Issue #16511 has been updated by jeremyevans0 (Jeremy Evans).


I believe this approach would break the following code:

```ruby
def debug_log(arg, output: $stderr)
  output.print(arg.inspect)
  output.print("\n")
end
def bar(*args, **opts)
  debug_log(args)
  debug_log(opts)
  # do something
end
bar(:baz, quux:1)
```

With the master branch, `{quux:1}` is considered a keyword argument by `bar`, but passed as a positional argument to `debug_log`.  This works fine as you would expect it to.  With the `KwHash` approach, `{quux:1}` ends up being passed as a keyword argument to `debug_log`, resulting in `ArgumentError`.  This results in even greater backwards compatibility issues than the ruby2_keywords by default approach.

In general, whether something is a keyword argument or a positional argument is something that should be decided on a per-call site basis, it shouldn't be a property of an object.  `ruby2_keywords` is strictly a work around to support older Ruby versions without requiring separate code paths, to ease supporting multiple Ruby versions at once.  When targeting only Ruby 3+, there would be no reason to use it (except performance when using CRuby).

The `KwHash` approach would result in the problems that keyword argument separation was designed to avoid, since you would have cases where positional arguments to methods are treated as keyword arguments depending on how they were created.  With the current master branch, that can only happen in limited cases on an opt-in basis when using `ruby2_keywords`.  Presumably, because it is opt-in, there will not be cases where positional arguments are ever mistreated as keyword arguments, without the deliberate misuse of `ruby2_keywords`.

----------------------------------------
Feature #16511: Subclass of Hash for keyword arguments
https://bugs.ruby-lang.org/issues/16511#change-83931

* Author: Dan0042 (Daniel DeLorme)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
As an alternative to #16463 and #16494 I'd like to propose this approach, which I believe allows a **much** more flexible path for migration of keyword arguments.

The idea is to have a subclass of Hash (let's name it "KwHash") which provides a clean, object-oriented design with various benefits.

I'll try to describe the idea by breaking it down into figurative steps. Imagine starting with ruby 2.6 and then:

### Step 1

When a double-splat or a brace-less hash is used, instead of a Hash it creates a KwHash.

```ruby
def foo(x) x end
foo(k:1).class      #=> KwHash
foo(**hash).class   #=> KwHash
[k:1].last.class    #=> KwHash
[**hash].last.class #=> KwHash
{**hash}.class      #=> Hash
```

At this point we haven't introduced any real change. Everything that worked before is still working the same way, with the ONLY exception being code like `kw.class == Hash` which now returns false. But no one actually writes code like that; it's always `kw.is_a?(Hash)`, which still returns true.

### Step 2

When there is ambiguity due to optional vs keyword argument, we rely on the last argument being Hash or KwHash to disambiguate.

```ruby
def foo(x=nil, **kw)
  [x,kw]
end
foo({k:1}) #=> [{k:1},{}]
foo(k:1)   #=> [nil,{k:1}]
```

This is the _minimum_ amount of incompatibility required to solve ALL bugs previously reported with keyword arguments. (#8040, #8316, #9898, #10856, #11236, #11967, #12104, #12717, #12821, #13336, #13647, #14130, etc.) 

### Step 3

Introduce additional incompatibility to improve clarity of design. Here we deprecate the automatic conversion of Hash to keyword argument; only KwHash is accepted. And always use the last KwHash argument if the method supports keyword arguments. With a deprecation/warning phase, of course. But importantly, all the changes required to silence these warnings are _compatible with 2.6_.

```ruby
def foo(x, **kw); end
foo(k:1)      # ArgumentError because x not specified
foo(1, {k:1}) # ArgumentError because too many arguments; Hash cannot be converted to KwHashs
opts = [k:1].first
foo(opts)     # opts is a KwHash therefore used as keyword argument; ArgumentError because x not specified
foo(1, opts)  # opts is a KwHash therefore used as keyword argument
```

At this point we have achieved _full_ **dynamic** keyword separation, as opposed to the current _almost-full_ **static** approach. I want to make the point here that, yes, keyword arguments **are** separated, it's just a different paradigm. With static separation, a keyword argument is defined lexically by a double-splat. With dynamic separation, a keyword argument is when the last argument is a KwHash.

Any form of delegation works with no change required. This preserves the behavior of 2.6 but only for KwHash objects. This is similar to having 2.7 with `ruby2_keywords` enabled by default. But also different in some ways. _Most importantly_, it allows the case shown in #16494 to work by default:

```ruby
array = [x:1]
array.push(x:2)
array.map{ |x:| x } #=> [1,2]
```

The current approach does not allow this to work at all. The solution proposed in #16494 has all the same flaws as Hash-based keyword arguments; what happens to `each{ |x=nil,**kw| }` ? The subclass-based solution allows a KwHash to be converted to... keywords. Very unsurprising.

Given that ruby is a dynamically-typed language I feel that dynamic typing of keywords if a more natural fit than static typing. But I realize that many disagree with that, which is why we continue to...

### Step 4

Introduce additional incompatibility to reach static/lexical separation of keyword arguments. Here we require that even a KwHash should be passed with a double-splat in order to qualify as a keyword argument.

```ruby
def bar(**kw)
end
def foo(**kw)
  bar(kw)   #=> error; KwHash passed without **
  bar(**kw) #=> ok
end
```

At this point we've reached the same behavior as 2.7. Delegation needs to be fixed, but as we know the changes required to silence these warnings are **not** compatible with 2.6. So here we introduce a way to _silence **only** these "Step 4" warnings_, for people who need to remain compatible with 2.6. And we keep them as warnings instead of errors until ruby 2.6 is EOL.

So instead of having to update a bunch of places with `ruby2_keywords` right now, it's a single flag like `Warning[:ruby3_keywords]`. Once ruby 2.6 is EOL these become controlled by `Warning[:deprecated]` which tells people they **have** to fix their code. Which is just like the eventual deprecation of `ruby2_keywords`, just without the busy work of adding `ruby2_keywords` statements in the first place.

The question remains of how to handle #16494 here. Either disallow it entirely, but I think that would be a shame. Or just like #16494 suggests, allow hash unpacking in non-lambda Proc. Except that now it can be a KwHash instead of a Hash, which at least preserves dynamic keyword separation.

## Putting it all together

The idea is _not_ to reimplement keyword argument separation; all that is needed is to implement the things above that are not in 2.7:
* Create a KwHash object when a double-splat is used.
* If a warning is due to a KwHash instead of a Hash, make it a different kind of warning that can be toggled off separately from the Hash warnings (and that will stay as warnings until 2.6 is EOL)

I think that's all, really...

### Pros
* Cleaner way to solve #16494
* Better compatibility (at least until 2.6 is EOL)
   * delegation
   * storing an argument list that ends with a KwHash
   * destructuring iteration (#16494)
* We can avoid the "unfortunate corner case" as described in the [release notes](https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/)
   * in 2.7 only do not output "Step 4" warnings, leave delegation like it was
   * in 2.8 the "Step 3" warnings have been fixed and a Hash will not be converted to keyword arguments
   * delegation can now safely be fixed to use the `**` syntax
* ruby2_keywords is not required, which is desirable because
   * it's a hidden flag _hack_
   * it requires to change the code now, and change it _again_ when ruby2_keywords is deprecated; twice the work; twice the gem upgrades
   * it was supposed to be used only for people who need to support 2.6 or below, but it's being misunderstood as an acceptable way to fix delegation in general
   * there's the non-zero risk that ruby2_keywords will never be removed, leaving us with a permanent "hack mode"
      * dynamic keywords are by far preferable to supporting ruby2_keywords forever
* Likely _better performance_, as the KwHash class can be optimized specifically for the characteristics of keyword arguments.
* More flexible migration
   * Allow more time to upgrade the hard stuff in Step 4
   * Can reach the _same_ goal as the current static approach
   * Larger "support zone" https://xkcd.com/2224/
   * Instead of wide-ranging incompatibilities all at once, there's the _possibility_ of making it finer-grained and more gradual
      * rubyists can _choose_ to migrate all at once or in smaller chunks
   * It hedges the risks by keeping more possibilities open for now.
   * It allows to cop-out at Step 3 if Step 4 turns out too hard because it breaks too much stuff

### Cons
* It allows to cop-out at Step 3 if Step 4 turns out too hard because it breaks too much stuff




-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:ruby-core-request / ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>