[ruby-core:103346] [Ruby master Feature#17785] Allow named parameters to be keywords

Issue #17785 has been updated by byroot (Jean Boussier).


Arguably it's a bit of a stretch, but how would you handle: `foo(class_, class:)`?

What if instead of mangling the variable name, there would be a way to tell the parser to interpret the next word as a regular name rather than a keyword? e.g.:

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

`\` being a common escaping character I think it the one that would make the most sense.

And that would allow to make it work with regular parameters as well:

```ruby
def diff(start, \end)
  \end - start
end
```

Even though this use case is much less important, except for documentation purposes.

----------------------------------------
Feature #17785: Allow named parameters to be keywords
https://bugs.ruby-lang.org/issues/17785#change-91436

* 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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