Bug #20482: nil variables in a guard clause of a standalone => or in expression

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin23]
• Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[Soilent (Konstantin x)]

The following expression produces a TypeError, which is quite unexpected:

[1, 2] in a, b if b == 2*a

x.rb:1:in `*': nil can't be coerced into Integer (TypeError)

[1, 2] in a, b if b == 2*a
                         ^
        from x.rb:1:in `<main>'

The expression above should be equivalent to the following one, which works as expected:

case [1, 2]
in a, b if b == 2*a
  true
else
  false
end
# => true

Apologies in advance if this is intentional or has been reported before.

[nobu (Nobuyoshi Nakada)]

One-line pattern matching can not have a guard clause, so the if in the first example is a modifier if and is evaluated before the pattern matching.
In short, it should be intentional, I think.

[Soilent (Konstantin x)]

Thank you, nobu! That explains it.

Just to clarify, is this how the expression is parsed?

( [1, 2] in a, b ) if b == 2*a

And since this is a modifier if, the parser creates variables a and b first, which is why they are defined in the test expression?

[nobu (Nobuyoshi Nakada)]

Soilent (Konstantin x) wrote in #note-2:

Thank you, nobu! That explains it.

Just to clarify, is this how the expression is parsed?

( [1, 2] in a, b ) if b == 2*a

Yes, you can confirm the AST by ruby --dump=parsetree -e '1 in a if a' (the code is simplified).

And since this is a modifier if, the parser creates variables a and b first, which is why they are defined in the test expression?

Yes, a and b are created by [1, 2] in a, b and available in the if condition.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset git|b911d2222f907d3fad397938e8f513ecfb4635b8.

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[Bug #20482] [DOC] Clarify about pattern maching guard clause

Guard clauses can only be used in case pattern matching statements,
not in =>/in operators.