From: merch-redmine@...
Date: 2019-03-25T22:50:19+00:00
Subject: [ruby-core:91986] [Ruby trunk Feature#14183] "Real" keyword argument

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

File vm_args.diff added

Since I think it is best to back proposed behavior changes with a proposed implementation, attached is a patch based on mame's keyword-argument-separation branch that implements my proposal:

* Same behavior as mame's for methods that accept keyword arguments
* For methods that do not accept keyword arguments
    * Allow use of braceless hash without warning (keep backwards compatibility)
    * Allow **keyword splats
        * If keyword splat is empty, do not add positional argument (new behavior)
        * Otherwise, add hash as positional argument (keep backwards compatibility)

mame, if you have the time, could you try this patch with your internal Rails app, using the same checkout that resulted in about 120k warnings, and see how many warnings it causes and whether not adding a positional argument for an empty keyword splat breaks any code?

I'm not sure the patch is the best approach possible.  I have limited knowledge of and experience with the VM internals.  This patch is the minimum change necessary, it doesn't remove the rb_no_keyword_hash variable, even though I don't think the variable is needed if we do not pass positional arguments for empty keyword splats.

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-77318

* 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)


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