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Maybe I’m confused, but I dont see how Promise.then() corresponds to bind? If I understand correctly, the point of the bind function is you pass a callback whic

Maybe I’m confused, but I dont see how Promise.then() corresponds to bind? If I understand correctly, the point of the bind function is you pass a callback which itself return the monad type. But the Promise.then() callback should not return a monad but just the regular result value of invoking the callback. So in essence Promise.then() is like Array.map() while bind is like Array.flatMap() Edit: It seems you are correct, you can indeed return a promise in the callback and it compose correctly. But in your example the callback does not return a promise and is therefore not an example of monadic bind.

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 ago
  This 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 ago
    It'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...

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 ago
This 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 ago
  It'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...