[ruby-core:119189] [Ruby master Feature#20738] Removing a specific entry from a hash literal

Issue #20738 has been updated by mame (Yusuke Endoh).


History. Long ago in Ruby, such a special value actually existed. It was `nil`.

```ruby
# ruby 1.4.6
h = { 1 => 2 }

p h  #=> {1=>2}

# This removes the key 1
h[1] = nil

p h  #=> {}
```

However, there were more and more cases where we want to treat `nil` as an ordinary value. Finally, `nil` has lost this speciality.

Considering this history, the special value is not a good idea.

----------------------------------------
Feature #20738: Removing a specific entry from a hash literal
https://bugs.ruby-lang.org/issues/20738#change-109773

* Author: ursm (Keita Urashima)
* Status: Open
----------------------------------------
Sometimes I want to decide whether or not to add a particular entry to a hash depending on a condition. If the entire hash does not use nil, I can use Hash#compact.

```ruby
{
  foo: 1,
  bar: bar? ? 2 : nil
}.compact
```

But if I want to remove only a specific entry while leaving the other nil, it is somewhat cumbersome. I have to either assign the hash once and change it destructively, or use Hash#reject.

``` ruby
h = {
  foo: 1,
  baz: nil
}

h[:bar] = 2 if bar?
```
``` ruby
{
  foo: 1,
  bar: bar? ? 2 : :drop,
  baz: nil
}.reject {|_, v| v == :drop }
```

As a suggestion, how about a special value that indicates an invalid key for the hash? With this, the above example could be written like this:

``` ruby
{
  foo: 1,
  bar: bar? ? 2 : Hash::DROP,
  baz: nil
} #=> {foo: 1, baz: nil}
```



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