ruby-core

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


Here's an alternative proposal, with the basic idea that behavior for historical ruby 1.6+ code that doesn't use keyword arguments remains the same.

## OK: Historical ruby 1.6+ (maybe before) usage (hash argument with omitted braces)

~~~ ruby
def foo(h)
  # h # => {:k => 1}
end
foo(:k => 1)
foo(k: 1) # ruby 1.9+ syntax
~~~

## OK: Ruby 2.0 keyword usage that will keep working

~~~ ruby
def foo(k: 1) # or foo(**h)
end
foo(:k => 1)
foo(k: 1)
foo(**{k: 1})
~~~

## NG: Using ** splat as hash argument

~~~ ruby
def foo(h)
end
foo(**{k: 1})
~~~

## NG: Using hash argument instead of keyword arguments

~~~ ruby
def foo(k: 1) # or foo(**h)
end
foo({k: 1})
~~~

My reasoning for this is that historical behavior for methods that do not use keyword arguments should not be broken to fix problems caused by keyword arguments.  I reviewed all issues mentioned in this ticket:

#8040: method keyword arguments
#8316: method keyword arguments
#9898: method regular argument, caller uses **
#10856: method regular argument, caller uses ** on empty array
#11236: method keyword arguments
#11967: method keyword arguments
#12104: proc usage, unrelated to keyword argument vs regular argument
#12717: method keyword arguments
#12821: method keyword arguments
#13336: method keyword arguments
#13467: method keyword arguments
#14130: method keyword arguments

As you can see, all of the problems are with using keyword arguments in the method definition or with ** at the call site when a method regular argument is used.  There are no issues when the method takes a regular argument and ** is not used at the call site, with the historical behavior and syntax of specifying a hash argument with omitted braces.  I see no reason to break the ruby 1.6+ historical behavior when keyword arguments are not involved.

Regarding the following program mentioned by mame:

~~~ ruby
def foo(*ary)
end
foo(kw: 1)
~~~

there is a lot of historical ruby code that does:

~~~ ruby
def foo(*ary)
  options = ary.pop if ary.last.is_a?(Hash)
  # ...
end
~~~

For that reason I think it would be best if `foo(kw: 1)` continued to work in such cases, since there are no problems in terms of the keyword arguments being used (no keyword arguments in method definition implies argument syntax is a hash with omitted braces).

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-73108

* 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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