formula building example

gistfile1.txt

class Formula
  attr_accessor :value

  def initialize(v)
    @value = v
  end

  def to_s
    value
  end

  [:=~, :+, :-].each do |operator_name|
    define_method(operator_name) do |operand|
      self.class.new("#{self} #{operator_name} #{operand}")
    end
  end
end

class FormulaInterpreter
  class << self
    def instance
      @instance ||= new
    end
  end

  def method_missing(method_name, *args)
    super unless args.empty?
    @variables ||= {}
    @variables[method_name] ||= Formula.new(method_name.to_s)
  end
end

def formula(&block)
  FormulaInterpreter.instance.instance_eval(&block)
end

f = formula { a =~ b + c - d + b + 1 }
puts f
# => 'a =~ b + c - d + b + 1'