Method Decorators for Ruby Code
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    There was a look of sheer panic in the dame's eye as she pulled her
  clutch to her chest.  I'd seen it a thousand times before, and she was
  right to be afraid - the things in her purse could easily pay the rent
  on my dingy little office for a couple of years.  A Decorator of about
  thirty, she'd clearly never considered the threats inherent to the real
  world - not every gentleman Caller had good intentions.  Gently, I
  explained how she might protect it, wrapping it in layers security.
  Cocooning it.    Encasing it.
    At length, she handed the purse over, and I breathed a sigh of relief.
  Unprotected valuables make me nervous.  That's why I took this job.  I
  make things safe.

Encase is an library of extensible library of method decorators, primarily focussed on promoting and improving the discovery of bugs. The first step towards this end is implementing a robust type-checking system, to automatically validate the inputs and outputs of each method call - this is implimented via the Contract decorator.


$ cd vendor
$ git clone git://



Contracts give you a way to express explicit type validations on your Ruby methods. This makes the expectations around your code much more clear, and helps (with the aid of robust unit tests) provide guarantees about the type safety of your code.

Getting started is easy:

class StrictTyping
  include Encase::Contracts

  Contract Integer, Returns[Integer]
  def double(n)
    n * 2

The special type Returns can be used to express type constraints around the return value of the method. Because this is such a common use case, there is a shorthand for expressing that constraint.

Contract Integer => Integer
def double(n)
  n * 2

This is a familiar system of constraints to those who've used C, Java, or Haskell before, but part of Ruby's strength comes from duck typing. In that spirit, we can write contracts that validate not only classes but any object that supports Ruby's case equality operator (i.e. ===)

Contract /^\d+$/ => Integer
def int(str)

Contracts are capable of much more, including validating the signature of blocks (and other procs!), destructuring arrays and hashes, typechecking splatted arguments, checking logical conjunctions of types and more!

Contracts also offer two callbacks, executed after each constraint validation, which can be overridden to better fit the environment your application runs in.

class Encase::Contract
  # Exceptions are too harsh in Production; maybe we'll just add a message
  # to the logfile...
  def failure(data)
    Logger.log_error("Failed Validation: " + data.inspect)
    return true

In Production?

Some of the included decorators may impart a non-trivial runtime performance penalty which may be undesirable in a production environment. For those cases, every decorator class includes a disable method which will cause all instances of that class (and all subclasses) to bypass all non-critical behavior.

Similarly, some decorators include explicit extension points, making it easy to redefine the behavior of the decorator to be better suited to your production environment.


Copyright (c) 2012 Pieter van de Bruggen.

(The GIFT License, v3)

Permission is hereby granted to use this software and/or its source code for whatever purpose you should choose. Seriously, go nuts. Use it for your pet open-source project, your "financial independence" webapp, or your distributed financial databases.

I don't care, it's yours. Change the name on it if you want -- in fact, if you start significantly changing what it does, I'd rather you did! Make it your own little work of art, complete with a stylish flowing signature in the corner. All I really did was give you the canvas. And my blessing.

Know always right from wrong, and let others see your good works.