From: Gavin Sinclair <gsinclair@...>
Date: 2003-07-19T23:22:20+09:00
Subject: Proposal: Array#to_h, to simplify hash generation

Hi -talk,

Ruby has wonderful support for chewing and spitting arrays.  For
instance, it's easy to produce an array from any Enumerable using
#map.  With hashes, however, it's a bit more cumbersome.

For example, the following method is typical of my code:

    # return { filename -> size }
  def get_local_gz_files
    files = {}
    Dir["*.gz"].each do |filename|
      files[filename] = File.stat(filename).size
    end
    files
  end

The pattern is: create an empty hash, populate it, and return it.  Now
Ruby is a wonderfully expressive and terse language.  Accordingly, the
two lines devoted to initialising and returning the hash in the above
code seem wasted.

If Ruby had Array#to_h, then I could rewrite it as:

    # return { filename -> size }
  def get_local_gz_files
    Dir["*.gz"].map { |filename|
      [ filename, File.stat(filename).size ]
    }.to_h
  end


The proposed implementation of Array#to_h is per the following code:

  class Array
    def to_h
      hash = {}
      self.each do |elt|
        raise TypeError unless elt.is_a? Array
        key, value = elt[0..1]
        hash[key] = value
      end
      hash
    end
  end


For the final justification, note that this is the logical reverse of
Hash#to_a:

  h = {:x => 5, :y => 10, :z => -1 }
  a = h.to_a                            # => [[:z, -1], [:x, 5], [:y, 10]]

  # And now, for my next trick...
  a.to_h == h                           # => true (gosh, that actually worked)

  
Thoughts?

Gavin