[ruby-core:106645] [Ruby master Feature#17785] Allow named parameters to be keywords

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


matheusrich (Matheus Richard) wrote in #note-11:
> Since we have `__method__`, maybe adding something like `__params__`?

I dislike this approach because it will introduce lots of complexity for Ruby implementations, and will likely make the language slower because arguments need to be retained longer than without it.
In the worst case it could even introduce non-obvious memory leaks (because one cannot know if `__params__` would be used, potentially in an `eval` or aliases or so).

JavaScript's `arguments` is a well known PITA for implementations and often seen as hurting optimizations.

----------------------------------------
Feature #17785: Allow named parameters to be keywords
https://bugs.ruby-lang.org/issues/17785#change-95316

* 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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