[ruby-core:117068] [Ruby master Feature#20326] Add an `undefined` for use as a default argument.

Issue #20326 has been updated by jeremyevans0 (Jeremy Evans).


The problem with this is you could pass `undefined` just the same as `nil`.  You would end up in a similar situation to JavaScript, which has both `null` and `undefined`.

The correct way to handle this is by setting a local variable inside the default argument value:

```ruby
class Array
  def count(item = (item_not_set = true))
    if item_not_set
      # item argument was not passed
    else
      # item argument was passed
    end
  end
end
```

Admittedly, this is a bit ugly, but the need to differentiate between passed default value and no supplied argument is fairly rare.  Maybe there is a nicer way of supporting the same idea, that can still differentiate between passing the default value and not supplying an argument?


----------------------------------------
Feature #20326: Add an `undefined` for use as a default argument.
https://bugs.ruby-lang.org/issues/20326#change-107141

* Author: shan (Shannon Skipper)
* Status: Open
----------------------------------------
Variations around `UNDEFINED = Object.new` are a fairly common pattern to see used as default arguments to distinguish between `nil` and no argument provided. For example, a Ruby implementation of Array#count might look something like:

``` ruby
class Array
  UNDEFINED = Object.new
  def UNDEFINED.inspect = 'UNDEFINED'
  UNDEFINED.freeze

  def count(item = UNDEFINED)
    if item == UNDEFINED
      # ...
    end
  end
end
```

I'd like to propose adding an `undefined` module function method on Kernel to remove the boilerplate for this fairly common use case. An `__undefined__` method or `__UNDEFINED__` keyword would be alternatives to `undefined`. An `undefined?` helper would also be an optional nicety:

``` ruby
class Array
  def count(item = undefined)
    if item.undefined?
      # ...
    end
  end
end
```

A Ruby implementation might look like:

``` ruby
module Kernel
  UNDEFINED = Object.new
  def UNDEFINED.inspect = -'undefined'
  UNDEFINED.freeze
  private_constant :UNDEFINED

  def undefined? = self == UNDEFINED

  module_function

  def undefined = UNDEFINED
end
```




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