bilby.js

Serious functional programming library for JavaScript.

Features

Multimethods

Multimethods belong to an immutable environment:

var env = λ.environment()
    .method('length', λ.isArray, function(a) {
        return a.length;
    })
    .method('length', λ.isString, function(s) {
        return s.length;
    })
    .property('empty', function(o) {
        return !this.length(o);
    });

env.empty([]) == true;
env.empty([1, 2, 3]) == false;

Helpers

Curried and uncurried application:

var add = λ.curry(function(a, b) {
    return a + b;
});
add(1)(2) == 3;
add(1, 2) == 3;

Lenses for immutable updates of nested data structures:

var person = {
        name: "Brian",
        address: {
            number: 100,
            street: "Pitt St",
            state: "QLD"
        }
    },
    addressLens = λ.objectLens('address'),
    stateLens = λ.objectLens('state'),
    personStateStore = stateLens.compose(addressLens).run(person),
    newPerson = personStateStore.setter("TAS");

newPerson.address.state == "TAS";
person.address.state == "QLD";

Functional data structures

Option:

λ.some(3).getOrElse(0) == 3;
λ.none.getOrElse(0) == 0;

Either:

λ.left(1).fold(add(1), add(2)) == 2;
λ.right(1).fold(add(1), add(2)) == 3;

Operator Syntax

Monads:

λ.Do()(
    λ.some(1) >= function(x) {
        return x < 0 ? λ.none : λ.some(x + 2);
    }
).getOrElse(0) == 3;

Kleislis:

λ.Do()(
    function(x) {
        return x < 0 ? λ.none : λ.some(x + 1);
    } >> function(x) {
        return x % 2 != 0 ? λ.none : λ.some(x + 1);
    }
)(1).getOrElse(0) == 3;

Functors:

λ.Do()(
    λ.some(1) < add(2)
).getOrElse(0) == 3;

Applicatives:

λ.Do()(
    λ.some(add) * λ.some(1) * λ.some(2)
).getOrElse(0) == 3;

Semigroups:

λ.Do()(
    λ.some(1) + λ.some(2)
).getOrElse(0) == 3;

QuickCheck

Automates generating arguments for a function and shrinking them when a test failure is found:

// Any string appended with its reverse is a palindrome.
exports.isPalindromeTest = function(test) {
    var report = λ.forAll(
        function(s) {
            var reversed = s.split('').reverse().join('');
            return isPalindrome(s + reversed);
        },
        [String]
    );

    test.ok(report.success, report.fold(
        "OK",
        function(inputs, tries) {
            return "Failed after " + tries + " tries: " + inputs.toString();
        }
    ));

    test.done();
};

Usage

node.js:

var λ = require('bilby');

Browser:

<script src="bilby-min.js"></script>