[ruby-core:103350] [Ruby master Feature#17785] Allow named parameters to be keywords

Issue #17785 has been updated by Eregon (Benoit Daloze).


I like @byroot's idea to solve the more general issue and not just this specific instance.

marcandre (Marc-Andre Lafortune) wrote in #note-3:
> Clearly, `class_` is much simpler and much faster than `binding.local_variable_get(:class)`...

It can be the same performance with a JIT and escape analysis (i.e., it's the same on TruffleRuby).

----------------------------------------
Feature #17785: Allow named parameters to be keywords
https://bugs.ruby-lang.org/issues/17785#change-91441

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
We should allow named parameters to be keywords and use add a trailing `_` to the corresponding variable:

```ruby
def check(arg, class:)
  arg.is_a?(class_)
end

check(42, class: Integer) # => true
```

Currently, if we want such an API we have to use `**rest`:

```ruby
def check(arg, **rest)
  class_ = rest.fetch(:class) { raise ArgumentError('missing keyword: :class')}
  if rest.size > 1
    unknown = rest.keys - [:class]
    raise ArgumentError("unknown keyword(s): :#{unknown.join(', :')})
  end

  arg.is_a?(class_)
end
```

This is very verbose, much less convenient, much less readable, prevents `steep` from generating the proper signature, etc.

We should do the same for pattern match.



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