[ruby-core:120109] [Ruby master Bug#20931] Using `in` as an expression requires extra parentheses

["mame (Yusuke Endoh) via ruby-core" <ruby-core@ml.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.

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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)
```



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