maybe.js

This library contains a JS implementation of the Maybe monad.

It's a bit more flexible, and comes with a few more extras than the Haskell implementation, given JavaScript's type system limitations and the language's general flexibiity.

Installation

If you're intending to use this library from Node, you can just install it with NPM:

$ npm install --save maybe-js

For other environments, you can just grab maybe.js or maybe.min.js directly.

Both will work with AMD, Node, or directly in the browser.

Why

null sucks. undefined sucks. Checking for either of them sucks.

Why not make things a bit easier?

Usage

The tour:

var object = {
  attr: "hello",
  fn: function() {
    return this.attr + " world!";
  }
};

var wrapped = maybe(object);
// => Maybe([object Object])

wrapped.value;
// => object

wrapped.get("attr");
// => Maybe(hello)

wrapped.bind("fn");
// => Maybe(hello world!)

wrapped.get("invalid_attr")
// => Maybe(empty)

wrapped.bind("invalid_fn")
// => Maybe(empty)

wrapped("attr");
// => Maybe(hello)

wrapped("fn");
// => Maybe(hello world!)

wrapped.tap(function(x) {
  x.attr = "bye"
}).get("attr");
// => Maybe(bye)

maybe().or("hello");
// => Maybe(hello)

maybe().or(function() { return "generated value"; });
// => Maybe(generated value)

maybe("hello").isNothing(); // => false
maybe().isNothing();        // => true

maybe("hello").isValue(); // => true
maybe().isValue();        // => false

maybe.nothing;
// => Maybe(empty)

var stringify = maybe.lift(JSON.stringify)
stringify([1, 2, 3]);
// => Maybe([1,2,3])

maybe(value)

Wraps value in a Maybe.

If value is undefined, the Maybe is said to be empty.

var value = "hello";
maybe(value);
// => Maybe(hello)

maybe();
// => Maybe(empty)

maybe(value).value

Returns the value stored in the Maybe.

maybe("hello").value;
// => "hello"

maybe().value;
// => undefined

maybe(value).get(key)

Accesses a property on a Maybe, returning a new Maybe encapsulating the property. If no value is present, an empty Maybe is returned.

var obj = { attr: "hello" };
maybe(obj).get("attr");
// => Maybe(hello);

maybe(obj).get("not_an_attr");
// => Maybe(empty);

maybe(value).bind(fn, ...args)

Calls a function on a Maybe's value. The return value is wrapped in another Maybe.

fn may be a Function, or a String. If it's a String, it's assumed to be a instance function of the value. If it's a Function, it's called with the Maybe's value prepending other arguments.

If an instance function isn't present, an empty Maybe is returned.

var obj = {
  fn: function() { return "return value"; }
};

function toString(thing) {
  return Object.prototype.toString.call(thing);
}

var wrapped = maybe(obj);

wrapped.bind(toString);
// => Maybe([object Object])

wrapped.bind("fn");
// => Maybe(return value)

wrapped.bind("not_a_fn");
// => Maybe(empty)

maybe(opt, ...args)

A shorthand for both of the above.

If a function, or a string property name that resolves to a function, the function will be called.

Otherwise, will attempt to return a wrapped property.

var obj = {
  attr: "hi",
  fn: function() { return "return value"; }
};

function toString(thing) {
  return Object.prototype.toString.call(thing);
}

var wrapped = maybe(obj);

wrapped("attr");
// => Maybe(hi)

wrapped("fn");
// => Maybe(return value)

wrapped(toString);
// => Maybe([object Object])

wrapped("not_a_property");
// => Maybe(empty)

maybe(value).tap(fn, thisArg)

Yields the Maybe's value for modification. Returns a Maybe with the modified value.

Support an optional argument that will be the this value in the function.

var obj = {
  attr: "hello"
};

maybe(obj).tap(function(x) {
  x.attr += " world!";
}).get("attr");
// => Maybe(hello world!)

maybe(value).or(alternate)

If a Maybe is empty, returns a new Maybe with the specified value.

maybe(undefined).or("new value");
// => Maybe(new value)

maybe(value).isNothing()

Checks if the Maybe is empty or not. Returns a boolean.

maybe("thing").isNothing()   // => false
maybe(undefined).isNothing() // => true

maybe(value).isValue()

Checks if the Maybe has a value or not. Returns a boolean.

maybe("thing").isValue()   // => true
maybe(undefined).isValue() // => false

maybe.nothing

An empty Maybe.

maybe.lift(fn)

Wraps a function such that its return value becomes a Maybe.

var stringify = maybe.lift(JSON.stringify)
stringify([1, 2, 3]);
// => Maybe([1,2,3])

License

MIT. For more details, see the LICENSE file.