Feature #12461: Hash & keys to make subset.

• Status: Rejected
• Assignee: -
• Target version: -

[ioquatix (Samuel Williams)]

It would be nice if Hash had a way to create a sub-hash with a limited set of keys. This would be useful for keyword functions in particular, because sometimes you have a large hash of things and you only want to send a subset to another function.

A definition somewhat like:

class Hash
    def &(keys)
        keys.each_with_object(self.class.new) {|key, hash| hash[key] = self[key] if has_key?(key)}
    end
end

You can see some similar definitions here: https://github.com/rails/rails/blob/b670fadb978c8a12c3414ed842cd49e4fde2cec0/activesupport/lib/active_support/core_ext/hash/slice.rb#L21

It might be nice to support slice and slice! in a similar logical sense to Array rather than proposed & operator above - that would just be nice shortcut.

[ioquatix (Samuel Williams)]

Just to add to this, I also found an implementation here:

https://github.com/rubyworks/facets/blob/master/lib/core/facets/hash/slice.rb

Now I wonder if this warrants being in the standard library. I believe it probably does since it appears lots of people are using it, and it would probably be significantly faster written in C.

[shyouhei (Shyouhei Urabe)]

• Related to Feature #8499: Importing Hash#slice, Hash#slice!, Hash#except, and Hash#except! from ActiveSupport added

[matz (Yukihiro Matsumoto)]

• Status changed from Open to Closed

I am against having Hash#&. Array#& takes two arrays. The proposed Hash#& takes a hash and key array.
if you want Hash#slice to be added to the core, submit a new issue. But note that now we have real
keyword arguments so that we have less need for manipulating keyword hashes explicitly.

Matz.

[usa (Usaku NAKAMURA)]

• Tracker changed from Bug to Feature
• Status changed from Closed to Rejected