Skip to content


Subversion checkout URL

You can clone with HTTPS or Subversion.

Download ZIP
Ruby Imperative Random Data Generator and Quickcheck
branch: master

Merge pull request #25 from niftyn8/rspec

Unbreak rspec extension
latest commit db76214c20
Howard Yeh authored


Imperative Random Data Generator and Quickcheck

You can use Rant to generate random test data, and use its Test::Unit extension for property-based testing.

Rant is basically a recursive descent interpreter, each of its method returns a random value of some type (string, integer, float, etc.).

Its implementation has no alien mathematics inside. Completely side-effect-free-free.


$ gem install rantly

$ irb -rrantly
> Rantly { [integer,float] } # same as Rantly.value { integer }
=> [20991307, 0.025756845811823]
> Rantly { [integer,float]}
=> [-376856492, 0.452245765751706]
> Rantly(5) { integer } # same as { integer }
=> [-1843396915550491870, -1683855015308353854, -2291347782549033959, -951461511269053584, 483265231542292652] 

Data Generation

Getting Random Data Values

  call the generator n times, and collect values
  call a random block n times
  call a random block once, and get its value.

To collect an array of random data,

# we want 5 random integers
> Rantly(5) { integer }
=> [-380638946, -29645239, 344840868, 308052180, -154360970]

To iterate over random data,

> Rantly.each(5) { puts integer }
=> nil

To get one value of random data,

> Rantly { integer }
=> 278101042

The optional argument limit is used with generator guard. By default, if you want to generate n items, the generator tries at most n * 10 times.

This almost always succeeds,

> Rantly(5) { i = integer; guard i > 0; i }
=> [511765059, 250554234, 305947804, 127809156, 285960387]

This always fails,

> Rantly(10) { guard integer.is_a?(Float) }
Rantly::TooManyTries: Exceed gen limit 100: 101 failed guards)

Random Generating Methods

The API is similiar to QuickCheck, but not exactly the same. In particular choose picks a random element from an array, and range picks a integer from an interval.

Simple Randomness

  random positive or negative integer. Fixnum only.
  random integer between lo and hi.
  random float
  true or false
  No-op. returns value.
  Pick one value from among vals.

Meta Randomness

A rant generator is just a mini interpreter. It’s often useful to go meta,

  If gen is a Symbol, just do a method call with send.
  If gen is an Array, the first element of the array is the method name, the rest are args.
  If gen is a Proc, instance_eval it with the generator.

> Rantly { call(:integer) }
=> -240998958

> Rantly { call([:range,0,10]) }
=> 2

> Rantly { call( { [integer] })}
=> [522807620]

The call method is useful to implement other abstractions (See next subsection).

  Pick a random arg among args, and Rantly#call it.

50-50 chance getting an integer or float,

> Rantly { branch :integer, :float }
=> 0.0489446702931332
> Rantly { branch :integer, :float }
=> 494934533


  Takes a list of 2-tuples, the first of which is the weight, and the second a Rantly#callable value, and returns a random value picked from the pairs. Follows the distribution pattern specified by the weights.

Twice as likely to get a float than integer. Never gets a ranged integer.

> Rantly { freq [1,:integer], [2,:float], [0,:range,0,10] }

If the “pair” is not an array, but just a symbol, freq assumes that the weight is 1.

# 50-50 between integer and float
> Rantly { freq :integer, :float }

If a “pair” is an Array, but the first element is not an Integer, freq assumes that it’s a Rantly method-call with arguments, and the weight is one.

# 50-50 chance generating integer limited by 10, or by 20.
> Rantly { freq [:integer,10], [:integer 20] }

Sized Structure

A Rantly generator keeps track of how large a datastructure it should generate with its size attribute.

 returns the current size
 sets the size for the duration of recursive call of block. Block is instance_eval with the generator.

Rantly provides two methods that depends on the size

  returns a sized array consisted of elements by Rantly#calling random branches.
  returns a sized random string, consisted of only chars from a char_class.
  returns a sized random hash. The generator block should generate tuples of keys and values (arrays that have two elements, the first one is used as key, and the second as value).

The avaiable char classes for strings are:


# sized 10 array of integers
> Rantly { array(10) { integer }}
=> [417733046, -375385433, 0.967812380000118, 26478621, 0.888588160450082, 250944144, 305584916, -151858342, 0.308123867823313, 0.316824642414253]

If you set the size once, it applies to all subsequent recursive structures. Here’s a sized 10 array of sized 10 strings,

> Rantly { sized(10) { array {string}} }
=> ["1c}C/,9I#}", "hpA/UWPJ\\j", "H'~ERtI`|]", "%OUaW\\%uQZ", "Z2QdY=G~G!", "H<o|<FARGQ", "g>ojnxGDT3", "]a:L[B>bhb", "_Kl=&{tH^<", "ly]Yfb?`6c"]

Or a sized 10 array of sized 5 strings,

> Rantly {array(10){sized(5) {string}}}
=> ["S\"jf ", "d\\F-$", "-_8pa", "IN0iF", "SxRV$", ".{kQ7", "6>;fo", "}.D8)", "P(tS'", "y0v/v"]

Generate a hash that has 5 elements,

> Rantly { dict { [string,integer] }}
 "-Mp '."=>653206579583741142,
 "= :/\\,"=>433790301059833691}

The dict generator retries if a key is duplicated. If it fails to generate a unique key after too many tries, it gives up by raising an error:

> Rantly { dict { ["a",integer] }}
Rantly::TooManyTries: Exceed gen limit 60: 60 failed guards)

Property Testing

Rantly extends Test::Unit and MiniTest::Test (5.0)/MiniTest::Unit::TestCase (< 5.0) for property testing. The extensions are in their own modules. So you need to require them explicitly:

require 'rantly/testunit_extensions' # for 'test/unit'
require 'rantly/minitest_extensions' # for 'minitest'

They define:

  The block is used to generate random data with a generator. The method returns a Rantly::Property instance, that has the method 'check'.

Rantly also extends RSpec in a similar way. Again you need to require it:

require 'rantly/rspec_extensions'

It’s like this, using the gem ‘shoulda’

# checks that integer only generates fixnum.
should "generate Fixnum only" do
   property_of  { integer }.check { |i| assert i.is_a?(Integer) }

The check block takes the generated data as its argument. One idiom I find useful is to include a parameter of the random data for the check argument. For example, if I want to check that Rantly#array generates the right sized array, I could say,

should "generate right sized array" do
  property_of {
    len = integer
  }.check { |(len,arr)|
    assert_equal len, arr.length

That’s about it. Enjoy :)


Copyright © 2009 Howard Yeh. See LICENSE for details.

Something went wrong with that request. Please try again.