From: ruby-core@...
Date: 2018-09-18T03:44:15+00:00
Subject: [ruby-core:89052] [Ruby trunk Feature#14183] "Real" keyword argument

Issue #14183 has been updated by marcandre (Marc-Andre Lafortune).


Very interesting.

akr (Akira Tanaka) wrote:
> The flag will be true if method call uses k => v, k: v, "k": v or **h and
> all keys of the Hash object constructed from them are symbol.
> (I think hash separation is not good idea.)

I agree that hash separation is not a good idea. I'm wondering if `(k = :a) => v` should be accepted. It is the only case that is not syntactical.

> This makes the confusion of positional/keyword arguments solvable.
> But I don't say the confusion is solvable easily (or by-default).
> Programmers must use the flag carefully.

IIUC, the only 100% correct way to forward a method call (including the `keyword_given?` flag) would be:

```
def forward(*args, &block)
  if keyword_given?
    options = args.pop
    target(*args, **options, &block)
  else
    target(*args, &block)
  end
end
```

That is assuming the current `**{}` creating a positional argument.

Assuming Ruby 2.x compatiblity, there's no way of a general forward with `**capture` though. We'd need a method `lash_argument_converted_to_keyword?`...

```
def forward(*args, **options, &block)
  if keyword_given?
    target(*args, **options, &block)
  else
    args << options if last_argument_converted_to_keyword?
    target(*args, &block)
  end
end
```

Maybe the API could be combined in a single method `keyword_style`, returning one of `[nil, :keyword, :hash_to_keyword, :keyword_to_hash]`:

```
def foo(*); keyword_style; end
def bar(**); keyword_style; end
h = {}

foo      # => nil
foo(**h) # => :keyword_to_hash
bar(**h) # => :keyword
bar(h)   # => :hash_to_keyword
```

<evil> Or `keyword_given?` could return `nil` in case a hash was converted to a keyword argument like in `bar(h)` </evil>

If we distinguish `**nil` and `**{}` as in #15078, then there's no need to even call `keyword_given?` and normal forwarding works I imagine... But that might be quite incompatible.

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-74078

* Author: mame (Yusuke Endoh)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: Next Major
----------------------------------------
In RubyWorld Conference 2017 and RubyConf 2017, Matz officially said that Ruby 3.0 will have "real" keyword arguments.  AFAIK there is no ticket about it, so I'm creating this (based on my understanding).

In Ruby 2, the keyword argument is a normal argument that is a Hash object (whose keys are all symbols) and is passed as the last argument.  This design is chosen because of compatibility, but it is fairly complex, and has been a source of many corner cases where the behavior is not intuitive.  (Some related tickets: #8040, #8316, #9898, #10856, #11236, #11967, #12104, #12717, #12821, #13336, #13647, #14130)

In Ruby 3, a keyword argument will be completely separated from normal arguments.  (Like a block parameter that is also completely separated from normal arguments.)
This change will break compatibility; if you want to pass or accept keyword argument, you always need to use bare `sym: val` or double-splat `**` syntax:

```
# The following calls pass keyword arguments
foo(..., key: val)
foo(..., **hsh)
foo(..., key: val, **hsh)

# The following calls pass **normal** arguments
foo(..., {key: val})
foo(..., hsh)
foo(..., {key: val, **hsh})

# The following method definitions accept keyword argument
def foo(..., key: val)
end
def foo(..., **hsh)
end

# The following method definitions accept **normal** argument
def foo(..., hsh)
end
```

In other words, the following programs WILL NOT work:

```
# This will cause an ArgumentError because the method foo does not accept keyword argument
def foo(a, b, c, hsh)
  p hsh[:key]
end
foo(1, 2, 3, key: 42)

# The following will work; you need to use keyword rest operator explicitly
def foo(a, b, c, **hsh)
  p hsh[:key]
end
foo(1, 2, 3, key: 42)

# This will cause an ArgumentError because the method call does not pass keyword argument
def foo(a, b, c, key: 1)
end
h = {key: 42}
foo(1, 2, 3, h)

# The following will work; you need to use keyword rest operator explicitly
def foo(a, b, c, key: 1)
end
h = {key: 42}
foo(1, 2, 3, **h)
```

I think here is a transition path:

* Ruby 2.6 (or 2.7?) will output a warning when a normal argument is interpreted as keyword argument, or vice versa.
* Ruby 3.0 will use the new semantics.



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