From: merch-redmine@...
Date: 2019-08-16T14:36:10+00:00
Subject: [ruby-core:94388] [Ruby master Feature#14183] "Real" keyword	argument

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


mame (Yusuke Endoh) wrote:
> > This Hash might contain numbers or strings as keys, and as such doesn't feel like "keywords arguments" to me.
> 
> I agree somewhat.  But, `foo("key" => 42)` actually looks a keyword argument, doesn't it?
> 
> But honestly I'm not so sure.  @jeremyevans0 what do you think?

I don't have a strong opinion about allowing non-Symbols for keyword hash keys.

With your original proposal, there is a definite need, since in order to get nice calling syntax, you want to allow the conversion of all `def a(opts={})` to `def a(**opts)`.  With my proposal, you can have a nice calling syntax with both `def a(opts={})` and `def a(**opts)`, so I don't think there is need.  If you want to support non-Symbol keys, you would just continue to use `def a(opts={})`.

That doesn't imply it is bad to allow non-Symbol keys for `def a(**opts)`.  It does change the original purpose of keyword arguments, but it adds flexibility, and would allow for easier switching between `def a(opts={})` and `def a(**opts)`.

mame (Yusuke Endoh) wrote:
> I think that Jeremy's branch is a great start.  So, @jeremyevans0 , how about merging it experimentally?  And then, if we face some actual issues, we can discuss each of them.  If you are not against, I'd like to propose the merge at the next dev-meeting.

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.

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-80803

* 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)


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