This is a good overview of promises and async / await. The initial thesis of this seems to be that you need promises or async / await to avoid callback hell which I disagree with. You can still write good code, not use promises or async / await and not run into callback hell.
The example given in the article of callback hell isn't even a very good one. Why are we using an asynchronous method to fetch users, another asynchronous method to filter them and yet a third one to get their likes and return those. If that's the ultimate goal then, depending on your backend, you can should be able to make that in a single call.
But let's ignore the poor example in the article. Let's say we need to make 3 calls to 3 asynchronous functions and the result ultimately needs to be returned after all 3 functions are finished executing. Do you just nest them like the article? Absolutely not.
Essentially my point is: you can always architect around callback hell and make it better. I've already gotten into projects where I would now consider them "promise hell" where every method returns a promise of a promise of a promise of a promise of a promise and it's just maddening to debug.
Alternatives to callback hell are always context specific. There is no one-shot-cure-all, in my opinion (not even promises or async / await) but there are plenty of steps you can take.
- Can I separate or combine at least two of these asynchronous functions?
- Can I use a pseudo parallel or serial processing pattern ala async.js?
- Can I use a messaging pattern where each message is handled separately with a single callback ala msngr.js or postal.js?
- Can I simply create a response building pattern where many asynchronous methods write to the same response object and, when it's complete, return it?
- Does it make sense to use a promise here? What about the async / await pattern?
I'm pretty new at promises, but doesn't any appearance of a "promise of a promise" indicate a trivial mis-use of promises? Isn't the whole point of promises that they can be chained together without nesting?
Indeed. And if you generalize this idea only slightly, you get ...... monads!
Maybe promises will be the vehicle that makes understanding the usefulness of monads available for the masses.
Alternatively, promises may fail the acceptance test for the same reason as monads - because the average developer simply doesn't care / doesn't get it.
To expound on this connection: the basic idea is that a Promise has a `.then(fn)` method call which takes this `fn`, which returns a value. If that value is not a Promise, then it is lifted to a new promise; either way that promise is what's returned by the .then().
In Haskell, for type safety, you have to explicitly write this conversion and the function which does it has the somewhat-confusing name `return`. The `.then()` method then has the name `>>=`, pronounced "bind".
If you have these, you have a template for type-safe functional metaprogramming. Given any x, you can produce "a program which produces an x" (return), and given any function from an x to a program which produces a y, you can bind that onto a program which produces an x, to produce a program which produces a y:
return :: x -> ProgramWhichProduces x
(>>=) :: ProgramWhichProduces x -> (x -> ProgramWhichProduces y) -> ProgramWhichProduces y
This was huge historically because you can still metaprogram in a purely functional language, even though you cannot do things which have side-effects. So I can give you a pure primitive like `println :: String -> ProgramWhichProduces Nothing` and you can build `println("Hello, world!")` with it, and there are no side-effects in running that function on that input, so this is purely functional. In this way the act of actually running a `ProgramWhichProduces x` as an executable is deferred until the language is no longer in play: when the compiler is run, it looks for a `ProgramWhichProduces Nothing` called `main` and writes that out as the actual executable generated by the compiler.
Monad refers then to precisely this design pattern of saying "I have some trivial embedding `x -> m x` into my problem domain `m` and I have some way to combine an `m x` with an `x -> m y` to produce an `m y`, hey, that's a monad!" ... One example is list comprehensions; there is a trivial way to embed any element as a singleton list [x] and then you can always use any function x -> [y] (called a "transducer" in Clojure) to process a list [x], concatenating together all of the [y]'s you get along the way. With ES6 generators this looks like:
function* forEach(iterator_x, ys_from_x) {
for (let x of iterator_x) {
yield* ys_from_x(x);
}
}
Hey, that's a monad. You can write a list comprehension like:
// equivalent to Python's [f(x, y) for x of list_x for y of list_y if predicate(x, y)]
forEach(list_x, x => forEach(list_y, y => predicate(x,y) ? [f(x, y)] : []))
In Haskell you can write all monads with a special syntax called `do`-notation; you can write the above in any of three ways:
[f x y | x <- list_x, y <- list_y, predicate x y]
-- which desugars to...
do
x <- list_x
y <- list_y
if predicate x y then [undefined] else []
return (f x y)
-- which desugars to almost exactly the JS above...
list_x >>= \x ->
list_y >>= \y ->
(if predicate x y then [undefined] else []) >>= \_ ->
return (f x y)
So that's the general pattern of monads and do-notation in a short, sweet lesson.
If you are not trying to use Haskell (which has monads as a workaround for certain problems) then what is the point having monads? The average developer doesn't care because they are working in an environment where monads aren't needed.
Trying to drag monads into arbitrary languages like Javascript isn't some kind of best practice, it's cargo culting.
Monad is a mathematical term for certain forms of (categorical) abstraction. Yes, Haskell actively uses the term "monad" in the language itself to discuss these things, but that doesn't make them a "Haskell thing" any more than Fortran has mathematical formulas and thus all math is a "Fortran thing".
Monads are also not a "workaround for certain problems", contrary to what you hear sometimes, but an attempt to generalize a number of seemingly unrelated concepts into a uniform interface.
Uniform interfaces are good for programmers. This is the magic that monads bring to other languages as we get a handle of what the monad is for and what it does. You don't have to know what a monad is to make use of it. You don't need to know that Promises are a "continuation monad" to take advantage of the fact that async/await are simple tools to transform the "continuation monad" into something resembling synchronous code, which is a big win.
(Meanwhile Haskell points out that you can go simpler than async/await for your monad bindings if you like and use the same constructs for other monads too, but language developers are taking this one step at a time and trying to hit a happy medium where they have the power that monad binding gives but the discoverability and debuggability of more bespoke solutions to individual monads...)
Needless to say, it's not cargo culting so much as learning more about the very mathematical fundamentals of programming and using those fundamentals to build better languages with better features for better programs.
Haskell doesn't have monads as a work-around. They are a useful abstraction because they capture a common semantic pattern found in many programming concepts. Nullable types, error handling, sequential execution, and asynchronous event handling are all concepts that can be described monadically.
By describing them as monads and making the monadic pattern a core idea of the language, you can write algorithms that operate on any monad, so that you can write one function and have it be useful for both managing the execution of IO and error handling in a generic way.
People have an aversion to monads because a the concept of a monad has no grounds in reality, kind of like complex numbers. It is an abstraction that is useful not because it describes anything, but because it is simply a useful way to think.
> If you are not trying to use Haskell (...) then what is the point having monads?
I'm not sure how to answer this question, as my previous comment actually is the answer to your question.
Promises are a great example. They are a mechanism that provides better encapsulation and is still easier to compose than using pure callbacks.
(IIRC, Haskell made a similar transition: Monads haven't been in Haskell forever, many other ways have been tried before, but in the end turned out to be quite cumbersome and ugly in larger code bases. The introduction of monads was generally perceived as relieve in Haskell, and are becoming more and more popular in other non-lazy functional languages as well, such as OCaml.)
Check the Elm language. It built on top of monad ideas (but the language author avoids using the term for good reasons). It make programming web pages very compostable, scalable and maintainable. It shows that monads are really good fit with event-driven programming.
You might as well ask what's the point of having Iterable<> when you're programming in C :)
It's not that you strictly _need_ monads or Iterable interfaces, it's that they represent an abstract concept which allows you to solve problems in a generic and type-safe way
I wouldn't probably "drag monads" into Javascript, but I'd surely appreciate Functors when programming in Javascript :)
A Monad is a general-purpose mathematical structure, not just a workaround. GP gave you an example of something they're useful for, which you completely ignored.
> Isn't the whole point of promises that they can be chained together without nesting?
Yes, it's an excedingly common misunderstanding and probably one of the main causes of people being "turned off" by promises. It's kind of frustrating to see so many blog posts making this mistake when critiquing them.
Correct but once you get some functions returning promises and other functions returning actual values then your own code base can become confusing. I constantly find myself in these types of code bases trying to find function definitions so I know what it returns.
You can try and make 100% of your asynchronous methods return promises but what happens when you need to work with a library that can't use promises? Do you write a callback for that? Now you have a mixture and sure a few of these no big deal but when the code base balloons and you start having one offs all over the place now you need to figure out what returns what and code specifically to that.
Maybe if promises were in ECMAScript from the beginning then everyone would have used them and it wouldn't be a huge deal but right now it can get a bit frustrating, in my opinion. The stack traces of these nested promises are never fun to dive into as well though that likely depends on whether you're using a polyfill or not.
I'm not quite sure I understand you, but it sounds like a case for `Promise.resolve(value)`[1] – that's what I do whenever I don't know if something will be a Promise or an already-resolved value. `Promise.resolve(value)` returns a Promise that immediately resolves with `value` or whatever `value` resolves to if it's a Promise – no need to do any checks yourself, no need to nest Promises inside Promises inside Promises.
For example, imagine you have a charting library and you want to draw a chart, but you don't know if the data is available already or if it's being loaded asynchronously. You can just write:
// `drawChart(data)` is a synchronous function
// `data` may be the data itself or a promise that resolves to the data
Promise.resolve(data).then(drawChart)
For me, this is actually the killer feature of Promises, avoiding callback hell is just a nice side-effect. Once you start thinking of Promises as placeholders for values that may or may not be known already (rather than just an abstraction of the `callback(err, value)` pattern), you can write some really elegant code.
Fair enough that would take care of my issue for the most part. Native implementations of promises should work well with that. Seems most of the polyfills end up using setTimeout() which is kinda slow[1] and one of the reasons I don't typically like working with them (waiting for more ubiquitous support).
>This is a good overview of promises and async / await. The initial thesis of this seems to be that you need promises or async / await to avoid callback hell which I disagree with. You can still write good code, not use promises or async / await and not run into callback hell.
Even if you could, you shouldn't. It's busy work -- manually doing the computer's work.
What I think people sometimes forget about async/Promises vs callbacks is that callbacks-style define a code structure that is hard to maintain by multiple developers, since it leaves a lot of power to the way you write your code. Not every team has 5 experienced JS developers, so at one point implementing a small feature by a team member would become : `Hey, let's just nest another callback in there. It takes 1 line of code` instead of implementing it in a readable way.
Promises solve this, but indeed there are a couple of problems there with debugging. Honestly I think they give more readable code than whatever callback implementation you invent to avoid the callback hell. Libraries as bluebird can make your promise operations really short and even one liners, without performance hit.
Finally now we have async, which I hope will be popular enough in the future to become at least as much popular as Promises are now.
Even when implemented in a readable way, callbacks can still become hard to maintain. When you use properly separate functions (instead of anonymous ones), you typically have to embed those methods inside one another.
I'll take an example from the callbackhell website that was linked previously:
document.querySelector('form').onsubmit = formSubmit
function formSubmit (submitEvent) {
var name = document.querySelector('input').value
request({
uri: "http://example.com/upload",
body: name,
method: "POST"
}, postResponse)
}
function postResponse (err, response, body) {
var statusMessage = document.querySelector('.status')
if (err) return statusMessage.value = err
statusMessage.value = body
}
What happens when I want to do something other than `postResponse` when `formSubmit` is finished? I would have to start thinking about using partial application and providing specific callbacks, which is okay, but what if the callback levels are multiples deep? The code still starts to get muddy really quickly.
With promises / async, the asynchronous code doesn't need to know anything about what functions want to run afterwards, so the concerns stay completely separated.
Exactly. You need to keep really good control over your code structure to avoid callback hell. And again at one point you will end up in a situation where you will need to declare functions inside a main function, etc.
That's why I noted that I've never seen a callback implementation that is more readable than promises.
The example given in the article of callback hell isn't even a very good one. Why are we using an asynchronous method to fetch users, another asynchronous method to filter them and yet a third one to get their likes and return those. If that's the ultimate goal then, depending on your backend, you can should be able to make that in a single call.
But let's ignore the poor example in the article. Let's say we need to make 3 calls to 3 asynchronous functions and the result ultimately needs to be returned after all 3 functions are finished executing. Do you just nest them like the article? Absolutely not.
Essentially my point is: you can always architect around callback hell and make it better. I've already gotten into projects where I would now consider them "promise hell" where every method returns a promise of a promise of a promise of a promise of a promise and it's just maddening to debug.
Alternatives to callback hell are always context specific. There is no one-shot-cure-all, in my opinion (not even promises or async / await) but there are plenty of steps you can take.
- Can I separate or combine at least two of these asynchronous functions?
- Can I use a pseudo parallel or serial processing pattern ala async.js?
- Can I use a messaging pattern where each message is handled separately with a single callback ala msngr.js or postal.js?
- Can I simply create a response building pattern where many asynchronous methods write to the same response object and, when it's complete, return it?
- Does it make sense to use a promise here? What about the async / await pattern?