No, Promise is not a monad

If you want to see how sloppy your thinking is, try writing. If you want to see how sloppy your writing is, try writing math.
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“Monad” is a well-defined mathematical object that must satisfy axioms known as the monad laws: Left Identity, Right Identity, and Associativity. One way to implement a monad is to define the functions return and bind such that they obey the monad laws. (And once you have that, you can derive other functions, as well.)

return :: a -> m a is the “unit” operation for the monad m. Essentially, return takes any value and puts it into the monadic context. The candidate for an analogous function on the Promise API is resolve. However, because of the OO nature of JavaScript we can’t use Promise.resolve as a pure function. I cannot simply write .then(Promise.resolve) – that gives me this delightful error: “Receiver of Promise.resolve call is not a non-null object”. So to be fair, we can use the lambda x => Promise.resolve(x) as our return.

bind :: m a -> (a -> m b) -> m b enables you to compose, flattening as you go. The analogous Promise function for bind is then.

Note that in the examples below, the symbol in an expression like a ≡ b indicates that you can confidently replace one side of with the other side, and your program will behave the same way.

The question is: Does Promise satisfy the monad laws?

Left Identity

Promise.resolve(a).then(f) ≡ f(a)

Left Identity appears to hold as long as the function f that takes an a and returns a Promise (i.e. a -> Promise b). But not quite. What if the type a is itself a Promise? Suppose f is

const f = p => p.then(n => n * 2)

In this example f is a function from Promise Number -> Promise Number. Let’s plug in a Promise Number for a to match up with the expected input for f. Then we can evaluate the expression:

const a = Promise.resolve(1)
const output = Promise.resolve(a).then(f)    
// output :: RejectedPromise TypeError: p.then is not a function

The value that gets passed to f is implicitly unwrapped. It is a TypeError to pass a function to then that takes a Promise for an argument. Note that we can evaluate the right-hand side of the equivalence to get a different result:

const output = f(a)  
// output :: ResolvedPromise 2

Therefore, Left Identity does not hold for every function of the form a -> Promise b: It only holds if a is not itself a Promise.

Right Identity

p.then(x => Promise.resolve(x)) ≡ p

This holds (where p is some Promise).


p.then(f).then(g) ≡ p.then(x => f(x).then(g))

As with Left Identity above, Associativity is only satisifed if f and g play nice. If f or g take a Promise argument, then, for the same reason as shown above, this law will not hold.


Failing to satisfy one law would have been sufficient to disqualify Promise from being a monad. The examples above show that Promise satisfies only one out of three: Right Identity.

Is this a problem?

Laws matter. They can give you confidence about the properties of the code you write, and how you can rewrite it. They permit you to port concepts across languages – to the extent the language can support them. On the other hand, if you are using JavaScript, then lawfulness may not be a priority.

Buzz de Cafe 10 April 2018