Basic example of folktale core.arity with mocha, chai, and jsverify

test.js

const jsc = require('jsverify');
const {assert} = require('chai');

const pass = () => assert.isOk(true);
const fail = () => assert.fail();

const {nullary, unary, binary, ternary} = require('core.arity');
const {curry} = require('core.lambda');

describe('arity', () => {
    describe('curried', () => {
        const add = (x, y) => x + y;
        const curried = curry(2, add);

        jsc.property('curried add is same as non-curried', jsc.number, jsc.number, 
          (x, y) => curried(x)(y) === add(x, y)
        ),
        jsc.property('curried add can curry or not curry', jsc.number, jsc.number, 
          (x, y) => curried(x)(y) === curried(x, y)
        ),
        it('curried functions can fail when given too many arguments', () => {
            try {
                curried(1, 2, 3);
                fail();
            } catch(e) {
                pass();
            }
        }),
        jsc.property('binary can handle too many arguments', jsc.number, jsc.number, jsc.number,
          (x, y, z) => binary(curried)(x, y, z) === curried(x, y)
        );
    }),
    describe('nullary', () => {
        const mkK = x => () => x;

        jsc.property('make K returns value given', jsc.number,
          x => mkK(x)() === x
        ),
        jsc.property('nullary restricts it to a constant function', jsc.string, jsc.bool,
          (s, b) => nullary(mkK(s))(b) === s
        );
    }),
    describe('unary', () => {
        const I = x => x;

        jsc.property('I returns value given', jsc.datetime,
          x => I(x) === x
        ),
        jsc.property('unary restricts it to an unary function', jsc.string, jsc.bool,
          (s, b) => unary(I)(s, b) === s
        );
    }),
    describe('binary', () => {
        const multiply = (x, y) => x * y;

        jsc.property('multiply returns x * y', jsc.number, jsc.number,
          (x, y) => multiply(x, y) === x * y
        ),
        jsc.property('binary restricts it to a binary function', jsc.number, jsc.number, jsc.string,
          (x, y, s) => binary(multiply)(x, y, s) === y * x
        );
    }),
    describe('ternary', () => {
        const tri = (pred, t, f) => pred ? t : f;

        jsc.property('tri returns t if predicate is true else f', jsc.bool, jsc.nat, jsc.nat,
          (pred, t, f) => tri(pred, t, f) === (pred ? t : f)
        ),
        jsc.property('ternary restricts it to a ternary function', jsc.bool, jsc.number, jsc.string, jsc.datetime,
          (pred, n, s, d) => ternary(tri)(pred, n, s) !== d
        );
    });
})