fantasy-laws

Property-based tests to verify the lawfulness of [Fantasy Land][1] -compliant
algebraic data types.

Installation

Add fantasy-laws, [jsverify][2], [sanctuary-show][3], and
[sanctuary-type-classes][4] 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:

``javascript
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. Import fantasy-laws, jsverify, sanctuary-show, and
sanctuary-type-classes:

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

2. Import the type to be tested:

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

3. Define an ["arbitrary"][5] for the type:

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

4. Provide the fixed parameters to laws.Group:

`javascript
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:

`javascript
// 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:

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

[1]: https://github.com/fantasyland/fantasy-land
[2]: https://github.com/jsverify/jsverify
[3]: https://github.com/sanctuary-js/sanctuary-show
[4]: https://github.com/sanctuary-js/sanctuary-type-classes
[5]: https://github.com/jsverify/jsverify#arbitrary-data