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Contracts let you clearly – even beautifully – express how your code behaves, and free you from writing tons of boilerplate, defensive code.

You can think of contracts as assert on steroids.


gem install contracts

Hello World

A contract is one line of code that you write above a method definition. It validates the arguments to the method, and validates the return value of the method.

Here is a simple contract:

  Contract Num => Num
  def double(x)

This says that double expects a number and returns a number. Here's the full code:

require 'contracts'
include Contracts

Contract Num => Num
def double(x)
  x * 2

puts double("oops")

Save this in a file and run it. Notice we are calling double with "oops", which is not a number. The contract fails with a detailed error message:

./contracts.rb:34:in `failure_callback': Contract violation: (RuntimeError)
    Expected: Contracts::Num,
    Actual: "oops"
    Value guarded in: Object::double
    With Contract: Contracts::Num, Contracts::Num
    At: main.rb:6
    ...stack trace...

Instead of throwing an exception, you could log it, print a clean error message for your user...whatever you want. contracts.ruby is here to help you handle bugs better, not to get in your way.


Check out this awesome tutorial.

Use Cases

Check out this screencast.


Using contracts.ruby results in very little slowdown. Check out this blog post for more info.

Q. What Rubies can I use this with?

A. It's been tested with 1.8.7, 1.9.2, 1.9.3, 2.0.0, 2.1, 2.2, and jruby (both 1.8 and 1.9 modes).

If you're using the library, please let me know what project you're using it on :)


Contracts literally saves us hours of pain at Snowplow every day

Alexander Dean, creator of Snowplow

Contracts caught a bug that saved us several hundred dollars. It took less than 30 seconds to add the contract.

Michael Tomer


Inspired by

Copyright 2012-2015 Aditya Bhargava. Major improvements by Alexey Fedorov.

BSD Licensed.