Bug #20931: Using `in` as an expression requires extra parentheses

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 3.3.1
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[stephenprater (Stephen Prater)]

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 }

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?

puts(t in {a: 1, c: 1}) # syntax error
puts((t in {a: 1, c: 1}) # false

Especially since this works fine:

puts(case t; in { a: 1, c:1 }; true; else false; end)

[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:

$ 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 | 

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

[stephenprater (Stephen Prater)]

That works for me - thanks for the explanation.

[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 interpretations: 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.