Verifications for Monad laws according to fantasy-land.
Last updated 7 years ago by killdream .
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$ cnpm install laws 
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Build Status NPM version Dependencies Status experimental

Claire properties for verifying Monad and other algebraic structures' laws, conforming to the Fantasy Land specification.


var Maybe = require('monads.maybe')
var laws  = require('laws')

laws.functors.identity(Maybe.Just).asTest({ verbose: true, times: 100 })()
// + OK passed 100 tests.


The easiest way is to grab it from NPM. If you're running in a Browser environment, you can use Browserify

$ npm install laws

Using with CommonJS

If you're not using NPM, Download the latest release, and require the folktale.laws.umd.js file:

var laws = require('laws')

Using with AMD

Download the latest release, and require the folktale.laws.umd.js file:

require(['folktale.laws'], function(laws) {

Using without modules

Download the latest release, and load the folktale.laws.umd.js file. The properties are exposed in the global Laws object:

<script src="/path/to/folktale.laws.umd.js"></script>

Compiling from source

If you want to compile this library from the source, you'll need Git, Make, Node.js, and run the following commands:

$ git clone git://
$ cd laws
$ npm install
$ make bundle

This will generate the dist/folktale.laws.umd.js file, which you can load in any JavaScript environment.

Getting Started

This library provides properties for verifying the correctness of an implementation of algebraic structures according to the Fantasy Land specification. They do so by generating random inputs and checking if the algebraic laws holds for your structure.

In order to use these properties, your algebraic library needs to implement the Eq typeclass, defined as:

class Eq a where
  -- | True if both structures are equivalent
  isEqual :: a -> a -> Bool

Here's an example of such implementation, for a Maybe(a) monad:

var Maybe = {
  /* (...) */
  isEqual: function(b) { 
    return this.isNothing?  b.isNothing
    :      this.isJust?     this.value === b.value
  /* (...) */

The Laws

  • Applicatives
    1. Identity
    2. Composition
    3. Homomorphism
    4. Interchange
  • Chains
    1. Associativity
  • Functors
    1. Identity
    2. Composition
  • Monads
    1. Left identity
    2. Right identity
  • Monoids
    1. Left identity
    2. Right identity
  • Semigroups
    1. Associativity

Using the laws

To verify if a data structure conforms to the laws, you need to partially apply the law to a function that constructs a structure holding a single value. For example, if you have an Identity container and want to check if it conforms to the Semigroup's law of associativity:

// :: a -> Id a
function makeId(a) {
  return new Id(a)

laws.semigroup.associativity(makeId).asTest({ verbose: true })()
// + OK passed 100 tests

Note that applying the law to the constructor function gives you back a Claire property. There are different ways of using this object, but the easiest one is to use the asTest(configuration) method to return a test function. When invoked (with no parameters), this test function will repeatedly generate random inputs to test if your implementation behaves correctly according to the laws.

If any of the inputs invalidates the property, an error is thrown with detailed information about why the property was invalidated. You can control both the amount of details in the reports, and the number of random tests that are performed by passing a configuration object to the asTest method. By default the reports are concise and only 100 random tests are performed.

class Configuration where
  verbose :: Bool       -- * whether to output a detailed report or not
  times   :: Int        -- * number of random tests to perform

Do note that if verbose is false, no message will be printed to the standard output by default.

Platform support

This library assumes an ES5 environment, but can be easily supported in ES3 platforms by the use of shims. Just include es5-shim :)


Copyright (c) 2013 Quildreen Motta.

Released under the MIT licence.

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