From: zverok.offline@...
Date: 2019-12-06T23:14:04+00:00
Subject: [ruby-core:96136] [Ruby master Feature#14183] "Real" keyword	argument

Issue #14183 has been updated by zverok (Victor Shepelev).


> I think everyone agrees that keyword argument separation is disruptive, but it is no more disruptive to blocks than it is to method calls in general.

(inb4, I am obviously not saying "the change is bad, undo! undo!" Just trying to think out loud/provide some additional things to consider.)

The problem with blocks is indeed Array/Enumerable (and similar code). I am not sure how widespread this approach is, but in our production code (lots of small hashes, too many and to short-living to wrap in objects), we frequently do this:
```ruby
words
  .select { |paragraph_id:, text:, **| paragraph_id > first_paragraph_id && !text.include?('foo') }
  .map { |text:, timestamp:, **| {at: timestamp.to_i, content: "#{text} [#{timestamp}]"} }
  .group_by { |at:, **| at % 1000 }

# Or even:
large_config_hash.then { |this_value:, that_value: DEFAULT, **| do_something(this_value, that_value) }
```
This allows constructing readable chains of expressive transformations, which `Hash#fetch`/`Hash#[]` would make arguably much less clear.

It seems keyword argument separation just clearly prohibits this technique without any alternative (of the same expressiveness).

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-83005

* Author: mame (Yusuke Endoh)
* Status: Closed
* 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.

---Files--------------------------------
vm_args.diff (4.19 KB)
vm_args_v2.diff (4.18 KB)


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