fantasy-laws

fantasy-laws

Property-based tests to verify the lawfulness of Fantasy Land -compliant algebraic data types.

Installation

Add fantasy-laws, jsverify, sanctuary-show, and sanctuary-type-classes to "devDependencies" in package.json, then run npm install.

Usage

Usage is best explained by example. The following code defines a Sum type which is intended to satisfy Setoid, Semigroup, Monoid, and Group:

function Sum(value) {
  if (!(this instanceof Sum)) return new Sum (value);
  this.value = value;
}

//  Sum.fantasy-land/empty :: () -> Sum
Sum['fantasy-land/empty'] = function() { return Sum (0); };

//  Sum#fantasy-land/equals :: Sum ~> Sum -> Boolean
Sum.prototype['fantasy-land/equals'] = function(other) {
  return Z.equals (this.value, other.value);
};

//  Sum#fantasy-land/concat :: Sum ~> Sum -> Sum
Sum.prototype['fantasy-land/concat'] = function(other) {
  return Sum (this.value + other.value);
};

//  Sum#fantasy-land/invert :: Sum ~> () -> Sum
Sum.prototype['fantasy-land/invert'] = function() {
  return Sum (-this.value);
};

The following steps demonstrate how to test the Group laws:

• Import fantasy-laws, jsverify, sanctuary-show, and sanctuary-type-classes:

  import laws from 'fantasy-laws';
  import jsc from 'jsverify';
  import show from 'sanctuary-show';
  import Z from 'sanctuary-type-classes';
  

• Import the type to be tested:

  import Sum from '../Sum.js';
  

• Define an "arbitrary" for the type:

  //    SumArb :: Arbitrary Sum
  const SumArb = jsc.number.smap (Sum, sum => sum.value, show);
  

• Provide the fixed parameters to laws.Group:

  const {leftInverse, rightInverse} = laws.Group (Z.equals, Sum);
  

• Provide the appropriate number of arbitraries to the function associated with a particular law to produce a thunk:

  //    testLeftInverse :: () -> Undefined !
  const testLeftInverse = leftInverse (SumArb);
  
  //    testRightInverse :: () -> Undefined !
  const testRightInverse = rightInverse (SumArb);
  

• To run the tests, invoke the thunk or use a test runner such as Mocha:

  suite ('Group laws', () => {
    test ('left inverse', testLeftInverse);
    test ('right inverse', testRightInverse);
  });