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I had a caveat in my original post re. the implicit wrapping of the `Promise.then()` callback's (pure) return value in a new Promise, and how this differs from
by ryandv 2y ago
I had a caveat in my original post re. the implicit wrapping of the `Promise.then()` callback's (pure) return value in a new Promise, and how this differs from Haskell's monadic bind; I had hoped to make a loose analogy while pointing out the differences for the sake of illustration. However it is indeed also possible to return a Promise from `.then()`'s callback, which is closer to Haskell's bind: [0]
The behavior of the returned promise (call it p) depends on the handler's
execution result, following a specific set of rules. If the handler function:
* returns a value: p gets fulfilled with the returned value as its value.
[...]
* returns an already fulfilled promise: p gets fulfilled with that promise's value as its value.
... then you can obtain a solution closer to the Haskell translation by using the behaviour of the second cited bullet point from the MDN article:
const promise1 = Promise.resolve(123);
promise1.then(v => Promise.resolve(v * 2)).then((value) => {
console.log(value);
// Expected output: 246
});
[0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
- bazoom42 2y agoThis article argues that while promises support the required operations for monad, they do not support the monad laws: https://www.siawyoung.com/promises-are-almost-monads/ https://www.siawyoung.com/promises-are-almost-monads/
- ryandv 2y agoIt's a good point. `Promise::resolve()` "flattens nested layers of promise-like objects (e.g. a promise that fulfills to a promise that fulfills to something) into a single layer." [0] The example was meant to be more of an illustrative analogy than an exact correspondence. [0] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/resolve https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...