N-Dimensional list comprehension

list_comprehension-Nd.rb

# N-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 for ranges, &condition
      first_range = ranges.values.first.to_a
      if ranges.length > 1
        @matrix = first_range.product(*ranges.values[1...ranges.length].collect { |r| r.to_a }) 
      else
        @matrix = first_range.product
      end

      ranges.keys.each { |sym| @variables.add_to_scope sym }

      @matrix.each do |tuple|
        ranges.keys.each_with_index do |sym, i|
          @variables.set sym, tuple[i]
        end
        if block_given?
          @result << @variables.instance_eval(&@expression) if @variables.instance_eval(&condition)
        else
          @result << @variables.instance_eval(&@expression)
        end
      end

      @result
    end
  end
end

p list { x * y }.for(:x => 0...3, :y => 0...3) { x != 0 and y != 0 })