Contraction

A code-by-contract library for ruby, using RDoc documentation to enforce the contract requirements

Basic usage

Just include Contraction at the bottom of your class/module:

require 'rubygems'
require 'contraction'

class MySuperCoolClass
  ##
  # The foobar function takes a string and an integer between 1 and 100 and
  # joins them, returning the result.
  #
  # @param [String] foo A string to which we add the number
  # @param [Fixnum] bar A number to add to foo { bar >= 0 and bar <= 100 }
  # @return [String] The concatonation of foo and bar { return.start_with?(foo) and return.include?(bar.to_s) }
  def foobar(foo, bar)
    "#{foo} #{bar.to_s}"
  end

  include Contraction
end

And you're done.

You define your normal documentation (you do document your code, don't you?) using regular RDoc syntax. For the params and returns, however, you can add an extra bit of information at the end. Any code put in between curly braces ({}) is evaluated as the contract for that param or return. You just use the names of the params for their values, and return for the return value. If you provide a type for either the param or the return, type-checking will be enforced as well. A full-qualified type is recommended (Foo::Bar instead of Bar.)

Ruby and the splendiferous always-open class

Because a class in Ruby can never really be "fully loaded", there are strange cases where Contraction may not build contracts for all the methods that you want it to. For example, if some third-party code modifies your class to add methods at run-time, after Contraction has already been loaded. In this case, if you would like contracts to be enabled for those classes, you can update the annotated methods by calling:

class MySuperCoolClass
  ...

  include Contraction
end

...

MySuperCoolClass.update_contracts

Please bear in mind that you will re-incur the overhead of parsing the RDoc docs for each method.

Warning about speed/overhead

This will slow things down. All-in-all you're looking at about an 8x increase in overhead vs just calling a function. It is not recommended that you use this for every method in a deeply-nested code-path. This overhead is more-or-less in-line with other Ruby design-by-contract libraries, however, and with the added benefit of free documentation. It is recommended that you have some concept of environment (staging, dev, production, etc.), and only include Contraction in development-like environments:

require 'rubygems'
require 'contraction'

class MySuperCoolClass
  ...

  include Contraction if Rails.env.development?
end