From: merch-redmine@...
Date: 2019-12-06T22:59:45+00:00
Subject: [ruby-core:96135] [Ruby master Feature#14183] "Real" keyword	argument

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


zverok (Victor Shepelev) wrote:
> So, there is no way to use block's keyword arguments to unpack a hash with symbol keys when passing it to the block?.. That's very disruptive change, this technique is super-useful when working with complex structured data :(

Block calls are treated just like any other method call, and you would generally fix the keyword argument separation issue on the caller side:

```ruby
def foo(h)
  yield **h
end

foo({foo: 1, bar: 2}) do |foo:, bar:|
  # foo = 1, bar = 2
end
```

The issue in the case shown by @koic is that he does not control the caller.  Switching from keyword arguments to `Hash#fetch` calls is probably easiest in his case.  He could also override the each method for the array object, but that is probably not a good idea unless there were many cases where you need this and you could use a separate class for it.

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.

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-83004

* 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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