Issue #14183 has been updated by mame (Yusuke Endoh).


jeremyevans0 (Jeremy Evans) wrote:
> Changing the callee side will not fix all cases. The `where` method supports more than just symbols keys in hashes.  `where('table.id'=>1)` is supported, for example.  Accepting a keyword args splat and then appending it to the array of arguments just decreases performance for no benefit.

As an experiment, I'm now trying to check Ruby's existing APIs, and noticed that some methods had the issue: Kernel#spawn, JSON::GenericObject.from_hash, etc.  It might be good to provide a variant of define_method for this case as a migration path:

```
define_last_hash_method(:foo) do |opt|
  p opt
end

foo(k: 1)     #=> {:k=>1}
foo("k"=>1) #=> {"k"=>1}
```


> Are you assuming that all methods that use hash arguments will end up wanting to use keyword arguments at some point?  I think that is unlikely.  If keyword arguments are never added to the method in the future, then you have broken backwards compatibility now for no benefit.

I don't think that all methods will have keyword arguemnts eventually.  However, I assume that we can never predict which methods will have.


> > I'd like to make it safe to extend an existing method definition with a keyword parameter.
> 
> Attempting to avoid backwards compatibility problems is a noble goal that I think we share.  Part of that is avoiding future backwards compatibility problems.  Another part of that is avoiding current backwards compatibility problems.  A change that causes more current backwards compatibility problems than the future backwards compatibility problems it is designed to avoid is a step in the wrong direction, in my opinion.

In general, I agree.  For this specific topic, however, the current spec and implementation are really a mess; the current backward compatibility problem is relatively easy to fix; the future backwards compatibility problem is hard to avoid and will become painful more and more.  We should now pay the debt for the future.

But this is just my opinion.  I really appreciate and respect your opinion.  I'd like to tell matz your opinion as fairly as I can.

----------------------------------------
Feature #14183: "Real" keyword argument
https://bugs.ruby-lang.org/issues/14183#change-73835

* 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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