fantasy-laws

Property-based tests to verify the lawfulness of Fantasy Land1 -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:```
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`:
```javascript
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:

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

- Define an "arbitrary"5 for the type:

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

- Provide the fixed parameters to
`laws.Group`

:

```
```javascript
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:
```javascript
// 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:

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