Bug #20041: Array destructuring and default values in parameters

• Status: Assigned
• Assignee: tenderlovemaking (Aaron Patterson)
• Target version: -
• Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN

[tenderlovemaking (Aaron Patterson)]

It's possible to set the default value of a parameter to a previous parameter. For example:

def foo(a, b = a)
  b
end

foo([1, 2]) # => [1, 2]

However, if the parameters are destructured, the destructring happens after default parameter assignment. For example:

def foo((x, y), b = x)
  [x, y, b]
end

foo([1, 2]) # => [1, 2, nil]

Is this expected behavior? I would have expected the parameters to be "evaluated" from left to right, and the array destructuring to happen before the default parameter assignment.

Thanks!

[matz (Yukihiro Matsumoto)]

It is not intended behavior, but under the current implementation, fixing this issue would make the VM far more complex, so we left this behavior untouched, when we found this for the first time. If someone come up with the nice idea, we will reconsider.

Matz.

[tenderlovemaking (Aaron Patterson)]

matz (Yukihiro Matsumoto) wrote in #note-1:

It is not intended behavior, but under the current implementation, fixing this issue would make the VM far more complex, so we left this behavior untouched, when we found this for the first time. If someone come up with the nice idea, we will reconsider.

Matz.

Sounds good, thank you. I think I have an idea, but it'll take time for the patch.

[tenderlovemaking (Aaron Patterson)]

• Assignee set to tenderlovemaking (Aaron Patterson)

[hsbt (Hiroshi SHIBATA)]

• Status changed from Open to Assigned