Not sure if this is the best channel to ask, but d...

# compiler

[Waldemar Kornewald]

Not sure if this is the best channel to ask, but does anyone know some compiler plugin which can turn a class with suspend (or non-suspend) functions and also a whole module or individual functions into non-suspend (or suspend) versions, so one doesn’t have to copy-paste lots of code and keep the two versions in sync by hand? The function coloring problem keeps hitting us every now and then and inline can’t always be used to work around it.

[efemoney]

What function coloring problem and how does inline work around it?? Can you post a code example?

[Waldemar Kornewald]

With the “function coloring problem” I was referring to the post by Bob Nystrom which received some attention: https://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/
Basically, the core issue is the infectiousness of suspend (or async), i.e. it infects the caller which now also has to use suspend or at least launch. This means your code gets split into two incompatible worlds and you need to duplicate code under different names to make it usable for both worlds. Infectiousness of any kind is always bad (there are also architectural decisions which lead to infectious patterns).

For instance, I’ve written a parser library, but it’s not using suspend because it needs to be usable without coroutines. Now I need the same API for streaming parsing and the most natural solution would be to use suspend . I don’t want to duplicate all that code because this risks introducing too many bugs/inconsistencies during maintenance. It’s a mostly mechanical transformation that the compiler could do for me. Or at least a compiler plugin could help.

With inline you can partially work around this:

inline fun foo(block: () -> Unit) { ... }

Although you didn’t declare foo and block as suspend the inline makes them compatible. So you can call foo { synchronousCall() } but also foo { suspendCall() } . The compiler will do the right thing and turn foo into a suspend function only when needed while in all other cases it’s a synchronous non-suspend function. Ideally, the compiler would be able to do this with every function and we could get rid of suspend completely.

[efemoney]

This means your code gets split into two incompatible worlds

Your code is already in two incompatible worlds.

suspend fun means this function can “suspend” execution of its caller while it does its work. It comes with many concepts but that basic one is the building block for ALL coroutines.
normal fun cannot and will not have the same behavior. You cannot model what suspend fun model using normal functions.

For instance, I’ve written a parser library, but it’s not using suspend because it needs to be usable without coroutines. Now I need the same API for streaming parsing and the most natural solution would be to use suspend

Which brings me to this part. You’ve written a blocking parser library. The natural solution is actually not to duplicate the API & implement streaming in coroutines but to implement streaming using blocking code just like you have the base API. You cannot have non-blocking (suspendable) code with an API that is blocking. Normal functions just cannot model suspendable computations.

With inline you can partially work around this:

This is a complete misconstruing of what inline does. It’s simple and should not be taken to mean anything other than what it does which is inline its function body at the call site! Basically the body of the function runs within the context of the caller. If that is a suspendable context then the body of the inlined lambda can call suspend functions. If the context is not suspendable then you cannot call suspend functions. It is not a way to workaround anything neither does it make two incompatible things compatible.

The compiler will do the right thing and turn foo into a suspend function only when needed while in all other cases it’s a synchronous non-suspend function.

No. The compiler does no such thing. See screen shot, if foo “_*became suspendable*_” via compiler magic, it will have a suspend icon in the gutter beside line number 6 but it doesnt because it is neither a normal function nor a suspend function. Its an inline function with defined semantics.

[Waldemar Kornewald]

Your code is already in two incompatible worlds.

suspend fun means this function can “suspend” execution of its caller while it does its work. It comes with many concepts but that basic one is the building block for ALL coroutines.

normal fun cannot and will not have the same behavior. You cannot model what suspend fun model using normal functions.

That’s not true. This is just a limitation of the language. The compiler could solve this automatically, but it would be more effort and maybe slower to analyze. At the very least you can solve it like in Java’s Project Loom or Go. There you don’t have this two worlds problem.

The two worlds problem is not a fundamental limitation of asynchronous programming. It’s only a limitation of the Kotlin approach.

Which brings me to this part. You’ve written a blocking parser library. The natural solution is actually not to duplicate the API & implement streaming in coroutines but to implement streaming using blocking code just like you have the base API. You cannot have non-blocking (suspendable) code with an API that is blocking. Normal functions just cannot model suspendable computations

I want to have a very natural API for the parser which is on par with PEGs, but is just normal Kotlin code. The blocking version already achieves this goal. By putting suspend everywhere I can keep the natural API and make it non-blocking. What’s your alternative suggestion? I don’t want to have RxJava-style operator chains like the other parser libraries because then the code becomes ugly and limited.

This is a complete misconstruing of what inline does. It’s simple and should not be taken to mean anything other than what it does which is inline its function body at the call site! Basically the body of the function runs within the context of the caller. If that is a suspendable context then the body of the inlined lambda can call suspend functions. If the context is not suspendable then you cannot call suspend functions. It is not a way to workaround anything neither does it make two incompatible things compatible.

I know what inline really means. However, even in the stdlib Kotlin uses inline in a lot of places which look a lot like they were chosen on purpose to be compatible with both suspend and non-suspend. And it’s the only real workaround for making functions work in both worlds. That’s why I mentioned inline.

No. The compiler does no such thing. See screen shot, if foo “_*became suspendable*_” via compiler magic, it will have a suspend icon in the gutter beside line number 6 but it doesnt because it is neither a normal function nor a suspend function. Its an inline function with defined semantics.

The compiler simply inlines the block. The function foo doesn’t really exist anymore after inlining, so from a pedantic point of view foo doesn’t become suspendable because there is no foo at all. However, the point is that you can use foo with sus even though foo is not marked as suspend. So from a practical point of view, foo is really compatible with two worlds only because it uses inline. This is exactly what I said right from the start.