[ruby-core:91308] [Ruby trunk Feature#15563] #dig that throws an exception if an key doesn't exist

Issue #15563 has been updated by 3limin4t0r (Johan Wentholt).


My scenario would be the similar as described by k0kubun.

```Ruby
# The connection translates the request to JSON and parses the response
# from JSON into the correct objects. In this case a nested hash structure.
response = connection.send(request)

# assign shortcuts
report = response
         .fetch('Era.Common.NetworkMessage.ConsoleApi.Reports.RpcGenerateReportResponse')
         .fetch('report')

column_data   = report.fetch('data').fetch('columns')
column_labels = report.fetch('rendering').fetch('table').fetch('columns')

# build report
report_data = column_data.each_with_object({}) do |column, data|
  column_id       = column.fetch('header').fetch('column_id')
  data[column_id] = column.fetch('values')
end

report = column_labels.each_with_object({}) do |column, data|
  label       = column.fetch('label').fetch('literal')
  column_id   = column.fetch('column_id')
  data[label] = report_data.fetch(column_id)
end
```

From the above scenario you can see that having this new functionality would help out a lot.

The reason I use `#fetch` here is because the API to which I'm talking might change it's structure. Getting an error as soon as possible reduces debug time, since the code won't move on with a `nil` value (that probably raises an exception somewhere later).

----------------------------------------
Feature #15563: #dig that throws an exception if an key doesn't exist
https://bugs.ruby-lang.org/issues/15563#change-76554

* 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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