[ruby-core:117084] [Ruby master Feature#20326] Add an `undefined` for use as a default argument.

Issue #20326 has been updated by shan (Shannon Skipper).


zverok (Victor Shepelev) wrote in #note-6:
> Unfortunately, such APIs are sometimes unavoidable, especially in implementing various generic algorithms (and not business logic).

I agree it's unfortunately unavoidable too often, so I resign myself to `default = UNDEFINED` or `default = default_not_set = true` like you mention above.

zverok (Victor Shepelev) wrote in #note-6:
> That being said, I am not sure I am in favor of having nil/undefined distinction in Ruby, and never proposed it myself (while considering it several times).

I'm not sold on `undefined` either, but would love some better way to detect default arguments that aren't set.

Crimson on Ruby Discord suggested something similar to `block_given?` might be nice, like an `arg_given?(arg)`.

```
def fetch(item, default = nil)
  if arg_given?(:default)
```


----------------------------------------
Feature #20326: Add an `undefined` for use as a default argument.
https://bugs.ruby-lang.org/issues/20326#change-107158

* Author: shan (Shannon Skipper)
* Status: Feedback
----------------------------------------
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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