[ruby-core:107754] [Ruby master Feature#18603] Allow syntax like obj.method(arg)=value

Issue #18603 has been updated by sawa (Tsuyoshi Sawada).


Notice that, while `[]` in `[]=` cannot be omitted, `()` in method invocation can be omitted. Are you expecting `()` in the proposed feature to be omittable or not? Either way, I think it would introduce complexity. I think that is a crucial difference between `[]=` and the proposed feature; the latter is not as simple as the former.

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Feature #18603: Allow syntax like obj.method(arg)=value
https://bugs.ruby-lang.org/issues/18603#change-96679

* Author: hmdne (hmdne -)
* Status: Open
* Priority: Normal
----------------------------------------
I propose here to allow a syntax like:

```ruby
obj.method(arg) = value
```

It would be translated to the following:

```ruby
obj.__send__(:method=, arg, value)
```

The lack of this syntax kind of limits the ability to design DSLs in Ruby in my opinion. I don't think this would bring any conflicts with existing parser rules.

My proposal would be to put the value at the last argument, akin to how `[]=` works. So, for example this code would work:

```ruby
module Indexable
  def dig=(*path, last, value)
    if path.empty?
      self[last] = value
    else
      first = path.shift
      self[first]&.dig(*path, last) = value
    end
  end
end

Hash.include Indexable
Array.include Indexable
```

The kwargs may be supported similarly to how they work on `[]=`, ie. becoming a penultimate Hash argument. While maybe not perfect, it is consistent with how `[]=` works and I imagine most usecases won't require kwargs.



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