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    mnd - JS monads for the masses

    Bare-bones JS monads


    Because monads are a step on the path to personal enlightenment, achieved via a shift in thought procedure.


    Well, first, try npm i -S mnd. That's a great start.

    No, really. How?

    Okay, so, I'm cheating. Monads have many "methods" attached to them, but we're going to focus on two. Why? Because we're humanoids doing JS, not functional automata doing Haskell. We want to do better, but we still want our code readable for junior devs. But wait, before we talk about mnd, let's get one thing front and center: you're already using monads in JS, at least one, which is called - you guessed it - Promise. I'm not going to explain why, but read on, you'll understand really soon.

    First, a problem without monads:

    // What if we don't have a user?
    // What if our user doesn't have an address?
    // etc, etc...
    function getUserState(user) {
    // Let's try again.
    function getUserState(user) {
      return (
        user &&
        user.address && &&
      ) || 'Default';
    // Oh. This looks unwieldy. If only there was a better way.

    Let's try this with monads.

    import { Maybe } from 'mnd';
    // This returns an instance of the Maybe monad, holding the value of obj[p].
    // prop :: string -> Object[string:T] -> Maybe T
    const prop => p => obj => Maybe.of(obj[p]);
    // getUserState :: User -> Maybe string
    function getUserState(user) {
      return Maybe.of(user) // we might have a user
                  .map(prop('address')) // if we do, they might have an address
                  .map(prop('country')) // it they have, it might have country
                  .map(prop('state')); // which in turn, might have a state
    // User -> Identity string
        state => state, // if the user has a state, let's get it
        () => 'default-value' // if the user doesn't have one, for whichever reason,
                             // let's use a default one

    Do you kind of see why Promise is a monad? So let's correlate the two APIs:

    Promise mnd
    Promise.resolve(x) Identity.of(x)
    Promise.reject(y) Monad.of(x)
    p.then(onResolve, onReject), onReject)
    p.catch(...) nothing, use the onReject cb

    So, as you see, you create a monad of type T with T.of(value) (usually), and you pass it forward using its .map(onResolve, onReject) method, which returns one of three objects:

    • If the object returned by onResolve/onReject (x) is not a monad, an Identity.of(x) is returned.
    • If x is a monad, it is returned.
    • If whichever callback being called throws, the thrown object (t) is returned as an Either.left(t) (see below).

    Another monad method is .pass, which acts like .map, with one difference: instead of accepting two callbacks, it accepts one callback, which it calls regardless of the monad's resolution. The callback is called with the monad's inner value, and its resolution state. For example:

    const withDefault = defaultValue => (value, isResolved) =>
      isResolved ? value : defaultValue;
    const o = { name: 'foo'};
    Maybe.of(o.age).pass(withDefault(5)).map(console.log); // prints 5

    What's inside?

    import { Monad, Identity, Maybe, Either, Async, monad, do_ } from 'mnd';
    import mtester from 'mnd/t';

    Monad - the never monad

    • Create: x => Monad.of(x)
    • Resolves map: never.

    Identity - I am X

    • Create: x => Identity.of(x)
    • Resolves map: always resolves with its internal value.

    Maybe - well... I might exist, but...

    • Create: x => Maybe.of(x)
    • Resolves map: resolves with internal value, as long as it's neither null nor undefined.

    Either - either this worked, or it didn't

    • Create with intent to resolve: x => Either.right(x)
    • Create with intent to reject: x => Either.left(x)
    • Wrap a block with the Either monad: Either.wrap(() => /* unsafe code */).map(...)

    Async - Promise to mnd

    Wrapping a Promise (a native one!) with Async.of returns a monad compatible with the rest of mnd's monads.

    monad(resolverFn) -> MonadType

    Use this function to create new monad types that are created using .of(x) and are resolved into .map with original value x, based on passing original value x to the resolverFn.

    Example: const Integer = monad(n => n >= 0)


    Basically, this is a shortcut. This is like co for monads, only it returns a monad, instead of a Promise.

    Monadic Tests

    Because of reasons, you're probably going to want to test some of your monads. mnd comes with a builtin monadic test helper under mnd/t.

    Usage with tape:

    const test = require('tape');
    const mtest = require('mnd/t')(test);
    const { monadicFunction } = require('./app');
    // if the monad is rejected, the test autofails
    mtest('Test my monadic function', t =>
      monadicFunction(5).map(value =>, 32)));


    • How do I simply retrieve the internal value of a monad, without .map?
    • The same way you do that with a Promise, and for the same reason: you don't, because it defeats the point.
    • All this is nice and well, but I'm gonna need some regular JS interop here.
    • Are promises cool? I hope you think they are. All monads have a .toPromise() method. Done.


    npm i mnd

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