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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.
0.17.x = Ruby 3.x only
0.17.x only supports Ruby 3.x
Looking for Ruby 2.x support?
gem install contracts
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' class Example include Contracts::Core include Contracts::Builtin Contract Num => Num def double(x) x * 2 end end puts Example.new.double("oops")
Save this in a file and run it. Notice we are calling
"oops", which is not a number. The contract fails with a detailed error message:
ParamContractError: Contract violation for argument 1 of 1: Expected: Num, Actual: "oops" Value guarded in: Example::double With Contract: Num => Num At: main.rb:8 ...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.
Check out this screencast.
To get started do the following:
- Install required gems for development
- Run our test suite
bundle exec rspec
- Run our code style checks
bundle exec rubocop
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
3.1. (In case this list becomes outdated see
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.
Inspired by contracts.coffee.