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The contracts.ruby tutorial


contracts.ruby brings code contracts to the Ruby language. Code contracts allow you make some assertions about your code, and then checks them to make sure they hold. This lets you

  • catch bugs faster
  • make it very easy to catch certain types of bugs
  • make sure that the user gets proper messaging when a bug occurs.


gem install contracts


A simple example:

Contract Num, Num => Num
def add(a, b)
  a + b

Here, the contract is Contract Num, Num => Num. This says that the add function takes two numbers and returns a number.

Copy this code into a file and run it:

require 'contracts'

class Math
  include Contracts

  Contract Num, Num => Num
  def self.add(a, b)
    a + b

puts Math.add(1, "foo")

You'll see a detailed error message like so:

./contracts.rb:60:in `failure_callback': Contract violation: (RuntimeError)
    Expected: Contracts::Num,
    Actual: "foo"
    Value guarded in: Object::add
    With Contract: Contracts::Num, Contracts::Num
    At: foo.rb:6

That tells you that your contract was violated! add expected a Num, and got a string ("foo") instead. By default, an exception is thrown when a contract fails. This can be changed to do whatever you want. More on this later.

You can also see the contract for a function with the functype method:

=> "add :: Num, Num => Num"

This can be useful if you're in a REPL and want to figure out how a function should be used.

Built-in Contracts

Num is one of the built-in contracts that contracts.ruby comes with. The built-in contracts are in the Contracts namespace. The easiest way to use them is to include the Contracts module in your class/module.

contracts.ruby comes with a lot of built-in contracts, including the following:

  • Num – checks that the argument is Numeric
  • Pos – checks that the argument is a positive number
  • Neg – checks that the argument is a negative number
  • Nat – checks that the argument is a natural number
  • Bool – checks that the argument is true or false
  • Any – Passes for any argument. Use when the argument has no constraints.
  • None – Fails for any argument. Use when the method takes no arguments.
  • Or – passes if any of the given contracts pass, e.g. Or[Fixnum, Float]
  • Xor – passes if exactly one of the given contracts pass, e.g. Xor[Fixnum, Float]
  • And – passes if all contracts pass, e.g. And[Fixnum, Float]
  • Not – passes if all contracts fail for the given argument, e.g. Not[nil]
  • ArrayOf – checks that the argument is an array, and all elements pass the given contract, e.g. ArrayOf[Num]
  • SetOf – checks that the argument is a set, and all elements pass the given contract, e.g. SetOf[Num]
  • HashOf – checks that the argument is a hash, and all keys and values pass the given contract, e.g. HashOf[Symbol => String]
  • Maybe – passes if the argument is nil, or if the given contract passes
  • RespondTo – checks that the argument responds to all of the given methods, e.g. RespondTo[:password, :credit_card]
  • Send – checks that all named methods return true, e.g. Send[:valid?]
  • Exactly – checks that the argument has the given type, not accepting sub-classes, e.g. Exactly[Numeric].

To see all the built-in contracts and their full descriptions, check out the RDoc.

More Examples

Hello, World

Contract String => nil
def hello(name)
  puts "hello, #{name}!"

You always need to specify a contract for the return value. In this example, hello doesn't return anything, so the contract is nil. Now you know that you can use a constant like nil as the end of a contract. Valid values for a contract are:

  • the name of a class (like String or Fixnum)
  • a constant (like nil or 1)
  • a Proc that takes a value and returns true or false to indicate whether the contract passed or not
  • a class that responds to the valid? class method (more on this later)
  • an instance of a class that responds to the valid? method (more on this later)

A Double Function

Contract Or[Fixnum, Float] => Or[Fixnum, Float]
def double(x)
  2 * x

Sometimes you want to be able to choose between a few contracts. Or takes a variable number of contracts and checks the argument against all of them. If it passes for any of the contracts, then the Or contract passes. This introduces some new syntax. One of the valid values for a contract is an instance of a class that responds to the valid? method. This is what Or[Fixnum, Float] is. The longer way to write it would have been:

Contract, Float) =>, Float)

All the built-in contracts have overridden the square brackets ([]) to give the same functionality. So you could write

Contract Or[Fixnum, Float] => Or[Fixnum, Float]


Contract, Float) =>, Float)

whichever you prefer. They both mean the same thing here: make a new instance of Or with Fixnum and Float. Use that instance to validate the argument.

A Product Function

Contract ArrayOf[Num] => Num
def product(vals)
  total = 1
  vals.each do |val|
    total *= val

This contract uses the ArrayOf contract. Here's how ArrayOf works: it takes a contract. It expects the argument to be a list. Then it checks every value in that list to see if it satisfies that contract.

# passes
product([1, 2, 3, 4])

# fails
product([1, 2, 3, "foo"])

Another Product Function

Contract Args[Num] => Num
def product(*vals)
  total = 1
  vals.each do |val|
    total *= val

This function uses varargs (*args) instead of an array. To make a contract on varargs, use the Args contract. It takes one contract as an argument and uses it to validate every element passed in through *args. So for example,

Args[Num] means they should all be numbers.

Args[Or[Num, String]] means they should all be numbers or strings.

Args[Any] means all arguments are allowed (Any is a contract that passes for any argument).

Contracts On Arrays

If an array is one of the arguments and you know how many elements it's going to have, you can put a contract on it:

# a function that takes an array of two elements...a person's age and a person's name.
Contract [Num, String] => nil
def person(data)
  p data

If you don't know how many elements it's going to have, use ArrayOf.

Contracts On Hashes

Here's a contract that requires a Hash. We can put contracts on each of the keys:

# note the parentheses around the hash; without those you would get a syntax error
Contract ({ :age => Num, :name => String }) => nil
def person(data)
  p data

Then if someone tries to call the function with bad data, it will fail:

# error: age can't be nil!
person({:name => "Adit", :age => nil})

You don't need to put a contract on every key. So this call would succeed:

person({:name => "Adit", :age => 42, :foo => "bar"})

even though we don't specify a type for :foo.

Peruse this contract on the keys and values of a Hash.

Contract HashOf[Symbol, Num] => Num
def give_largest_value(hsh)

Which you use like so:

# succeeds
give_largest_value(a: 1, b: 2, c: 3) # returns 3

# fails
give_largest_value("a" => 1, 2 => 2, c: 3)

Contracts On Functions

Lets say you are writing a simple map function:

def map(arr, func)

map takes an array, and a function. Suppose you want to add a contract to this function. You could try this:

Contract ArrayOf[Any], Proc => ArrayOf[Any]
def map(arr, func)

This says that the second argument should be a Proc. You can call the function like so:

p map([1, 2, 3], lambda { |x| x + 1 }) # works

But suppose you want to have a contract on the Proc too! Suppose you want to make sure that the Proc returns a number. Use the Func contract. Func takes a contract as it's argument, and uses that contract on the function that you pass in.

Here's a map function that requires an array of numbers, and a function that takes a number and returns a number:

Contract ArrayOf[Num], Func[Num => Num] => ArrayOf[Num]
def map(arr, func)
  ret = []
  arr.each do |x|
    ret << func[x]

Earlier, we used Proc, which just says "make sure the second variable is a Proc". Now we are using Func[Num => Num], which says "make sure the second variable is a Proc that takes a number and returns a number". Better!

Try this map function with these two examples:

p map([1, 2, 3], lambda { |x| x + 1 }) # works
p map([1, 2, 3], lambda { |x| "oops" }) # fails, the lambda returns a string.

NOTE: This is not valid:

Contract ArrayOf[Num], Func => ArrayOf[Num]
def map(arr, &func)

Here I am using Func without specifying a contract, like Func[Num => Num]. That's not a legal contract. If you just want to validate that the second argument is a proc, use Proc.

Returning Multiple Values

Treat the return value as an array. For example, here's a function that returns two numbers:

Contract Num => [Num, Num]
def mult(x)
  return x, x+1

Synonyms For Contracts

If you use a contract a lot, it's a good idea to give it a meaningful synonym that tells the reader more about what your code returns. For example, suppose you have many functions that return a Hash or nil. If a Hash is returned, it contains information about a person. Your contact might look like this:

Contract String => Or[Hash, nil]
def some_func(str)

You can make your contract more meaningful with a synonym:

# the synonym
Person = Or[Hash, nil]

# use the synonym here
Contract String => Person
def some_func(str)

Now you can use Person wherever you would have used Or[Hash, nil]. Your code is now cleaner and more clearly says what the function is doing.

Defining Your Own Contracts

Contracts are very easy to define. To re-iterate, there are 5 kinds of contracts:

  • the name of a class (like String or Fixnum)
  • a constant (like nil or 1)
  • a Proc that takes a value and returns true or false to indicate whether the contract passed or not
  • a class that responds to the valid? class method (more on this later)
  • an instance of a class that responds to the valid? method (more on this later)

The first two don't need any extra work to define: you can just use any constant or class name in your contract and it should just work. Here are examples for the rest:

A Proc

Contract lambda { |x| x.is_a? Numeric } => Num
def double(x)

The lambda takes one parameter: the argument that is getting passed to the function. It checks to see if it's a Numeric. If it is, it returns true. Otherwise it returns false. It's not good practice to write a lambda right in your contract...if you find yourself doing it often, write it as a class instead:

A Class With valid? As a Class Method

Here's how the Num class is defined. It does exactly what the lambda did in the previous example:

class Num
  def self.valid? val
    val.is_a? Numeric

The valid? class method takes one parameter: the argument that is getting passed to the function. It returns true or false.

A Class With valid? As an Instance Method

Here's how the Or class is defined:

class Or < CallableClass
  def initialize(*vals)
    @vals = vals

  def valid?(val)
    @vals.any? do |contract|
      res, _ = Contract.valid?(val, contract)

The Or contract takes a sequence of contracts, and passes if any of them pass. It uses Contract.valid? to validate the value against the contracts.

This class inherits from CallableClass, which allows us to use [] when using the class:

Contract Or[Fixnum, Float] => Num
def double(x)
  2 * x

Without CallableClass, we would have to use .new instead:

Contract, Float) => Num
def double(x)
# etc

You can use CallableClass in your own contracts to make them callable using [].

Customizing Error Messages

When a contract fails, part of the error message prints the contract:

Expected: Contracts::Num,

You can customize this message by overriding the to_s method on your class or proc. For example, suppose we overrode Num's to_s method:

def Num.to_s
  "a number please"

Now the error says:

Expected: a number please,

Failure Callbacks

Supposing you don't want contract failures to become exceptions. You run a popular website, and when there's a contract exception you would rather log it and continue than throw an exception and break your site.

contracts.ruby provides a failure callback that gets called when a contract fails. For example, here we log every failure instead of raising an error:

Contract.override_failure_callback do |data|
  puts "You had an error"
  puts failure_msg(data)

failure_msg is a function that prints out information about the failure. Your failure callback gets a hash with the following values:

  :arg => the argument to the method,
  :contract => the contract that got violated,
  :class => the method's class,
  :method => the method,
  :contracts => the contract object

If your failure callback returns false, the method that the contract is guarding will not be called (the default behaviour).

Disabling contracts

If you want to disable contracts, set the NO_CONTRACTS environment variable. This will disable contracts and you won't have a performance hit. Pattern matching will still work if you disable contracts in this way! With NO_CONTRACTS only pattern-matching contracts are defined.

Method overloading

You can use contracts for method overloading! This is commonly called "pattern matching" in functional programming languages.

For example, here's a factorial function without method overloading:

Contract Num => Num
def fact x
  if x == 1
    x * fact(x - 1)

Here it is again, re-written with method overloading:

Contract 1 => 1
def fact x

Contract Num => Num
def fact x
  x * fact(x - 1)

For an argument, each function will be tried in order. The first function that doesn't raise a ContractError will be used. So in this case, if x == 1, the first function will be used. For all other values, the second function will be used.

This allows you write methods more declaratively, rather than using conditional branching. This feature is not only useful for recursion; you can use it to keep parallel use cases separate:

Contract lambda{|n| n < 12 } => Ticket
def get_ticket(age) age)

Contract lambda{|n| n >= 12 } => Ticket
def get_ticket(age) age)

Note that the second get_ticket contract above could have been simplified to:

Contract Num => Ticket

This is because the first contract eliminated the possibility of age being less than 12. However, the simpler contract is less explicit; you may want to "spell out" the age condition for clarity, especially if the method is overloaded with many contracts.

Contracts in modules

To use contracts on module you need to include both Contracts and Contracts::Modules into it:

module M
  include Contracts
  include Contracts::Modules

  Contract String => String
  def self.parse
    # do some hard parsing


Invariants are conditions on objects that should always hold. If after any method call on given object, any of the Invariants fails, then Invariant violation error will be generated.

NOTE: Only methods with contracts will be affected.

A simple example:

class MyBirthday <, :month)
  include Contracts
  include Contracts::Invariants

  invariant(:day) { 1 <= day && day <= 31 }
  invariant(:month) { 1 <= month && month <= 12 }

  Contract None => Fixnum
  def silly_next_day! += 1

birthday =, 12)

If you run it, last line will generate invariant violation:

./invariant.rb:38:in `failure_callback': Invariant violation: (RuntimeError)
   Expected: day condition to be true
   Actual: false
   Value guarded in: MyBirthday::silly_next_day!
   At: main.rb:9

Which means, that after #silly_next_day! all checks specified in invariant statement will be verified, and if at least one fail, then invariant violation error will be raised.

Auto-generate documentation using contracts

If you are generating documentation for your code with YARD, check out yard-contracts. It will automatically annotate your functions with contracts information. Instead of documenting each parameter for a function yourself, you can just add a contract and yard-contracts will generate the documentation for you!


Please submit any bugs here and I'll try to get them resolved ASAP!

See any mistakes in this tutorial? I try to make it bug-free, but they can creep in. File an issue.

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

See the wiki for more info.

Happy Coding!

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