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0 pointsbazoom421y ago0 comments

How is async-await monads? Isn’t it just syntax sugar over callbacks?

0 comments

Consider async-await a syntactic sugar over Promises (from JavaScript). Then, Promises constitute an instance of the Monad typeclass where monadic `bind` or (>>=) is `Promise.then()`, and `return` is `Promise::resolve()`.

Here is a translation of a modification of the example given in [1]:

    const promise1 = Promise.resolve(123);

    promise1.then(v => v * 2).then((value) => {
      console.log(value);
      // Expected output: 246
    });

into Haskell:

    ghci> let promise1 :: IO Int = return 123
    ghci> promise1 >>= (return . (* 2)) >>= print
    246

One key discrepancy worth pointing out is that in the `Promise.then()` API of JavaScript, the function provided to `then` (e.g. `v => v * 2` above) is implicitly composed with a call to `::resolve` in order to turn that function's pure return value, the `Number` 246, into a Promise resolving into the `Number` 246; in Haskell, this operation must be made explicit (hence the composed function `return . (* 2)` passed to the first application of (>>=) or `bind`).

You could say that the instance method `Promise.then()` expects an argument of type (a -> b), with the return value of type `b` being implicitly and automatically wrapped into a Monad `m b`, whereas Haskell's bind expects an argument of type (a -> m b) on that operator's right-hand side, with the return value explicitly wrapped into a Monad `m b` by the provided function argument itself.

[0] https://wiki.haskell.org/Monad

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

bazoom42OP1y ago

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.

ryandv1y 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/Refe...

1 more reply

wk_end1y ago

At that moment, the student became enlightened.

bazoom42OP1y ago

Callbacks are not the same as monads if that is what you are implying.

mrkeen1y ago

That's right, but only because it's a 1-to-many comparison error.

Callbacks are one particular monad, not monads in general: https://jsdw.me/posts/haskell-cont-monad/

2 more replies

j / k navigate · click thread line to collapse

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ryandv1y ago

Here is a translation of a modification of the example given in [1]:

    const promise1 = Promise.resolve(123);

    promise1.then(v => v * 2).then((value) => {
      console.log(value);
      // Expected output: 246
    });

into Haskell:

    ghci> let promise1 :: IO Int = return 123
    ghci> promise1 >>= (return . (* 2)) >>= print
    246

[0] https://wiki.haskell.org/Monad

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

bazoom42OP1y ago

So in essence Promise.then() is like Array.map() while bind is like Array.flatMap()

ryandv1y ago

    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/Refe...

1 more reply

wk_end1y ago

At that moment, the student became enlightened.

bazoom42OP1y ago

Callbacks are not the same as monads if that is what you are implying.

mrkeen1y ago

That's right, but only because it's a 1-to-many comparison error.

Callbacks are one particular monad, not monads in general: https://jsdw.me/posts/haskell-cont-monad/

2 more replies

j / k navigate · click thread line to collapse