[ruby-core:115299] [Ruby master Feature#19370] Anonymous parameters for blocks?

Issue #19370 has been updated by mame (Yusuke Endoh).


We discussed this issue and #19983 at the dev meeting.

* Only method arguments can be delegated by `foo(*)`.
* It should raise a SyntaxError to use `foo(*)` in a block that accepts `*` explicitly.
* It is allowed to use `foo(*)` in a block that does not accept `*` explicitly.

```ruby
def m(*)
  ->(*) { p(*) }    # SyntaxError
  ->(x, *) { p(*) } # SyntaxError
  ->(x) { p(*) }    #=> 1
  proc {|x| p(*) }  #=> 1
end

m(1).call(2)
```

The main discussion was as follows.

* `def m(*); ->(*) { p(*) }; end` is indeed confusing.
* We want to prohibit a case where takes `*` and `**` from different blocks, such as `->(&) { ->(**) { ->(*) { foo(*, **, &) } }`.
* We want to allow `def m(*) = @mutex.synchronize { m2(*) }` enclosed in a block that does not accept `*`. So we cannot prohibit to take arguments from outer blocks (or the method arguments).
* Should we allow to take block arguments in unambiguous cases, such as `def m; ->(*) { p(*) }; end` ?
  * @knu: In many cases, what we want to delegate is the method arguments.
  * @ko1: Once we prohibit it, we can allow the delegation of block arguments later, if it is really needed.

----------------------------------------
Feature #19370: Anonymous parameters for blocks?
https://bugs.ruby-lang.org/issues/19370#change-105217

* Author: zverok (Victor Shepelev)
* Status: Assigned
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
----------------------------------------
Just to clarify: are anonymous parameters delegation is planned to support in blocks? 

It would be a nice addition, if it is possible to implement:
```ruby
# data in form [request method, URL, params]:
[
  [:get, 'https://google.com', {q: 'Ruby'}, {'User-Argent': 'Google-Chrome'}],
  [:post, 'https://gist.github.com', 'body'],
  # ...
].each { |method, *| request(method.to_s.upcase, *) }
```
...and at the very least, consistent with what the method definition can have.

If they are NOT planned to be implemented, I believe that at least error messages should be made much clearer, because currently, this would happen while running the code above:
> no anonymous rest parameter (SyntaxError)

I understand the reason (the `request` clause doesn't "see" anonymous parameter of the **block**, and claims that current **method** doesn't have them), but it looks honestly confusing and inconsistent.



-- 
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