Why do I have to use `coroutineScope ` only inside another c

Question

Why do I have to use `coroutineScope ` only inside another coroutine or a suspending function What s wrong with `fun main = coroutineScope lt do the majic gt ` Do I need `runBlocking ` there What s the meaning of `runBlocking` with scoped coroutines if scopes wait for their children

Vsevolod Tolstopyatov [JB] · Answer

and should have the answer

voddan · Answer

< Vsevolod Tolstopyatov JB > I have read those That s where the question have came from

voddan · Answer

Could you elaborate on the why of this API

Vsevolod Tolstopyatov [JB] · Answer

gt those sections do not explain the why do they gt The main difference between runBlocking and coroutineScope is that the latter does not block the current thread while waiting for all children to complete You can think of `coroutineScope` as about syntactic sugar for ``` val job = launch coroutineContext parent = Job Here is the body of currentScope job join job throwExceptionIfPresent ```

Vsevolod Tolstopyatov [JB] · Answer

`coroutineScope` is more or less suspending version of `runBlocking` which provides dispatcher from current `coroutineContext` for all children

voddan · Answer

gt `coroutineScope` is more or less suspending version of `runBlocking` In other words `runBlocking` is like `coroutineScoped` with pre selected dispatcher that always blocks correct

Vsevolod Tolstopyatov [JB] · Answer

They have one key difference `runBlocking` blocks until given coroutine completes `coroutineScope` suspends until given coroutine and all children of the scope complete no matter in what dispatcher

Vsevolod Tolstopyatov [JB] · Answer

I think we can improve documentation and or guide about this distinction

voddan · Answer

Ok I see the technical difference now May I ask then isn t suspending is a synonym to blocking in the coroutine world

voddan · Answer

I suggest we rename `runBlocking` into `coroutineScope` without a receiver

voddan · Answer

Should I create an issue

Vsevolod Tolstopyatov [JB] · Answer

I don t think it s a good idea 1 `coroutineScope` will not compile because of resolution ambiguity 2 We prefer to explicitly reflect the fact that builder blocks a thread 3 After this change it will be possible to accidentally misuse suspending and blocking fun 4 `runBlocking` still should provide a scope otherwise everyone will be forced to use `GlobalScope` where it s not really necessary

voddan · Answer

gt 2 We prefer to explicitly reflect the fact that builder blocks a thread Any coroutine scope stops execution flow How it maps to threads is an implementation detail desided by the dispatcher gt 3 After this change it will be possible to accidentally misuse suspending and blocking fun How so If I create a coroutine scope it will wait for its children gt 4 `runBlocking` still should provide a scope otherwise everyone will be forced to use `GlobalScope` where it s not really necessary Not sure I follow here GlobalScope should almost never be used I don t see how unification of scope builders will change that

voddan · Answer

Do other implementations like Trio distinguish when one creates a scope inside the main thread or another coroutine

Vsevolod Tolstopyatov [JB] · Answer

yes <https trio readthedocs io en latest reference core html entering trio>

Vsevolod Tolstopyatov [JB] · Answer

gt How it maps to threads is an implementation detail It s not and it has nothing to do with dispatcher Blocking and suspension are not synonyms E g in `blockingFoo String` and `asyncFoo Future lt String gt ` whether function is async or not is not an implementation detail Same for blocking vs suspending the only difference is that compiler hides a complexity behind callbacks gt How so Because they have the same name slightly smiling face gt Not sure I follow here If `runBlocking` lambda won t have a receiver then it would be impossible to write `runBlocking launch ` because now scope is missing

Dico · Answer

Good question and a good answer I would say Good to know

voddan · Answer

< Vsevolod Tolstopyatov JB > thanks for the comments I ll think about them over night slightly smiling face