[ruby-core:94928] [Ruby master Misc#16157] What is the correct and *portable* way to do generic delegation?

[merch-redmine@jeremyevans.net]

Issue #16157 has been updated by jeremyevans0 (Jeremy Evans).


Dan0042 (Daniel DeLorme) wrote:
> With 2.7, people will be asked to migrate their generic delegations to use `pass_keywords`. Then at some point in the future when 2.6 is no longer supported, `pass_keywords` will be deprecated and they'll be asked to migrate their generic delegations a second time, to use `(*args, **kw)`
> 
> Is that the idea?

`pass_keywords` is proposed only to make it easier to continue to support older versions of Ruby with the same method definition.  I don't think we will be pushing people to use `pass_keywords`, merely making it available as an option.

You are correct that `pass_keywords` is currently envisioned as a temporary solution, not a permanent one.  However, there is nothing technically preventing it from being a permanent solution.

For 2.7+, delegation using `(*args, **kw)` should work the same way `(*args)` works in Ruby <2.7.  If you only want to migrate once and still want to support backwards compatibility, you can certainly switch to having separate definitions, using the current one for <2.7 and adding one for 2.7+.  Then you could drop the <2.7 definition when you drop support for Ruby <2.7.

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Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-81544

* Author: Dan0042 (Daniel DeLorme)
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
With the keyword argument changes in 2.7 we must now specify keyword arguments explicitly when doing generic delegation. But this change is not compatible with 2.6, where it adds an empty hash to the argument list of methods that do not need/accept keyword arguments.

To illustrate the problem:

```ruby
class ProxyWithoutKW < BasicObject
  def initialize(target)
    @target = target
  end
  def method_missing(*a, &b)
    @target.send(*a, &b)
  end
end

class ProxyWithKW < BasicObject
  def initialize(target)
    @target = target
  end
  def method_missing(*a, **o, &b)
    @target.send(*a, **o, &b)
  end
end

class Test
  def args(*a)   a  end
  def arg(a)     a  end
  def opts(**o)  o  end
end
                                          # 2.6        2.7              3.0
ProxyWithoutKW.new(Test.new).args(42)     # [42]       [42]             [42]        ok
ProxyWithoutKW.new(Test.new).arg(42)      # 42         42               42          ok
ProxyWithoutKW.new(Test.new).opts(k: 42)  # {:k=>42}   {:k=>42} +warn   [{:k=>42}]  incompatible with >= 2.7
ProxyWithKW.new(Test.new).args(42)        # [42, {}]   [42]             [42]        incompatible with <= 2.6
ProxyWithKW.new(Test.new).arg(42)         # error      42               42          incompatible with <= 2.6
ProxyWithKW.new(Test.new).opts(k: 42)     # {:k=>42}   {:k=>42} +warn   {:k=>42}    must ignore warning? cannot use pass_positional_hash in 2.6
```

I don't know how to solve this, so I'm asking for the **official** correct way to write portable delegation code. And by **portable** I mean code that can be used in gems that target ruby 2.6 and above.




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