fantasy-laws

Property-based tests to verify the lawfulness of Fantasy Land1 -compliant algebraic data types.

Installation

Add fantasy-laws, jsverify2, sanctuary-show3, and sanctuary-type-classes4 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. 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';
```

1. Import the type to be tested:

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

1. Define an "arbitrary"5 for the type:

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

1. Provide the fixed parameters to laws.Group:

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

1. 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);
```

1. 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);
});
```