Hi - I’m experimenting with `sequence {}` but gett...

# announcements

[Mark]

Hi - I’m experimenting with sequence {} but getting “Restricted suspending functions can only invoke member or extension suspending functions on their restricted coroutine scope”. I can’t seem to find any information about this. Any pointers please? https://pl.kotl.in/B1hRgy_2m Presumably I need to use a suspending extension function of CoroutineScope?

[gildor]

See @RestrictsSuspension annotation docs
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-restricts-suspension/index.html

[gildor]

because sequence is completely synchronous, you cannot use any arbitrary suspending functions there, only synchronous yield

[Mark]

Ok thanks. I was thinking/hoping it was just some kind of channel where the provider is lazily creating/sending items in the background and the receiver is picking up the items as it needs them

[gildor]

If it would work like that toList() would be lock the thread and it doesn’t sound as a good idea

[gildor]

instead you can use Channels

[Mark]

Yeah, the toList() was just playing around in case the compiler worked out that the sequence was never accessed

[gildor]

any operation on such sequence would be block thread, not only toList()

[Mark]

Okay, but a launch/async would deal with that

[gildor]

to provide fully asynchronous sequences, all sequence operators also should be suspending to avoid problem with blocking
And we have such API already it called Channels

[Mark]

Okay, so I guess sequence{} is only useful for when calculating the next item is reasonably fast (i.e. non IO-blocking)?

[gildor]

https://pl.kotl.in/S1xi_Ju2m

[gildor]

Yes, sequences are completely synchronous

[Mark]

Thanks for that. And with the default buffer, that would suspend so that items are only calculated when required?

[gildor]

by default produce uses Rendezvous channel

[Mark]

Ok, thanks Andrey, I’ll dive into that

[gildor]

and yeah, with Rendezvous channel send will suspend until someone consumes this value

[Mark]

It suspends before calculating the next value or after?

[gildor]

when you call send

[gildor]

so value should be calculated before this

[Mark]

Right, so it’s kind of got it there ready for the next consumption

[gildor]

channels are “hot”

[gildor]

There is a plan to provide also “cold” streams, which calculate values on request
https://github.com/Kotlin/kotlinx.coroutines/issues/254

[gildor]

actually this issue has many good explanations directly related to your original question