If Python's gonna have breaking syntax, why not work on bringing it more in line with other modern languages that don't require special breakout syntax for expressions and rely more on functional features?
Are we still maintaining that lambdas are hard but suggesting expression-scoped variables are easy?
We maintain that list comprehensions do all the things lambdas can without the clunky anonymous functions. All while clearly communicating that a sequence is being transformed without any obscure syntax.
Or, as I like to call it, "The same lie they've been telling for a decade, that everyone knows is false because lots of people learn JavaScript and do not find it excessively complicated."
It's pretty rare that I write multi expression anonymous functions in js, and the language appears to be doing everything in it's power to make them wholly unnecessary.
This isn't true at all. Async/await is just a nice way to think about chaining callbacks. They're still callbacks. You can await raw promises and lambdas. Callbacks are still used when they ought to be used, which is in situations where there isn't an "end" to the deferred computation.
> It's pretty rare that I write multi expression anonymous functions in js, and the language appears to be doing everything in it's power to make them wholly unnecessary.
I don't agree with that assessment and I think a lot of Javascript programmers would too. You still see multi-expression lambdas used a lot, and as I've said elsewhere async/await is just a way of monadically handling lambdas with the appropriate call signature.
Heck, the entire point of the PEP we're discussing is to try to make it easier to write what'd trivially be recognizable multi-expression lambdas into things like list comprehensions or for decls. What a victory! Now you have to read the entire list comprehension at least twice!
As for this pep, I don't really like it, I think it adds dubious value and overly terse syntax. But that has nothing to do with the general idea that multi expression lambdas are unnecessary.
The fact that the python community is proposing and approving PEPs like this is evidence they have a desire for multi expression lambda functionality. They've just made a decision they refused to go back on because of over a decade of pride. They end up rebuilding every use case as an ad hoc piece of syntax.
This is wrong. Please fully inform yourself before making incorrect claims that I'm being reductive.
Some callbacks may not return a value, but callbacks in general absolutely can and do. (A callback simply need conform to an interface, that interface can be whatever the enclosing code decides it should be).
Either that, or async/await is more powerful than callbacks, because most of the `async` functions I've seen have a return value (coroutines can and do yield results, and async/await is a way of implementing coroutines).
I also don't really think that this solves the same issue as multiline lambdas in all cases. Sure, one of the examples (the case of a double function invocation in a comprehension) can be solved with a multiline lambda (among other ways). But multiline lambdas don't particularly clean up `if (x := re.match(...)): process(x)`. Which I'd argue is the more motivating example.
So sure, this doesn't solve your problem with the language in the best way possible, but it was never really intended to, it just sort of can as a side effect. (I don't really think this is a good thing, mind you, but it is a thing).
Async "callbacks" in Javascript cannot have semantically meaningful return values, because a callback context cannot read the values. EVERY lambda has a return value in Javascript, but this is more an implementation detail of dynamic typing than a statement of intent. They don't have callers that accept values of any type, so they're effectively a function of (Context -> IO ()). They're executed strictly for side effects.
If you have a meaningful return value, it's not properly called a callback. You can google about this because obviously the "spec" is "how it is used in literature and the common vernacular." As an example, the first SO hit on Google I say has top rated answer saying " An async callback function can return a value, in other words, but the code that calls the function won't pay attention to the return value." https://stackoverflow.com/questions/6847697/how-to-return-va...). You can surely find more, this is what distinguishes constructions like "callbacks" from "hooks" (which often return booleans, but may modify internal state passed to them or execute side effects as a callback) and further from "Promises" (which compute a value once and allow useful synchronization on the completion of that computation).
> Either that, or async/await is more powerful than callbacks, because most of the `async` functions I've seen have a return value (coroutines can and do yield results, and async/await is a way of implementing coroutines).
Well of course they are. I don't think I said they were equal in every respect, did I? I was a bit overly specific in talking about how Promises are often just used as callbacks, so sorry if I was unclear.
Async-await is a continuation monad. That's strictly a more powerful construction than a side-effect-only callback.
> I also don't really think that this solves the same issue as multiline lambdas in all cases.
I agree. It solves one very small slice: not having multi-line lambdas for filters and maps, nor a compiler that can fuse them.
> But multiline lambdas don't particularly clean up `if (x := re.match(...)): process(x)`. Which I'd argue is the more motivating example.
This PEP is even more exquisitely awful if that is truly the dominate use case. "We saved a line for the sake of saving a line" is a brutally frivolous reason to introduce that much new machinery.
Async callbacks in javascript cannot return semantically meaningful values to sync code. But the classic "callback hell" is when you have async code calling other async code. In that case, a callback can absolutely return a meaningful value to the other async callback. And yes, that is a common and valid use.
>This PEP is even more exquisitely awful if that is truly the dominate use case.
I don't particularly disagree with you here.
But then it ceases to be a callback? It's a one-shot function called within the context of a callback. A use case promises are particularly good at enabling.
No, it is a synchronous callback called in an async context. I think the confusion here is that you think that "callback" and "asynchronous callback" are synonyms when they are not[1]. A callback is simply any function passed into another function to be executed at a later time. An async callback is such a function which will be called asynchronously.
In JS, asynchronous callbacks are not allowed to return values (or well they can return values they just cannot be meaningfully accessed). Synchronous callbacks, however, certainly are.
Yes indeed, promises are incredibly good at un-nesting said synchronous-inside-of-asynchronous code.
[1]: https://en.wikipedia.org/wiki/Callback_(computer_programming...
Because generally what I call those is "higher order functions." For example, by the definition you've offered Array.map takes a "synchronous callback" and that seems quite wrong.
> The then() method returns a Promise. It takes up to two arguments: callback functions for the success and failure cases of the Promise.
Both of these functions may be synchronous, and may return values. Thus
Promise.resolve(2)
.then(x => x + 1)
.then(x => x + 1)
.then(console.log);
Generally speaking, "callback" is only used in the context of the function being executed in an async context, but again there's nothing stopping a sync code from being executed in an async context, and indeed this is pretty common (anytime one invokes a promise, as a simple example).
[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
That promise chain is not synchronous. You can prove this by putting your code sample and a following console.log("meep") into a function. The console.log("meep") will always fire before your promise's final handler runs, when you call the function. That's because it's scheduled after your calling function returns.
Try it.
> Will print 4, not undefined.
That's indeed true, but it's a special property of promises that you can extend the construction of a callback with `then` calls. You can never return that value to the calling context. Creating the illusion that you can is what async/await is for. The promise context extended the computation to the final callback, which executed the side effect.
It would be a much neater implementation if the underlying resolve/reject functions that construct the promise were synchronously calling the functions supplied by then. That'd be awesome. Sadly, that's not possible with this implementation, because you can then-extend a resolved promise. But I confess I like how algebraic this implementation is.
It's not wrong to call a then-extension a "callback" if it's the final one in the chain. It's computed value is forever discarded, as ultimately is the value of the entire promise. All promises terminate with a callback who's value is discarded.
As I said, Promises are special and expose a continuation monad, in that they're a way to construct callbacks piece-wise. It's a very useful piece of functionality for JavaScript to have and enables the underlying syntactic transformations that make async/await work.
The promise chain is executed synchronously in an asynchronous context. You seem to misunderstand this nuance. `x => x + 1` is a synchronous function. It will always execute synchronously and will return a result when it is invoked and block (within its context) until that invocation is complete. However, its calling context may be paused/pre-empted.
Importantly though, the promise won't be pre-empted during the evaluation of a synchronous function:
function sleep(ms) {
var start = new Date().getTime(), expire = start + ms;
while (new Date().getTime() < expire) { }
return;
}
Promise.resolve(0).then(
(x) => {
for (i = 0; i < 15; i++) {
sleep(1000);
console.log(`doing stuff ${i}`);
}
}).then(console.log);
Promises aren't doing anything magical, they're simply syntactic sugar for flattening otherwise nested chains of this (where +1 is a standin for the action that this specific function is taking):
f = (cb, v) => cb(v + 1)
f = (cb, v) => cb(v + 1)
(v) => (f(console.log, v))
f((v) => (f(console.log, v)), 1)
f((v) => (f((v) => (f(console.log, v)), v)), 1)
Promise.resolve(1)
.then(f)
.then(f)
.then(f)
.then(console.log)
All of those are synchronous functions, executed synchronously, in an asynchronous context, and all of them are called callbacks by what is perhaps the most authoritative source on web programming today.