I really don't like this article. It has a catchy, profound-sounding title that people bandy about to argue against stuff they don't like.
All functions, even non-async functions, are colored. In any large system codebase you'll have functions that can only be called in certain situations, with the right setup, whatever, and if you're lucky this is communicated by types but regardless those restrictions can't be avoided. It's easy to call low-restriction functions from high-restriction ones and not the other way around.
Furthermore, it's not like the alternative to explicit await doesn't have issues too (that the article doesn't mention). There is inherent complexity, it's a tradeoff, you can't just syntax it away.
> You still can’t call a function that returns a future from synchronous code. (Well, you can, but if you do, the person who later maintains your code will invent a time machine, travel back in time to the moment that you did this and stab you in the face with a #2 pencil.)
Author makes up a lie.
Then lampshades it away with a colorful non sequitur.
---
The alternatives that people praise like golang, have other tradeoffs that are much worse because the async logic is now implicit. Your entire codebase is now a surface area that is at risk of being blocked by waiting on a channel; the the mitigation of this is through responsible use of coroutines, but then you're right back around to extra information about your code that is analogous to colring, except not as explicit as async/await.
If you haven’t taken a lock, any other code can start executing at any time, so any invariant you might have established on one line may no longer be true on the next line.
If you don’t depend on anything mutable that anyone else can modify then this is mitigated, but that’s a very specific discipline you have to abide by.
> Your entire codebase is now a surface area that is at risk of being blocked
The point of goroutines is that they can freely block when needed. It's not like async where you have to be paranoid at every moment about writing blocking code
You can only freely block the goroutines that you designed that way, there's plenty of ways of shooting yourself in the foot with goroutines without even touching “blocking” code (because everything is blocking).
Go doesn't have colored functions due to its nice fat runtime hiding all the async magic away for us.
That makes it a pleasure to code concurrent stuff for IMHO.
It does have its own similar problems though - does a function return an error? If so you are going to need to plumb the error return through all the callers. Does a function need a context.Context? Ditto.
This is a subtle point that I've seen missed repeatedly, but: The reason that "color" is important is that if you have a function ten layers down in your stack that is the wrong "color", you now have to change that top-level function. There is no other option.
Propagating errors up the stack is not the same, because the top-level function is not developing an error return because of the 10-level-nested function. It is developing one because the function it called has one, and apparently, it needs to return it to its local caller. It's a local consideration. It is true that it may be a recursive local consideration where this was true 10 times, but the reason it is different is that it doesn't have to be that way. It could have been the case that the function 7 layers down handled the error somehow and it stopped propagating up the stack. But at each point, the consideration was local, and as such, amenable to local solutions other than just tossing the error up. If you choose to "correctly" plumb the error through all your functions, well, good on you for apparently being willing to apply good software engineering practices even when it's annoying, but this is just normal day-to-day function activity stuff.
By contrast, in a function coloring situation, if the color is wrong 10 layers down, you must change the calling function. It's a non-local consideration. You don't get to decide not to change it. You can't encapsulate it. You don't get a choice. It pollutes the entire stack, forcibly.
Another way to look at it is, if the function 10 levels down developed what you think is a color, but there is a way for the function 9 levels down to hide the color from the rest of the stack, even via a hack like simply dropping an error you really need or hackily constructing an object of some type to pass in, then it is by definition not a color. A color change can't be stopped by any way of writing an intermediate function. It must be propagated all the way up the stack.
If you don't have this, you don't have "color". Like, some people will say that in their language that maybe there is some way to encapsulate "async". If you can, then you don't have an async color. Although I will say that if your "encapsulation" is basically to run it in a non-concurrent environment, that's really not encapsulation. It isn't really "encapsulation" if you're giving up an entire major feature of the language, because that is something very visible to the rest of the program.
Go's context.Context is similarly not a color. You can always just create a context.Background() and pass that down. If you didn't have any context already in hand, which means you must not care about any of the features context offers, then that is usually a fine thing to do. Context is trivially bypassed if you don't want it. It can be encapsulated within a portion of the stack without "polluting" the rest of the stack like any other function parameter.
The key aspect of color is that it is not optional. It isn't something that you can just decide to ignore and stop passing up, or trivially create a value for passing down to other functions. You have to change the "color". Async is a color in many environments. There aren't really that many colors in programming languages because they are very, very quickly inconvenient and we tend to squeeze them out. (Haskell really sticks out here as a language that is not only capable of creating arbitrary colors, but where this is an explicit tool used by the community rather than a limitation, and they even have ways of combining colors together deliberately.) Statement versus expression distinctions are another one, where a "statement" may not be usable in an "expression", and you'll note how languages have in general erased that one over time because it's really just a cost without much benefit.
I wish the key word was instead dontawait and was used inversely to how await is used. 99% of the time I'm using an async function, despite however slow it is, there's nothing for my code to do but wait for it to finish. But if for some reason I would like the next line of code to run before the current one is done, I'll let you know.
Like, why can't my sync function await something asynchronous? If it has to lock up the whole thread while that function executes, that's fine because that's how it was going to work anyway 99% of the time
At least in JavaScript, it's nice to be able to see explicitly where you can expect the function to yield, so it's clear when race conditions can occur, or if you're calling it in a loop, whether you should consider running things in parallel.
Plus, you probably don't want to lock up the whole thread if you're writing anything more than a quick script, like a web server or a GUI.
> Like, why can't my sync function await something asynchronous?
The answer, at least for Python, is that it is an intentional limitation because the alternatives introduce some quite bad trade-offs.
Option 1: your awaited promise goes into the main async event loop. This is bad because it means that your single-threaded sync method now needs to be thread-safe, and so does any sync code that calls your sync method despite it not even knowing that you're doing anything async. This is essentially unworkable without throwing away the option of writing non-thread-safe code.
Option 2: Your awaited promise goes into its own new event loop that only contains sibling and child promises. There's nothing technically stopping someone from doing this[1], but now you've lost a ton of the value of async because you will inevitably end up with a ton of siloed event loops that leave the process idle despite other async tasks existing that could run. Effective async code needs to share an event loop at as high of a level as possible, which means tainting as many methods with async as possible. At that point, you might as well enforce it at the language level and avoid the inevitable pain and fragmentation that comes from other devs across the ecosystem mixing sync and async code.
I think the downsides of option 2 are overstated here. In lots of cases you don't care about the "value of async", you just want code that works well enough and option 2 does accomplish that in anything that is not perf critical.
We need algebraic effects in more languages, this solves the function coloring problem. OCaml 5 has them and it seems to be doing quite well, combine that with the semantics of the borrow checker in the form of OxCaml and we might just have an ideal language. I'd like to see algebraic effects in Rust as well but sadly it seems their keyword generics initiative is languishing.
Colored functions are good. It reflects the language design on signaling what is important, and what are the properties it want the writer to pay attention.
Other examples of colored functions:
* Haskell: pure function and non-pure (IO monads) looks different.
* Rust: unsafe functions (or block) requires special markers.
The problem with checked exceptions afaik was far more in the execution than in the idea itself. And also late 90s-early 00s was different time in general.
My first ever EM showed me this piece ~10 years ago, and I still think about it a lot. One pattern I've adopted is to keep as much code to be synchronous as possible. On larger teams, especially when the slop-cannon is really going, I can at least depend on codeowners to tag me if someone tries to convert something to async (eg. adding a DB call somewhere), because they chain of things that need to be converted to async is so long. Then I can jump in and say "this entire chain of code is sync, if you want a DB call, do it somewhere else"
For Python backends I've seen good success with just making it company policy that everything is synchronous (normal-colored) and bypassing the developer overhead from async/await. Cooperative multitasking is a pain because, well, it requires cooperation. You can go pretty far by just adding more threads, processes, and replicas before it's worth the overhead.
You not only leave performance on the table (which depending on your use case/environment, may not matter if you can just throw more threads at it) but also some developer ergonomics.
asyncio.gather is a lot less code than having to manage a thread pool or something like Celery with all it's underlying infrastructure.
If you're in an ecosystem where a lot of the async boilerplate is free/cheap (ex: FastAPI) then the developer overhead of sprinkling awaits on your I/O bound calls is pretty low IMO.
> something like Celery with all it's underlying infrastructure.
Unpopular opinion, but combining this with the other "no thanks" sentiments in this subthread is the right answer. Your app is so complicated you need async? Then it's complicated enough that you can benefit from infrastructure. I don't want to watch coworkers try to badly rebuild message queue or scheduling semantics in an application code base. Just use infrastructure that's made by people who know what they are doing. That was problematic in 2015, but in 2026 it's a bit of docker, and it's not just about web/microservices. Very easy for sufficiently complex apps to simply leverage a local sandbox of celery, redis, graphdb's and whatever. Stand-alone is overrated since we don't have to do it anymore.. app devs should get more comfortable working with ensembles like this so they have access to best-in-class solutions.
You don't like infrastructure AND have such a need for performance AND don't want threads or multiprocess? Consider using another language. Async is mostly a solution in search of a problem, and the enduring popularity of TFA goes to show this has been the right conclusion for ~10 years.
I said nothing about "stand-alone" services. I'm all about using the right solution to the problem. We run on Kubernetes and have access to message queues. But if all I want to do is make a couple of HTTP calls concurrently, I don't think I should have to manage a thread/process pool to do so, or lean on a message queue or redis based RPC mechanism. In an async context I can do this with a single line of code.
Every rich client-side experience in your browser is written using async code in Javascript or Typescript, as is every electron app. Every developer at my company is comfortable with this pattern, and frameworks like FastAPI make this a similarly smooth experience when using Python.
If async was a solution in search of a problem, it wouldn't have been stolen from C# and added to Rust, Python, Kotlin, etc. The engineering effort required to bring this solution to all these languages is immense, so I'm clearly not the only person seeing value in it.
Performance aside (which I would argue is premature optimization, as most programs will not feel the theoretical overhead of threads), async is a bad approach for developer ergonomics. Threads are so much easier to work with and reason about than async. There are reasons to use async (like if you're in the rare case when thread overhead is noticeable), but developer ergonomics are absolutely not a reason.
Say I need the results from two expensive REST API calls, so I want to run them concurrently. Managing a thread pool you find a _better_ experience than
one, two = await asyncio.gather(callOne(), callTwo())
I just do not want to do async in Python. If you need async its questionable whether Python is a good choice at all, and if you use Python maybe look at another solution if at all possible (even using more processes and throwing hardware at it).
I really don't like this article. It has a catchy, profound-sounding title that people bandy about to argue against stuff they don't like.
All functions, even non-async functions, are colored. In any large system codebase you'll have functions that can only be called in certain situations, with the right setup, whatever, and if you're lucky this is communicated by types but regardless those restrictions can't be avoided. It's easy to call low-restriction functions from high-restriction ones and not the other way around.
Furthermore, it's not like the alternative to explicit await doesn't have issues too (that the article doesn't mention). There is inherent complexity, it's a tradeoff, you can't just syntax it away.
> You still can’t call a function that returns a future from synchronous code. (Well, you can, but if you do, the person who later maintains your code will invent a time machine, travel back in time to the moment that you did this and stab you in the face with a #2 pencil.)
Author makes up a lie.
Then lampshades it away with a colorful non sequitur.
---
The alternatives that people praise like golang, have other tradeoffs that are much worse because the async logic is now implicit. Your entire codebase is now a surface area that is at risk of being blocked by waiting on a channel; the the mitigation of this is through responsible use of coroutines, but then you're right back around to extra information about your code that is analogous to colring, except not as explicit as async/await.
Your entire codebase is already at risk of being blocked by a spinlock or CPU-intensive operation, so what's the difference?
If you haven’t taken a lock, any other code can start executing at any time, so any invariant you might have established on one line may no longer be true on the next line.
If you don’t depend on anything mutable that anyone else can modify then this is mitigated, but that’s a very specific discipline you have to abide by.
> Your entire codebase is now a surface area that is at risk of being blocked
The point of goroutines is that they can freely block when needed. It's not like async where you have to be paranoid at every moment about writing blocking code
You can only freely block the goroutines that you designed that way, there's plenty of ways of shooting yourself in the foot with goroutines without even touching “blocking” code (because everything is blocking).
Go doesn't have colored functions due to its nice fat runtime hiding all the async magic away for us.
That makes it a pleasure to code concurrent stuff for IMHO.
It does have its own similar problems though - does a function return an error? If so you are going to need to plumb the error return through all the callers. Does a function need a context.Context? Ditto.
I guess you can't win them all :-)
Same with the BEAM languages like Erlang, Elixir, and Gleam. Though it still bothers me that they call their green threads "processes".
That's mostly because BEAM uses an actor-style approach while predating the concept of actors, isn't it? Interesting artefact of history if so
And Haskell is an ensemble of rainbows. It's very fun and pretty to look at.
Type classes can smooth over some of it but it's not unusual to have to do some plumbing.
This is a subtle point that I've seen missed repeatedly, but: The reason that "color" is important is that if you have a function ten layers down in your stack that is the wrong "color", you now have to change that top-level function. There is no other option.
Propagating errors up the stack is not the same, because the top-level function is not developing an error return because of the 10-level-nested function. It is developing one because the function it called has one, and apparently, it needs to return it to its local caller. It's a local consideration. It is true that it may be a recursive local consideration where this was true 10 times, but the reason it is different is that it doesn't have to be that way. It could have been the case that the function 7 layers down handled the error somehow and it stopped propagating up the stack. But at each point, the consideration was local, and as such, amenable to local solutions other than just tossing the error up. If you choose to "correctly" plumb the error through all your functions, well, good on you for apparently being willing to apply good software engineering practices even when it's annoying, but this is just normal day-to-day function activity stuff.
By contrast, in a function coloring situation, if the color is wrong 10 layers down, you must change the calling function. It's a non-local consideration. You don't get to decide not to change it. You can't encapsulate it. You don't get a choice. It pollutes the entire stack, forcibly.
Another way to look at it is, if the function 10 levels down developed what you think is a color, but there is a way for the function 9 levels down to hide the color from the rest of the stack, even via a hack like simply dropping an error you really need or hackily constructing an object of some type to pass in, then it is by definition not a color. A color change can't be stopped by any way of writing an intermediate function. It must be propagated all the way up the stack.
If you don't have this, you don't have "color". Like, some people will say that in their language that maybe there is some way to encapsulate "async". If you can, then you don't have an async color. Although I will say that if your "encapsulation" is basically to run it in a non-concurrent environment, that's really not encapsulation. It isn't really "encapsulation" if you're giving up an entire major feature of the language, because that is something very visible to the rest of the program.
Go's context.Context is similarly not a color. You can always just create a context.Background() and pass that down. If you didn't have any context already in hand, which means you must not care about any of the features context offers, then that is usually a fine thing to do. Context is trivially bypassed if you don't want it. It can be encapsulated within a portion of the stack without "polluting" the rest of the stack like any other function parameter.
The key aspect of color is that it is not optional. It isn't something that you can just decide to ignore and stop passing up, or trivially create a value for passing down to other functions. You have to change the "color". Async is a color in many environments. There aren't really that many colors in programming languages because they are very, very quickly inconvenient and we tend to squeeze them out. (Haskell really sticks out here as a language that is not only capable of creating arbitrary colors, but where this is an explicit tool used by the community rather than a limitation, and they even have ways of combining colors together deliberately.) Statement versus expression distinctions are another one, where a "statement" may not be usable in an "expression", and you'll note how languages have in general erased that one over time because it's really just a cost without much benefit.
I wish the key word was instead dontawait and was used inversely to how await is used. 99% of the time I'm using an async function, despite however slow it is, there's nothing for my code to do but wait for it to finish. But if for some reason I would like the next line of code to run before the current one is done, I'll let you know.
Like, why can't my sync function await something asynchronous? If it has to lock up the whole thread while that function executes, that's fine because that's how it was going to work anyway 99% of the time
At least in JavaScript, it's nice to be able to see explicitly where you can expect the function to yield, so it's clear when race conditions can occur, or if you're calling it in a loop, whether you should consider running things in parallel.
Plus, you probably don't want to lock up the whole thread if you're writing anything more than a quick script, like a web server or a GUI.
> Like, why can't my sync function await something asynchronous?
The answer, at least for Python, is that it is an intentional limitation because the alternatives introduce some quite bad trade-offs.
Option 1: your awaited promise goes into the main async event loop. This is bad because it means that your single-threaded sync method now needs to be thread-safe, and so does any sync code that calls your sync method despite it not even knowing that you're doing anything async. This is essentially unworkable without throwing away the option of writing non-thread-safe code.
Option 2: Your awaited promise goes into its own new event loop that only contains sibling and child promises. There's nothing technically stopping someone from doing this[1], but now you've lost a ton of the value of async because you will inevitably end up with a ton of siloed event loops that leave the process idle despite other async tasks existing that could run. Effective async code needs to share an event loop at as high of a level as possible, which means tainting as many methods with async as possible. At that point, you might as well enforce it at the language level and avoid the inevitable pain and fragmentation that comes from other devs across the ecosystem mixing sync and async code.
[1] https://pypi.org/project/nest-asyncio/
As explained by Guido: https://github.com/python/cpython/issues/66435#issuecomment-...
I think the downsides of option 2 are overstated here. In lots of cases you don't care about the "value of async", you just want code that works well enough and option 2 does accomplish that in anything that is not perf critical.
Like the & at the end of a shell command?
We need algebraic effects in more languages, this solves the function coloring problem. OCaml 5 has them and it seems to be doing quite well, combine that with the semantics of the borrow checker in the form of OxCaml and we might just have an ideal language. I'd like to see algebraic effects in Rust as well but sadly it seems their keyword generics initiative is languishing.
Related, one of the former React maintainers wrote a primer on algebraic effects that's a good read: https://overreacted.io/algebraic-effects-for-the-rest-of-us/
I don't think effects alone solve function coloring problem, in worst case they make it worse because every library can have its own colors
Colored functions are good. It reflects the language design on signaling what is important, and what are the properties it want the writer to pay attention. Other examples of colored functions:
* Haskell: pure function and non-pure (IO monads) looks different. * Rust: unsafe functions (or block) requires special markers.
My mind went to Java's checked exceptions -- not sure if anyone today believes that coloring is still a good idea.
The problem with checked exceptions afaik was far more in the execution than in the idea itself. And also late 90s-early 00s was different time in general.
> Spidermouth the Night Clown
Thanks for my next horror shortfilm plot. Twist: he's the protagonist
My first ever EM showed me this piece ~10 years ago, and I still think about it a lot. One pattern I've adopted is to keep as much code to be synchronous as possible. On larger teams, especially when the slop-cannon is really going, I can at least depend on codeowners to tag me if someone tries to convert something to async (eg. adding a DB call somewhere), because they chain of things that need to be converted to async is so long. Then I can jump in and say "this entire chain of code is sync, if you want a DB call, do it somewhere else"
For Python backends I've seen good success with just making it company policy that everything is synchronous (normal-colored) and bypassing the developer overhead from async/await. Cooperative multitasking is a pain because, well, it requires cooperation. You can go pretty far by just adding more threads, processes, and replicas before it's worth the overhead.
You not only leave performance on the table (which depending on your use case/environment, may not matter if you can just throw more threads at it) but also some developer ergonomics.
asyncio.gather is a lot less code than having to manage a thread pool or something like Celery with all it's underlying infrastructure.
If you're in an ecosystem where a lot of the async boilerplate is free/cheap (ex: FastAPI) then the developer overhead of sprinkling awaits on your I/O bound calls is pretty low IMO.
> something like Celery with all it's underlying infrastructure.
Unpopular opinion, but combining this with the other "no thanks" sentiments in this subthread is the right answer. Your app is so complicated you need async? Then it's complicated enough that you can benefit from infrastructure. I don't want to watch coworkers try to badly rebuild message queue or scheduling semantics in an application code base. Just use infrastructure that's made by people who know what they are doing. That was problematic in 2015, but in 2026 it's a bit of docker, and it's not just about web/microservices. Very easy for sufficiently complex apps to simply leverage a local sandbox of celery, redis, graphdb's and whatever. Stand-alone is overrated since we don't have to do it anymore.. app devs should get more comfortable working with ensembles like this so they have access to best-in-class solutions.
You don't like infrastructure AND have such a need for performance AND don't want threads or multiprocess? Consider using another language. Async is mostly a solution in search of a problem, and the enduring popularity of TFA goes to show this has been the right conclusion for ~10 years.
I said nothing about "stand-alone" services. I'm all about using the right solution to the problem. We run on Kubernetes and have access to message queues. But if all I want to do is make a couple of HTTP calls concurrently, I don't think I should have to manage a thread/process pool to do so, or lean on a message queue or redis based RPC mechanism. In an async context I can do this with a single line of code.
Every rich client-side experience in your browser is written using async code in Javascript or Typescript, as is every electron app. Every developer at my company is comfortable with this pattern, and frameworks like FastAPI make this a similarly smooth experience when using Python.
If async was a solution in search of a problem, it wouldn't have been stolen from C# and added to Rust, Python, Kotlin, etc. The engineering effort required to bring this solution to all these languages is immense, so I'm clearly not the only person seeing value in it.
Performance aside (which I would argue is premature optimization, as most programs will not feel the theoretical overhead of threads), async is a bad approach for developer ergonomics. Threads are so much easier to work with and reason about than async. There are reasons to use async (like if you're in the rare case when thread overhead is noticeable), but developer ergonomics are absolutely not a reason.
Say I need the results from two expensive REST API calls, so I want to run them concurrently. Managing a thread pool you find a _better_ experience than
one, two = await asyncio.gather(callOne(), callTwo())
?
I just do not want to do async in Python. If you need async its questionable whether Python is a good choice at all, and if you use Python maybe look at another solution if at all possible (even using more processes and throwing hardware at it).
Purple.