Feature #13332: Forwardable#def_instance_delegator nil

• Status: Feedback
• Assignee: nobu (Nobuyoshi Nakada)
• Target version: -

[eike.rb (Eike Dierks)]

I'd like to suggest an extension to the signature of stdlib Forwardable#def_instance_delegator

currently (ruby 2.3.3) it is declared as:

  def_instance_delegator(accessor, method, ali = method)

I'd like to add a nil: option like this:

  def_instance_delegator(accessor, method, ali = method, nil_behaviour:NoMethodError)

Rationale:
As of ruby2.3, delegated methods fail with
#<NoMethodError: undefined method foo' for nil:NilClass>`
when the declared accessor returns nil

aka, when the delegator may be nil,
the Forwardable module can not be used,
but one has to ressort to implement the forward manually.

Adding an option to the signature could solve this.
I believe that this can be implemented in a fully backward compatible way.

I named this option :nil in my example,
(some people will cry out loud)

Specification scribble:

if accessor.nil?
  case nil_behaviour
  when nil then nil    # return nil, when accessor was nil
  when Error             # raise that Error with proper args
  when Proc             # invoke that Proc with proper args
  when String           # return that string
  else                        # return that object
end

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)

I wonder how common such case is.

And you can override delegator/accessor methods by yourself.

[ko1 (Koichi Sasada)]

• Assignee set to nobu (Nobuyoshi Nakada)

[matz (Yukihiro Matsumoto)]

• Status changed from Open to Feedback

The proposed solution seems to be too generic. Show us the use-case except for nil for nil'. For example, it is acceptable that allow_nil: true` keyword argument we see in ActiveSupport.

Matz.