[ruby-core:91286] [Ruby trunk Feature#15563] #dig that throws an exception if an key doesn't exist

Issue #15563 has been updated by k0kubun (Takashi Kokubun).


Personally I've hit real-world use-case for this feature many times.

I often manage structured configs with nested YAML files and load it from Ruby. With current ruby, to avoid unhelpful exception `NoMethodError`, I assert the existence of the deep keys using a `Hash#fetch` chain like this:

```ruby
config = YAML.load_file('config.yml')
config.fetch('production').fetch('environment').fetch('SECRET_KEY_BASE') #=> an exception like: KeyError: key not found: "SECRET_KEY_BASE"
```

If we have such a method, we would be able to write (let's say it's named `Hash#fetch_keys` instead of `#dig!`):

```ruby
config.fetch_keys('production', 'environment', 'SECRET_KEY_BASE')
```

and its good part is that we could get a more helpful error message like "key not found: production.environment.SECRET_KEY_BASE" whose nest information can't be get with `Hash#fetch` method chains.

---

By the way, if we had this, I would like to have a keyword argument `default:` like the second optional argument of `Hash#fetch`:

```ruby
env = 'production' # can be 'staging', 'development'
config.fetch_keys(env, 'environment', 'SECRET_KEY_BASE', default: '002bbfb0a35d0fd05b136ab6333dc459')
```

we want to safely manage the credentials only for production, so sometimes we don't want to manage credentials in (safely-managed originally-encrypted) YAML file for development environment and just want to return the unsafe thing as a default value.

----------------------------------------
Feature #15563: #dig that throws an exception if an key doesn't exist
https://bugs.ruby-lang.org/issues/15563#change-76536

* Author: 3limin4t0r (Johan Wentholt)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Ruby 2.3.0 introduced `#dig` for *Array*, *Hash* and *Struct*. Both *Array* and *Hash* have `#fetch` which does the same as `#[]`, but instead of returning the default value an exception is raised (unless a second argument or block is given). *Hash* also has `#fetch_values` which does the same as `#values_at`, raising an exception if an key is missing. For `#dig` there is no such option.

My proposal is to add a method which does the same as `#dig`, but instead of using the `#[]` accessor it uses `#fetch`.

This method would look something like this:

```Ruby
module DigWithException
  def dig_e(key, *others)
    value = fetch(key)
    return value if value.nil? || others.empty?

    if value.respond_to?(__method__, true)
      value.send(__method__, *others)
    else
      raise TypeError, "#{value.class} does not have ##{__method__} method"
    end
  end
end

Array.include(DigWithException)
Hash.include(DigWithException)
```

The exception raised is also taken from `#dig` (`[1].dig(0, 1) #=> TypeError: Integer does not have #dig method`). I personally have my issues with the name `#dig_e`, but I haven't found a better name yet.

There are also a few other things that I haven't thought out yet.

 1. Should this method be able to accept a block which, will be passed to the `#fetch` call and recursive `#dig_e` calls?  

    ```Ruby
    module DigWithException
      def dig_e(key, *others, &block)
        value = fetch(key, &block)
        return value if value.nil? || others.empty?

        if value.respond_to?(__method__, true)
          value.send(__method__, *others, &block)
        else
          raise TypeError, "#{value.class} does not have ##{__method__} method"
        end
      end
    end

    Array.include(DigWithException)
    Hash.include(DigWithException)
    ```

 2. I currently kept the code compatible with the `#dig` description.

    > Extracts the nested value specified by the sequence of *key* objects by calling `dig` at each step, returning `nil` if any intermediate step is `nil`.

    However, with this new version of the method one could consider dropping the *"returning `nil` if any intermediate step is `nil`"* part, since this would be the more strict version.

    ```Ruby
    module DigWithException
      def dig_e(key, *others)
        value = fetch(key)
        return value if others.empty?

        if value.respond_to?(__method__, true)
          value.send(__method__, *others)
        else
          raise TypeError, "#{value.class} does not have ##{__method__} method"
        end
      end
    end

    Array.include(DigWithException)
    Hash.include(DigWithException)
    ```

I'm curious to hear what you guys think about the idea as a whole, the method name and the two points described above.
 



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