Florian Frank flori@ping.de

This is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License Version 2 as published by the Free Software Foundation: www.gnu.org/copyleft/gpl.html

The latest version of protocol can be found at

• rubyforge.org/frs/?group_id=4778

The homepage of this library is located at

• protocol.rubyforge.org

This library offers an implementation of protocols against which you can check the conformity of your classes or instances of your classes. They are a bit like Java Interfaces, but as mixin modules they can also contain already implemented methods. Additionally you can define preconditions/postconditions for methods specified in a protocol.

This defines a protocol named Enumerating:

Enumerating = Protocol do
  # Iterate over each element of this Enumerating class and pass it to the
  # +block+.
  def each(&block) end

  include Enumerable
end

Every class, that conforms to this protocol, has to implement the understood messages (each in this example - with no ordinary arguments and a block argument). The following would be an equivalent protocol definition:

Enumerating = Protocol do
  # Iterate over each element of this Enumerating class and pass it to the
  # +block+.
  understand :each, 0, true

  include Enumerable
end

An example of a conforming class is the class Ary:

class Ary
  def initialize
    @ary = [1, 2, 3]
  end

  def each(&block)
    @ary.each(&block)
  end

  conform_to Enumerating
end

The last line (this command being the last line of the class definition is important!) of class Ary conform_to Enumerating checks the conformance of Ary to the Enumerating protocol. If the each method were not implemented in Ary a CheckFailed exception would have been thrown, containing all the offending CheckError instances.

It also mixes in all the methods that were included in protocol Enumerating (Enumerable‘s instance methods). More examples of this can be seen in the examples sub directory of the source distribution of this library in file examples/enumerating.rb.

It’s also possible to mix protocol specification and behaviour implementation like this:

Locking = Protocol do
  specification # not necessary, because Protocol defaults to specification
                # mode already

  def lock() end

  def unlock() end

  implementation

  def synchronize
    lock
    begin
      yield
    ensure
      unlock
    end
  end
end

This specifies a Locking protocol against which several class implementations can be checked against for conformance. Here’s a FileMutex implementation:

class FileMutex
  def initialize
    @tempfile = Tempfile.new 'file-mutex'
  end

  def path
    @tempfile.path
  end

  def lock
    puts "Locking '#{path}'."
    @tempfile.flock File::LOCK_EX
  end

  def unlock
    puts "Unlocking '#{path}'."
    @tempfile.flock File::LOCK_UN
  end

  conform_to Locking
end

The Locking#synchronize method is a template method (see en.wikipedia.org/wiki/Template_method_pattern), that uses the implemtented methods, to make block based locking possbile:

mutex = FileMutex.new
mutex.synchronize do
  puts "Synchronized with '#{file.path}'."
end

Now it’s easy to swap the implementation to a memory based mutex implementation instead:

class MemoryMutex
  def initialize
    @mutex = Mutex.new
  end

  def lock
    @mutex.lock
  end

  def unlock
    @mutex.unlock
  end

  conform_to Locking # actually Mutex itself would conform as well ;)
end

To check an object for conformity to the Locking protocol call Locking.check object and rescue a CheckFailed. Here’s an example class

class MyClass
  def initialize
    @mutex = FileMutex.new
  end

  attr_reader :mutex

  def mutex=(mutex)
    Locking.check mutex
    @mutex = mutex
  end
end

This causes a CheckFailed exception to be thrown:

obj.mutex = Object.new

This would not raise an exception:

obj.mutex = MemoryMutex.new

And neither would this

obj.mutex = Mutex.new # => #<Mutex:0xb799a4f0 @locked=false, @waiting=[]>

because coincidentally this is true

Mutex.conform_to? Locking # => true

and thus Locking.check doesn’t throw an exception. See the examples/locking.rb file for code.

You can add additional runtime checks for method arguments and results by specifying pre- and postconditions. Here is the classical stack example, that shows how:

StackProtocol = Protocol do
  def push(x)
    postcondition { top === x }
    postcondition { result === myself }
  end

  def top() end

  def size() end

  def empty?()
    postcondition { size === 0 ? result : !result }
  end

  def pop()
    s = size
    precondition { not empty? }
    postcondition { size === s - 1 }
  end
end

Defining protocols and checking against conformance doesn’t get in the way of Ruby’s duck typing, but you can still use protocols to define, document, and check implementations that you expect from client code.