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:

1. Require fantasy-laws, jsverify, sanctuary-show, and sanctuary-type-classes:

   const laws = require ('fantasy-laws');
   const jsc = require ('jsverify');
   const show = require ('sanctuary-show');
   const Z = require ('sanctuary-type-classes');
   

2. Require the type to be tested:

   const Sum = require ('../Sum');
   

3. Define an "arbitrary" for the type:

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

4. Provide the fixed parameters to laws.Group:

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

5. 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);
   

6. 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);
   });