module Contract
  module Contracts
    def push &contract
      stack.push contract
    end

    def stack
      @stack || @stack = []
    end

    private :stack
  end

  class ContractRequires
    include Contracts

    def assert *args
      stack.each {|r| r.call(*args)}
    end
  end

  class ContractEnsures
    include Contracts

    def assert arg
      stack.each {|e| e.call(arg)}
    end
  end

  class ContractedMethod
    def initialize &contracts
      self.instance_eval &contracts
    end

    def evaluation *args
      contract_requires.assert *args
      ret = @body.call *args
      contract_ensures.assert ret
    end

    def requires &contract
      contract_requires.push &contract
    end

    def ensures &contract
      contract_ensures.push &contract
    end

    def body &method_body
      @body = method_body
    end

    def contract_requires
      @contract_requires || @contract_requires = ContractRequires.new
    end

    def contract_ensures
      @contract_ensures || @contract_ensures = ContractEnsures.new
    end

    protected :requires, :ensures, :body, :contract_requires, :contract_ensures
  end
end

m = Contract::ContractedMethod.new do
  requires do |arg1, arg2|
    puts "requires"
    p arg1
    p arg2
  end
  ensures do |ret|
    puts "ensures"
    p ret
  end
  body do |arg1, arg2|
    puts "body"
    p arg1
    p arg2
    "result"
  end
end

m.evaluation("first arg", "second arg")