Hash#maph

hash_maph.rb

# an aproach to http://www.ruby-forum.com/topic/3446541

$VERBOSE = true

class Hash
  def maph
    return to_enum(__method__) unless block_given?
 
    {}.tap {|hash|
      each_pair do |key, value|
        key, value = yield key, value
        warn "already assigned key '#{key}'" if hash.has_key? key
        hash[key] = value
      end
    }
  end
end

list = [4, 5, 6, 4, 5, 6, 6, 7]
 
p list.group_by{|v|v}.maph{|k, v|[k, v.size]}

in ruby-1.9.3p0

{4=>2, 5=>2, 6=>3, 7=>1}