Why is 'transform_keys' method undefined?

Question

This example is taken directly from the Ruby 2.4.1 documentation, and I can confirm I am running 2.4.1:

({a: 1, b: 2, c: 3}).transform_keys {|k| k.to_s}

When I execute it, I receive the following error:

NoMethodError: undefined method `transform_keys' for {:a=>1, :b=>2, :c=>3}:Hash

Why is the transform_keys method not defined?

Answer 1

As observed in another question, it appears that http://ruby-doc.org currently (erroneously) generates the documentation for Ruby 2.4.1 based on Ruby trunk instead of the actually released 2.4.1 version.

Unfortunately, the Hash#transform_keys method is not yet released as part of any 2.4 release. It was developed and comitted to Ruby trunk with Feature #13583 but was not (yet) backported to the stable 2.4 branch.

As a workaround for that, you can use this method instead:

def transform_keys(hash)
  result = {}
  hash.each_pair do |key, value|
    result[yield(key)] = value
  end
  result
end

Equivalently (that is: a bit shorter but also a slightly slower) you could use this:

def transform_keys(hash)
  hash.keys.each_with_object({}) do |key, result|
    result[yield(key)] = hash[key]
  end
end

If you are bold, you can add this as a core-patch to the Hash class where you then just have to replace every mention of hash with self.

Note that ActiveSupport (i.e. Rails) brings a core-patch with this exact method since about forever. They use a mixture of both implementations.