Handshake

Handshake is an informal 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. Handshake is written by Brian Guthrie (btguthrie@gmail.com) and lives at handshake.rubyforge.org.

Here's an example of Handshake in action:

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

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