File: doc-main.txt [RDoc Documentation]

Protocol - Method Protocol Specifications in Ruby

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License

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

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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

Description

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.

Usage

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.

Template Method Pattern

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.

Preconditions and Postconditions

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.

Error modes in Protocols

You can set different error modes for your protocols. By default the mode is set to :error, and a failed protocol conformance check raises a CheckError (a marker module) exception. Alternatively you can set the error mode to :warning with:

 Foo = Protocol do
   check_failure :warning
 end

during Protocol definition or later

  Foo.check_failure :warning

In :warning mode no execptions are raised, only a warning is printed to STDERR. If you set the error mode via Protocol::ProtocolModule#check_failure to :none, nothing will happen on conformance check failures.

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