[ruby-core:77273] [Ruby trunk Bug#12521] Syntax for retrieving argument without removing it from double-splat catch-all

Issue #12521 has been updated by Tomohiro Hashidate.


I want this feature.
Keyword argument is safer and more explanatory than splat style.
But I sometimes want to pass whole arguments to other methods.

Example.
CLI gem that uses AWS api and accepts api key, secret key, region, and endpoint as command options.

~~~ ruby
class KMS
  def initialize(key_id, region:, access_key_id: nil, secret_access_key: nil)
    @client = Aws::KMS::Client.new({
      region: region,
      access_key_id: access_key_id,
      secret_access_key: secret_access_key,
    })
    @key_id = key_id
  end

  def encrypt(value)
    resp = @client.encrypt(key_id: @key_id, plaintext: YAML.dump(value))
    Base64.strict_encode64(resp.ciphertext_blob)
  end

  def decrypt(value)
    resp = @client.decrypt(ciphertext_blob: Base64.strict_decode64(value))
    YAML.load(resp.plaintext)
  end
end
~~~

If I can use `__keyargs__` (tentative name), I can omit redundant lines.

~~~ ruby
class KMS
  def initialize(key_id, region:, access_key_id: nil, secret_access_key: nil)
    @client = Aws::KMS::Client.new(__keyargs__) # simple
    @key_id = key_id
  end

  # ...
~~~

This example has few args, but I sometimes needs so many args when I kick complicated web API.

----------------------------------------
Bug #12521: Syntax for retrieving argument without removing it from double-splat catch-all
https://bugs.ruby-lang.org/issues/12521#change-60503

* Author: Guyren Howe
* Status: Closed
* Priority: Normal
* Assignee: 
* ruby -v: 
* Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
----------------------------------------
There is an interesting style of programming that is *almost* really easy to do in Ruby. It would work elegantly with a simple change. A double-colon keyword argument should be available that will still leave the same argument captured by a double-splat argument if present. With this available, it becomes easy to pass down a "context" through a call chain.

Consider this:

```
def controller name::, **context
  …
  log_then_render something:, name:, **context
end

def log_then_render **context
  log context
  complex_logic_then_render **context
end

def complex_logic_then_render name::, **context
…
Bunch of further calls

def render name::, something::, **context
  …
end
```

Now assume I decide render needs a foo argument, that I obtain in my controller. The only functions that are aware of or have any need for the argument are controller and render. With functions written in this style, I only need to modify the two functions that need to know about the argument:

```
def controller name::, **context
  …
  log_then_render something:, name:, foo: foo_value, **context
end

… no changes …

def render name::, something::, foo:: **context
  … now use foo …
end
```

This is, I accept, unusual. I've not seen a language that offers this sort of feature (I call them, for various reason I don't have time to go into now, FREST functions). I can basically implement this now with a decorator, but it's a little ugly and slow.

It just occurred to me that an alternative would be a triple-splat final argument (or some such) that gathers all the keywords.

There is a related problem with the way double-splat and regular keyword arguments interact that should be fixed anyway.



-- 
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