Functional Programming in Ruby with Contracts

I read Thomas Reynolds’ My Weird Ruby a week or two ago, and I loved it. I’d never heard of the Contracts gem, but after reading the post and the well-written docs, I couldn’t wait to try it out. I’d been doing some functional programming as part of our ongoing programming challenge series, and saw an opportunity to use Contracts to rewrite my Ruby solution to the One-Time Pad problem. Check out my rewritten encrypt program:

 #!/usr/bin/env ruby

require "contracts"
include Contracts

Char = -> (c) { c.is_a?(String) && c.length == 1 }
Cycle = Enumerator::Lazy

Contract [Char, Char] => Num
def int_of_hex_chars(chars)

Contract ArrayOf[Num] => String
def hex_string_of_ints(nums) { |n| n.to_s(16) }.join

Contract Cycle => Num
def get_mask(key)
 int_of_hex_chars key.first(2)

Contract [], Cycle => []
def encrypt(plaintext, key)

Contract ArrayOf[Char], Cycle => ArrayOf[Num]
def encrypt(plaintext, key)
 char = plaintext.first.ord ^ get_mask(key)
 [char] + encrypt(plaintext.drop(1), key.drop(2))

plaintext =
key = ARGV.last.chars.cycle.lazy

print hex_string_of_ints(encrypt(plaintext, key))

Pretty cool, yeah? Compare with this Haskell solution. Some highlights:


At its most basic, Contracts offers typechecking on function input and output. Give it the expected classes of the arguments and the return value, and you’ll get a nicely formatted error message if the function is called with something else, or returns something else.

Custom types with lambdas

Ruby has no concept of a single character data type – running "string".chars returns an array of single-character strings. We can simulate a native char type using a lambda, as seen on line #6, which says that the argument must be a string and must have a length of one.


If you’re expecting an array of a specific length and type, you can specify it, as I’ve done on line #9.

Pattern matching

Rather than one encrypt method with a conditional to see if the list is empty, we define the method twice: once for the base case (line #24) and once for the recursive case (line #29). This keeps our functions concise and allows us to do case-specific typechecking on the output.

No unexpected nil

There’s nothing worse than undefined method 'foo' for nil:NilClass, except maybe littering your methods with presence checks. Using Contracts, you can be sure that your functions aren’t being called with nil. If it happens that nil is an acceptable input to your function, use Maybe[Type] à la Haskell.

Lazy, circular lists

Unrelated to Contracts, but similarly inspired by My Weird Ruby, check out the rotating encryption key made with cycle and lazy on line #36.

* * *

As a professional Ruby developer with an interest in strongly typed functional languages, I’m totally psyched to start using Contracts on my projects. While you don’t get the benefits of compile-time checking, you do get cleaner functions, better implicit documentation, and more overall confidence about your code.

And even if Contracts or FP aren’t your thing, from a broader perspective, this demonstrates that experimenting with other programming paradigms makes you a better programmer in your primary language. It was so easy to see the utility and application of Contracts while reading My Weird Ruby, which would not have been the case had I not spent time with Haskell, OCaml, and Elixir.

David is Viget's managing development director. From our Durham, NC, office, he builds high-quality, forward-thinking software for PUMA, the World Wildlife Fund,, and many others.

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