returning non-Promise values from async functions and running them synchronously (or Promise.sync() idea)

Isiah Meadows isiahmeadows at gmail.com
Mon Feb 4 07:09:12 UTC 2019


I agree that sometimes-sync is always a nightmare, and I've
experienced this pain personally from a library API that once did
this. (I did succeed in getting it to eventually change.) I'm willing
to draw exception for things like APIs that wrap both sync and async
iterators, but those literally provide two separate entry points in
the form of `Symbol.iterator` vs `Symbol.asyncIterator` methods.

-----

Isiah Meadows
contact at isiahmeadows.com
www.isiahmeadows.com

On Mon, Feb 4, 2019 at 1:43 AM Jordan Harband <ljharb at gmail.com> wrote:
>
> Typically, APIs that are sometimes sync and sometimes async are called "zaļgo" - unpredictable, hard to maintain, hard to understand. The general best practice is that a function should always be async, or always sync, but never the twain shall meet.
>
> Relevant:  http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony
>
> On Sun, Feb 3, 2019 at 4:49 PM Isiah Meadows <isiahmeadows at gmail.com> wrote:
>>
>> You could move the async part into a helper function and call that from a sync function you instead expose. I've used this trick more than once, and although it is a bit of boilerplate, it works well enough.
>> On Sun, Feb 3, 2019 at 18:40 #!/JoePea <joe at trusktr.io> wrote:
>>>
>>> I often find myself enjoying async functions until the time comes when I don't want to await anything, and I want the call stack to be sync (i.e. I don't want to defer if I don't have to).
>>>
>>> But, async functions always return a Promise. So I find my self converting my async functions back into normal functions so that I can return non-Promise values, this way the caller can wait only if I return a promise, otherwise continue synchronously.
>>>
>>> Here's an example function in one of my classes:
>>>
>>> ```js
>>>             initWebGL() {
>>>                 if (this.__glLoading) return false
>>>                 this.__glLoading = true
>>>
>>>                 // order of the if-else matters!
>>>                 if (this.__glUnloading) return Promise.resolve().then(A.bind(this))
>>>                 else if (this.__glLoaded) return false
>>>
>>>                 A.call(this)
>>>
>>>                 function A() {
>>>                     // ... load stuff (omitted for brevity) ...
>>>                 }
>>>
>>>                 return true
>>>             }
>>> ```
>>>
>>> then the caller (f.e. a subclass) only needs to wait when a promise is returned:
>>>
>>> ```js
>>>         initWebGL() {
>>>             const superResult = super.initWebGL()
>>>             if (superResult instanceof Promise) return superResult.then(A.bind(this))
>>>             if (!superResult) return false
>>>
>>>             A.call(this)
>>>
>>>             function A() {
>>>                     // ... subclass loads stuff (omitted for brevity) ...
>>>             }
>>>
>>>             return true
>>>         }
>>> ```
>>>
>>> This is just one example, but in general I find many cases where I want to run synchronously most of the time, and only sometimes have to defer (whether by choice or not).
>>>
>>> So I am losing out on the nice `await` syntax just because I can not do what I want to do with `async`/`await`.
>>>
>>> What if async functions could have some way to continue synchronously, and return non-Promise values?
>>>
>>> Or, what if we could have some sort of Promise API that would cause any callers to synchronously away, like in the following example, so that the change is not a breaking change, and async functions would still return Promises but the Promise could perhaps continue synchronously:
>>>
>>> ```js
>>> async function example() {
>>>   return Promise.sync()
>>> }
>>> ```
>>>
>>> Then, in a caller, there would be no microtask deferral:
>>>
>>> ```js
>>> async function test() {
>>>   await example()
>>>   console.log('one')
>>>   // implicit return is a Promise.sync() here.
>>> }
>>>
>>> function main() {
>>>   test()
>>>   console.log('two')
>>> }
>>> ```
>>>
>>> The output in this case would be:
>>>
>>> ```
>>> "one"
>>> "two"
>>> ```
>>>
>>> Note that in that example, we are _sure_ that `two` is logged after `one` because those async functions only ever use `Promise.sync()`.
>>>
>>> It would be good practice to use `await` regardless, because one may not know if the async function they are calling will return a non-`Promise.sync` value. So the `main` function is better written as
>>>
>>> ```js
>>> async function main() {
>>>   await test()
>>>   console.log('two')
>>> }
>>> ```
>>>
>>> In this case, everything still happens synchronously, but if the author of `example()` changes the implementation to return a non-sync Promise, then things will still run in the expected order. F.e.
>>>
>>> ```js
>>> async function example() {
>>>   await fetch(...)
>>> }
>>> ```
>>>
>>> I keep finding scenarios where I want to avoid async unless I need async, but then I lose the convenient async/await syntax.
>>> _______________________________________________
>>> es-discuss mailing list
>>> es-discuss at mozilla.org
>>> https://mail.mozilla.org/listinfo/es-discuss
>>
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss


More information about the es-discuss mailing list