[Proposal] Hash#transform in Ruby

hash.rb

class Hash
  def transform(&block)
    new_hash = {}
    each_pair do |key, value|
      rtn = block.call(key, value)
      case rtn
      in nil | true then new_hash[key] = value
      in false then next
      in Array[[_, _], *]
        rtn.each do |k, v|
          new_hash[k] = v
        end
      in [k, v] then new_hash[k] = v
      else
        raise 'Unexpected return value'
      end
    end
    new_hash
  end
end

require 'minitest/autorun'

class HashTransformTest < MiniTest::Test
  def test_transform
    hash = {a: 1, b: 2, c: 3}
    new_hash = hash.transform do |key, value|
      [key.to_s, value * 2]
    end
    assert_equal({'a' => 2, 'b' => 4, 'c' => 6}, new_hash)

    new_hash2 = hash.transform do |key, value|
      [key.to_s, value * 2] if value.odd?
    end
    assert_equal({'a' => 2, :b => 2, 'c' => 6}, new_hash2)

    new_hash3 = hash.transform do |key, value|
      if value.even?
        false
      else
        [key.to_s, value * 2]
      end
    end
    assert_equal({'a' => 2, 'c' => 6}, new_hash3)

    new_hash4 = hash.transform do |key, value|
      [[key.to_s, value * 2], [key, value]]
    end
    assert_equal({'a' => 2, :a => 1, 'b' => 4, :b => 2, 'c' => 6, :c => 3}, new_hash4)
  end
end