Handshake

Handshake is an informal AOP and design-by-contract system written in pure Ruby. It’s intended to allow Ruby developers to apply simple, clear constraints to their methods and classes.

Features

Method signature contracts
Contracts on blocks and procs
Method pre- and post-conditions
Class invariants

Examples

Here’s an example of Handshake in action on a hypothetical BankAccount class:

class BankAccount
  attr_reader :balance

  class << self
    def less_than_balance?
      all? positive_number?, clause {|n| n <= balance}
    end
  end

  invariant { balance >= 0 }

  contract positive_number? => anything
  def initialize(balance)
    @balance = balance
  end

  contract less_than_balance? => positive_number?
  def withdraw(amount)
    new_balance = @balance - amount
    @balance = new_balance
    return new_balance
  end
end

Here’s an example that uses an invariant to enforce a constraint on a subclass of Array:

class NonEmptyArray < Array
  include Handshake
  invariant { not empty? }
end

Further specializing the subclass:

class NonEmptyStringArray < NonEmptyArray
  contract :initialize, [[ String ]] => anything
  contract :<<, String => self
  contract :+, many?(String) => self
  contract :each, Block(String => anything) => self
end

Handshake can also define pre- and post-conditions on your methods.

class Foo
  before do
    assert( not @widget.nil? )
  end
  def something_that_requires_widget
    ...
  end
end

See Handshake::ClassMethods for more documentation on exact syntax and capabilities. Handshake::ClauseMethods contains a number of helper and combinator clauses for defining contract signatures.

Caveats

Handshake works by wrapping any class that includes it with a proxy object that performs the relevant contract checks. It acts as a barrier between an object and its callers. Unfortunately, this means that internal calls, for example to private methods, that do not pass across this barrier, are unchecked. Here’s an example:

class UncheckedCall
  include Handshake

  contract String => Numeric
  def checked_public(str); str.to_i; end

  def checked_public_delegates(str)
    checked_private(str)
  end

  private
  contract String => Numeric
  def checked_private(str); str.to_i; end
end

In this example, we have a public checked method protected by a contract. Any external call to this method will be checked. The method marked as checked_public_delegates calls a private method that is itself protected by a contract. But because the call to that private method is internal, and does not pass across the contract barrier, no contract will be applied.

You can get around this problem by calling private methods on the special private method checked_self:

class UncheckedCall
  ...
  def checked_public_delegates(str)
    checked_self.checked_private(str)
  end
  ...
end

License (MIT)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.