Portrayal::Contracts (proof of concept). Portrayal gem is here: http://github.com/maxim/portrayal

0-proof-of-concept.rb

class Foo
  extend Portrayal

  keyword :word
  keyword :size, default: 'oops-not-an-integer' # Contract-violating default also gets blocked.
  public :word=, :size=

  contract('word must be String >5 chars') { String === word && word.size > 5 }
  contract('size must be an integer') { Integer === size }
  contract('size must equal length of word') { size == word.length }
end

1-usage-1-general.rb

Foo.portrayal.list_contracts # => ["word must be String >5 chars", "size must be an integer", "size must equal length of word"]

Foo.new(word: 'abcdef') # => ArgumentError: size must be an integer  <===== size's default "oops-not-an-integer" causes this
Foo.new(word: 'abcdef', size: 'x') # => ArgumentError: size must be an integer
Foo.new(word: 'abcdef', size: 1) # => ArgumentError: size must equal length of word

f = Foo.new(word: 'abcdef', size: 6) # => #<Foo @size=6, @word="abcdef">
f.word = 'abc' # => ArgumentError: word must be String >5 chars
f.word = 'abcdefg' # => ArgumentError: size must equal length of word
f.word # => 'abcdef'

f.size = 1 # => ArgumentError: size must equal length of word
f.size # => 6

f.word = 'xoxoxo' # => "xoxoxo"

f.update(word: 'abc', size: 1) # => ["word must be String >5 chars", "size must equal length of word"]
f.update(word: '123456', size: 1) # => ["size must equal length of word"]
f.update(word: '1234567', size: 7) # => nil
f # => #<Foo @size=7, @word="1234567">

1-usage-2-types.rb

module CommonTypes
  def int(name)
    contract("#{name} must be an integer") { Integer === send(name) }
    name
  end

  def odd(name)
    contract("#{name} must be odd") { send(name).odd? }
    name
  end
  
  def md5(name)
    contract("#{name} must be an md5 string") {
      string = send(name)
      String === string && string.match?(/\A[a-f0-9]{32}\z/)
    }

    name
  end
end

class Bar
  extend Portrayal
  extend CommonTypes

  md5 keyword :token
  odd int keyword :number # <- they're composable too!
end

Bar.new(token: "foo", number: 'x') # => ArgumentError: token must be an md5 string
Bar.new(token: "1ddcb92ade31c8fbd370001f9b29a7d9", number: 'x') # => ArgumentError: number must be an integer
Bar.new(token: "1ddcb92ade31c8fbd370001f9b29a7d9", number: 2) # => ArgumentError: number must be odd
Bar.new(token: "1ddcb92ade31c8fbd370001f9b29a7d9", number: 3) # => #<Bar @number=3, @token="1ddcb92ade31c8fbd370001f9b29a7d9">

Bar.portrayal.list_contracts
=> ["token must be an md5 string", "number must be an integer", "number must be odd"]

2-implementation.rb

module Portrayal::Contracts
  class Contract
    attr_reader :name, :block
    def initialize(name, blk); @name, @block = name, blk end
    def enforce(obj); raise ArgumentError, name unless pass?(obj) end
    def failure(obj); name unless pass?(obj) end
    def pass?(obj); !!obj.instance_exec(&@block) end
    def initialize_dup(src); @name, @block = src.name.dup, src.block.dup end
  end

  module SchemaExt
    attr_reader :contracts, :contract_module
    def initialize(*); @contracts = []; @contract_module = Module.new; super end
    def collect_failures(obj); contracts.map { |c| c.failure(obj) }.compact end
    def add_contract(name, blk); @contracts << Contract.new(name, blk) end
    def list_contracts; contracts.map(&:name) end
    def set_changes(obj, hash)
      obj.instance_variable_set(:@__portrayal_cskip, true)
      hash.each { |k, v| obj.send("#{k}=", v) }
    ensure
      obj.remove_instance_variable(:@__portrayal_cskip)
    end

    def initialize_dup(other)
      @contracts = other.contracts.map(&:dup)
      @contract_module = other.contract_module.dup
      super
    end

    def enforce_contracts(obj)
      return if obj.instance_variable_get(:@__portrayal_cskip)
      contracts.each { |c| c.enforce(obj) }
    end

    def update_contract_module!
      @contract_module.module_eval(render_contract_module_code)
    end

    def try_changes(obj, changes)
      sandbox = obj.dup
      sandbox.class.portrayal.set_changes(sandbox, changes)
      sandbox.class.portrayal.collect_failures(sandbox)
    end

    def render_contract_module_code
      writers = keywords.map { |k|
        <<-RUBY
        protected def #{k}=(v)
          return super if @__portrayal_cskip
          failures = self.class.portrayal.try_changes(self, #{k}: v)
          return super if failures.empty?
          raise ArgumentError, failures[0]
        end
        RUBY
      }.join("\n")

      <<-RUBY
      def initialize(*, **)
        super
        self.class.portrayal.enforce_contracts(self)
      end

      def update(changes)
        failures = self.class.portrayal.try_changes(self, changes)
        return failures unless failures.empty?
        self.class.portrayal.set_changes(self, changes)
        nil
      end
      #{writers}
      RUBY
    end
  end

  module ClassExt
    def inherited(c); super(c); c.include(c.portrayal.contract_module) end
    def contract(name, &block); portrayal.add_contract(name, block) end
    def keyword(*, **)
      name = super
      include portrayal.contract_module
      portrayal.update_contract_module!
      name
    end
  end

  ::Portrayal::Schema.prepend(SchemaExt)
  ::Portrayal.prepend(ClassExt)
end

That definitely helps!