>` and `in` are different, though.) As for why it requires parentheses in argument context, it's consistent with other single word English keywords such as `and`, `or`, `if`, and `unless`: <details> ```shell $ for keyword in and or if unless; do ruby -vc -e "puts(1 $keyword 1)"; done ruby 3.4.0dev (2024-12-04T21:26:31Z master c0e12bf8d2) +PRISM [arm64-darwin24] ruby: -e:1: syntax errors found (SyntaxError)

1 | puts(1 and 1) | ^~~ unexpected 'and'; expected a `)` to close the arguments | ^ unexpected ')', ignoring it | ^ unexpected ')', expecting end-of-input 2 | ruby 3.4.0dev (2024-12-04T21:26:31Z master c0e12bf8d2) +PRISM [arm64-darwin24] ruby: -e:1: syntax errors found (SyntaxError) 1 | puts(1 or 1) | ^~ unexpected 'or'; expected a `)` to close the arguments | ^ unexpected ')', ignoring it | ^ unexpected ')', expecting end-of-input 2 | ruby 3.4.0dev (2024-12-04T21:26:31Z master c0e12bf8d2) +PRISM [arm64-darwin24] -e:1: warning: literal in condition ruby: -e:1: syntax errors found (SyntaxError) 1 | puts(1 if 1) | ^~ unexpected 'if'; expected a `)` to close the arguments | ^ unexpected ')', ignoring it | ^ unexpected ')', expecting end-of-input 2 | ruby 3.4.0dev (2024-12-04T21:26:31Z master c0e12bf8d2) +PRISM [arm64-darwin24] -e:1: warning: literal in condition ruby: -e:1: syntax errors found (SyntaxError) 1 | puts(1 unless 1) | ^~~~~~ unexpected 'unless'; expected a `)` to close the arguments | ^ unexpected ')', ignoring it | ^ unexpected ')', expecting end-of-input 2 |

</details>

Allowing these limit examples to work as expected probably causes parsing ambiguity in some other cases, so they're rejected. But I'm no parser expert.

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Bug #20931: Using `in` as an expression requires extra parentheses
https://bugs.ruby-lang.org/issues/20931#change-110855

* Author: stephenprater (Stephen Prater)
* Status: Rejected
* ruby -v: 3.3.1
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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TBH - I'm not sure if this is a bug or not - but it certainly surprising behavior and I'd at least like to understand it.

Given a hash t - that can be pattern matched: `t = {a: 1, b:1 }`

``` ruby
r = t in {a: 1, c:1 } # returns `false`
r # {a: 1, c: 1} wat

Presumably this is because `=` binds higher than `in` - so that expression is equivalent to `(r = t) in {a: 1, c: 1}` But in that case - why does using the results of `in` require an additional set of parentheses to avoid a syntax error when the result of the expression is used as an argument to a method? ``` ruby puts(t in {a: 1, c: 1}) # syntax error puts((t in {a: 1, c: 1}) # false ``` Especially since this works fine: ``` ruby puts(case t; in { a: 1, c:1 }; true; else false; end) ``` -- https://bugs.ruby-lang.org/

Issue #20931 has been updated by mame (Yusuke Endoh). As for the limitation of `in`, there is a more easy-to-understand explanation. Consider `foo(a in 1, 2, 3)`. This is very ambiguous because there are three possible interpretation: `foo((a in 1), 2, 3)`, `foo((a in 1, 2), 3)`, and `foo((a in 1, 2, 3))`. Note that `a in 1, 2, 3` returns true when `a = [1, 2, 3]`. So parentheses are necessary. I understand that it is confusing to need double parentheses when you use `in` as a simple expression without following commas. But no good solution came to mind. ---------------------------------------- Bug #20931: Using `in` as an expression requires extra parentheses https://bugs.ruby-lang.org/issues/20931#change-110857 * Author: stephenprater (Stephen Prater) * Status: Rejected * ruby -v: 3.3.1 * Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- TBH - I'm not sure if this is a bug or not - but it certainly surprising behavior and I'd at least like to understand it. Given a hash t - that can be pattern matched: `t = {a: 1, b:1 }` ``` ruby r = t in {a: 1, c:1 } # returns `false` r # {a: 1, c: 1} wat ``` Presumably this is because `=` binds higher than `in` - so that expression is equivalent to `(r = t) in {a: 1, c: 1}` But in that case - why does using the results of `in` require an additional set of parentheses to avoid a syntax error when the result of the expression is used as an argument to a method? ``` ruby puts(t in {a: 1, c: 1}) # syntax error puts((t in {a: 1, c: 1}) # false ``` Especially since this works fine: ``` ruby puts(case t; in { a: 1, c:1 }; true; else false; end) ``` -- https://bugs.ruby-lang.org/