雑な感じの更新版。親のメソッドのことを考えるともうちょっと色々とやる必要があるはず。

contract.rb

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")