[ruby-core:94872] [Ruby master Misc#16157] What is the correct and *portable* way to do generic delegation?

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


Dan0042 (Daniel DeLorme) wrote:
> I've compiled your delegate-keyword-argument-separation branch to test this. BTW in your example it should be `respond_to?(:pass_positional_hash, true)`
> 
> The portability result is... Almost there, but not quite. In 2.6 and 2.7, `o.empty?` cannot distinguish between `foo()` and `foo({})`. This results in the discrepancies found below via this test script.

Thank you for working on this and providing the detailed example.  I think your analysis is correct.  We could handle this in 2.7 by introducing a method to check for whether positional->keyword hash conversion was done, but as we cannot change the behavior in 2.0-2.6, it looks like we cannot use the same method definition to delegate to all possible target method definitions with all possible arguments and be backwards compatible with <2.7.  For best compatibility, you will need separate method definitions for <2.7 and 2.7+:

```ruby
class Proxy < BasicObject
  def initialize(target)
    @target = target
  end
  if ::RUBY_VERSION < '2.7'
    def method_missing(*a, &b)
      @target.send(*a, &b) rescue $!
    end
  else
    def method_missing(*a, **o, &b)
      @target.send(*a, **o, &b) rescue $!
    end
    pass_positional_hash(:method_missing) if respond_to?(:pass_positional_hash, true)
  end
end

class Test
  def args(*a)   a  end
  def arg(a)     a  end
  def opts(**o)  o  end
  def arg0o(a=nil, **o) [a,o] end
  def arg1o(a, **o)     [a,o] end
end

e = Object.new
def e.write(*args) $stdout.print("warn! ") end
$stderr = e

opt = {}
hash = {k:42}
proxy =  Proxy.new(Test.new)
p proxy.args(42)
p proxy.arg(42)
p proxy.arg({k:42})
p proxy.arg({})
p proxy.opts(k: 42)
p proxy.arg0o(hash)
p proxy.arg0o(hash, **{})
p proxy.arg0o(hash, **opt)
```

This results in the following behavior for 2.6.4 and 2.7.0 with the delegate-keyword-argument-separation branch:

```
ruby 2.6.4p104    │ ruby 2.7.0-delegate-keyword-argument-separation
[42]              │ [42]
42                │ 42
{:k=>42}          │ {:k=>42}
{}                │ {}
{:k=>42}          │ {:k=>42}
[nil, {:k=>42}]   │ warn! warn! [nil, {:k=>42}]
[nil, {:k=>42}]   │ [{:k=>42}, {}]
[{:k=>42}, {}]    │ [{:k=>42}, {}]
```

The only behavior difference is for `**{}`, which was ignored by the parser in 2.6, and is not likely to appear in production code.  The only warning in 2.7 is for `proxy.arg0o(hash)`, which correctly warns in 2.7 as the behavior will change in 3.0 to be the same as `proxy.arg0o(hash, **opt)` in 2.6.

Note that could use metaprogramming to reduce the duplication:

```ruby
class Proxy < BasicObject
  def initialize(target)
    @target = target
  end

  kw = ", **kw" if ::RUBY_VERSION >= '2.7'
  class_eval(<<-END, __FILE__, __LINE__+1)
    def method_missing(*a#{kw}, &b)
      @target.send(*a#{kw}, &b) rescue $!
    end
  END
  pass_positional_hash(:method_missing) if respond_to?(:pass_positional_hash, true)
end
```

----------------------------------------
Misc #16157: What is the correct and *portable* way to do generic delegation?
https://bugs.ruby-lang.org/issues/16157#change-81488

* 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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