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


akr (Akira Tanaka) wrote:
> I have an idea to separate positional arguments and keyword arguments without incompatibility.

I like this idea of allowing per-method handling of arguments.  Just to confirm my understanding of the proposal:

~~~ ruby
def m(*a)
  [keyword_given?, a]
end

m # => [false, []]
m(1) # => [false, [1]]
m({:a=>1}) # => [false, [{:a=>1}]]
m(:a=>1) # => [true, [{:a=>1}]]
m(a: 1) # => [true, [{:a=>1}]]
m("a": 1) # => [true, [{:a=>1}]]
m(**{a: 1}) # => [true, [{:a=>1}]] or ArgumentError ?
m(**{}) # => [true, [{}]], [true, []], or ArgumentError ?
m('a'=>1, :a=>1) # => [false, [{'a'=>1, :a=>1}]]
a = :a
m(a=>1) # => [true, [{:a=>1}]]

def m2(*a, **kw)
  [keyword_given?, a, kw]
end

m2 # => [false, [], {}]
m2(1) # => [false, [1], {}]
m2({:a=>1}) # => [false, [{:a=>1}], {}]
m2(:a=>1) # => [true, [], {:a=>1}]
m2(a: 1) # => [true, [], {:a=>1}]
m2("a": 1) # => [true, [], {:a=>1}]
m2(**{a: 1}) # => [true, [], {:a=>1}]
m2(**{}) # => [true, [], {}]
m2('a'=>1, :a=>1) # => [false, [{'a'=>1, :a=>1}], {}]
a = :a
m2(a=>1) # => [true, [], {:a=>1}]

def m3(a)
  [keyword_given?, a]
end

m3 # => ArgumentError
m3(1) # => [false, 1]
m3({:a=>1}) # => [false, {:a=>1}]
m3(:a=>1) # => [true, {:a=>1}]
m3(a: 1) # => [true, {:a=>1}]
m3("a": 1) # => [true, {:a=>1}]
m3(**{a: 1}) # => [true, {:a=>1}] or ArgumentError ?
m3(**{}) # => [true, {}] or ArgumentError ?
m3('a'=>1, :a=>1) # => [false, {'a'=>1, :a=>1}]
a = :a
m3(a=>1) # => [true, {:a=>1}]

def m4(**kw)
  [keyword_given?, kw]
end

m4 # => [true, {}] or [false, {}] ?
m4(1) # => ArgumentError
m4({:a=>1}) # => ArgumentError
m4(:a=>1) # => [true, {:a=>1}]
m4(a: 1) # => [true, {:a=>1}]
m4("a": 1) # => [true, {:a=>1}]
m4(**{a: 1}) # => [true, {:a=>1}]
m4(**{}) # => [true, {}] or [false, {}] ?
m4('a'=>1, :a=>1) # => ArgumentError
a = :a
m4(a=>1) # => [true, {:a=>1}]
~~~

If the method does not explicitly declare any keyword arguments, and the caller uses `**hash` is used to explicitly pass a keyword argument, does that raise an ArgumentError or does it pass the hash as a positional argument and have `keyword_given?` return true?

If the method does not explicitly declare any keyword arguments, does it pass the splatted empty hash as a positional argument, does it ignore it, or does it raise an ArgumentError?

If the method explicitly declares keyword arguments (either required keyword, optional keyword, or keyword splat), and is called without keyword arguments, does `keyword_given?` return true or false?

If the method explicitly declares keyword arguments and an empty hash is splatted, does `keyword_given?` return true or false?

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-74068

* 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.



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