From: merch-redmine@...
Date: 2019-08-16T23:05:15+00:00
Subject: [ruby-core:94392] [Ruby master Feature#14183] "Real" keyword	argument

Issue #14183 has been updated by jeremyevans0 (Jeremy Evans).


jeremyevans0 (Jeremy Evans) wrote:
> I am in favor of the merge and eager to see matz's decision.  I'll rebase my branch against master today to make it easier for people to comparison test.

I've rebased my branch against master: https://github.com/jeremyevans/ruby/tree/keyword-argument-separation

After rebasing against master, I ran the specs on my branch.  I guess I didn't run the specs previously (only the tests), because the specs found a backwards compatibility issue on my branch:

```ruby
def m(a=1, b:) [a, b] end

m("a" => 1, b: 2)
# 2.6: [{"a"=>1}, 2]
# my branch: ArgumentError (unknown keyword: "a")
```

While my branch has the behavior we want in 3.0, my branch currently does not have a good transition path for this case.  Since this worked in 2.6, we probably want a warning and the same behavior, correct?  I think that means adding a keyword argument to positional hash split.  My branch already implements the positional hash to keyword argument split, but has not implemented the reverse split yet.  I will try to implement that before the next developer meeting.

Note that there is still an expected behavior change when using keyword splats:

```
def b(a=1, **b) [a, b] end
p b('a'=>1, :b=>3)
# 2.6: [{"a"=>1}, {:b=>3}]
# my branch: [1, {"a"=>1, :b=>3}]
```

I still believe this change is reasonable, because it doesn't make sense to have a warning in 2.7 for something that will be valid in 3.0.

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-80809

* 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.

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


-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request@ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>