Bug #20931
closedUsing `in` as an expression requires extra parentheses
Description
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)
Updated by alanwu (Alan Wu) about 2 months ago · Edited
- 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.
Updated by stephenprater (Stephen Prater) about 2 months ago
That works for me - thanks for the explanation.
Updated by mame (Yusuke Endoh) about 2 months ago · Edited
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.