[ruby-core:114137] [Ruby master Feature#19764] Introduce defp keyword for defining overloadable, pattern matched methods

Issue #19764 has been reported by zeke (Zeke Gabrielse).

----------------------------------------
Feature #19764: Introduce defp keyword for defining overloadable, pattern matched methods
https://bugs.ruby-lang.org/issues/19764

* Author: zeke (Zeke Gabrielse)
* Status: Open
* Priority: Normal
----------------------------------------
Pattern matching has become one of my favorite features of Ruby, if not my favorite. It
changed the way I write and express my thoughts through clean, maintainable code. And
I'd like to use it *more*.

I propose a new keyword, `defp`, for defining a method which applies pattern matching to
its arguments.

```ruby
defp call(String => s unless s in /^[a-z]/)
  puts "string: #{s.inspect} (capitalized)"
end

defp call(String => s)
  puts "string: #{s.inspect}"
end

defp call(Hash(foo:, bar:) => h)
  puts "hash: #{h.inspect}"
end

defp call(**nil)
  puts "no keyword args"
end

call("Example") # => string: "Example" (capitalized)
call("test")    # => string: "test"
call(foo: 1, bar: 2) 
# => hash: { :foo => 1, :bar => 2 }
```

Internally, this could be represented as the following `case..in` pseudocode:

```ruby
def call(...)
  case ...
  in String => s unless s in /foo/
    puts "string: #{s.inspect} (not foo)"
  in String => s
    puts "string: #{s.inspect}"
  in Hash(foo:, bar:) => h
    puts "hash: #{h.inspect}"
  in **nil
    puts "no keyword args"
  else
    raise NoMatchingMethod
  end
end
```

As you could imagine, this could be used to refactor a lot of code, making the developer's
intent much clearer. From [complex methods that use `case` statements](https://github.com/rails/rails/blob/593893c901f87b4ed205751f72df41519b4d2da3/actionpack/lib/action_dispatch/routing/url_for.rb#L173-L193)
for taking varied arguments (I'm sure all our code bases contain such `case` statements),
to defining smaller, simpler methods that handle particular argument patterns.

In addition, not only can this improve code quality, but it brings in method overloads,
and it also adds a way to define more typing to the language -- something that RBS has
tried to do, to mixed reactions -- but in a more Ruby-like way that Rubyists are
already learning *and loving*.

Thoughts?

Original idea by Victor Shepelev: https://zverok.space/blog/2023-05-05-ruby-types.html

Further discussion: https://news.ycombinator.com/item?id=35834351



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