1 dimensional list comprehension

list_comprehension-1d.rb

# 1 dimensional list comprehension

module Kernel
  def list &block
    Comprehension::List.new &block
  end
end

module Comprehension
  class List
    def initialize &block
      @result = []
      @expression = block if block_given?

      @variables = Object.new

      # creates a basic struct pattern
      def @variables.add_to_scope sym
        metaclass = class << self; self; end
        metaclass.send :attr_accessor, sym
      end

      def @variables.set sym, value
        self.send "#{sym}=".to_sym, value
      end
    end

    def ranges ranges, &condition
      ranges.each_pair do |sym, range|
        @variables.add_to_scope sym
        range.each do |i|
          @variables.set sym, i
          if block_given?
            if @variables.instance_eval &condition
              @result << @variables.instance_eval(&@expression)
            end
          else
            @result << @variables.instance_eval(&@expression)
          end
        end
      end

      @result
    end
  end
end

list{ x * 16 }.ranges(:x => 0..16) { x % 4 == 0 }
#=> [0, 64, 128, 192, 256]

word = "spam"

list{ [word[0,i], word[i, word.length]] }.ranges(:i => 0..word.length)
#=> [["", "spam"], ["s", "pam"], ["sp", "am"], ["spa", "m"], ["spam", ""]]