ErrorHandlingProtocol

Provides common error messages in the protocol of class Object.
For example, to report the error that a subclass should have implemented one of the superclass's messages.

Inspired in Smalltalk's Error Handling Protocol.
See Smalltalk-80: The Language And Its Implementation, Chapter 6, page 102.

Installation

Add this line to your application's Gemfile:

gem 'error_handling_protocol'

And then execute:

$ bundle

Or install it yourself as:

$ gem install error_handling_protocol

Usage

Just require 'error_handling_protocol'.

Subclass Responsibility

Report to the user that a method specified in the superclass of the receiver should have been implemented in the receiver's class.

Motivation

Ruby does not have a way to communicate a method should be implemented by a subclass.
Either the method is not defined at all in the superclass, or it is implemented by throwing some kind of error.

The first alternative is not good at all because the model fails to communicate whether the message is part of the hierarchy's procotol or not, and because the method was not defined in the superclass, a NoMethodError will be raised.

The problem with that is ambiguity: it is not clear if the object was not expected to respond to the message, or if someone forgot to implement it.
That means is the developer's job to read the class definition to clear that out, on a case-by-case basis.

The second alternative communicates better the fact that a message is part of the hierarchy's protocol by explicitly informing which method a subclass was supposed to implement, freeing the developer from doing that task manually. The downside is that the error raising collaborations will quickly be duplicated all over the codebase (i.e. fail 'implement me').

The :subclass_responsibility message addresses this issue by providing a reusable message developers can use to communicate better their intentions when designing classes.
The error raised when sent, SubclassResponsibilityError, provides detailed information about the method that should have been implemented, the subclass responsible to do it, and whether the method should be an instance or class one.

Example: communicating subclasses are responsible to implement an instance method

class A
    def m1
        self.subclass_responsibility
    end
end

class B < A
end

Sending the :m1 message to an instance of B yields:

B.new.m1

ErrorHandlingProtocol::SubclassResponsibilityError: B had the responsibility to implement :m1 instance method.

Example: communicating subclasses are responsible to implement a class method

class C
    def self.m2
        self.subclass_responsibility
    end
end

class D < C
end

Sending the :m2 message to D yields:

D.m2

ErrorHandlingProtocol::SubclassResponsibilityError: D had the responsibility to implement :m2 class method.

Contributing

1. Fork it ( http://github.com//error_handling_protocol/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

Developed & maintained by 10 Pines