How can I make interface functions to suspend function for e

Question

How can I make interface functions to suspend function for example ```interface BaseClass fun foo fun bar ``` ```class ChildClass BaseClass override fun foo override fun bar ``` My expectation is ```class ChildClass BaseClass override suspend fun foo override suspend fun bar ``` Is that possible

hfhbd · Answer

```interface BaseClass suspend fun foo suspend fun bar ```

chanjungskim · Answer

then every child class will override suspend fun funtions

hfhbd · Answer

Yes You can t override a non suspend function with a suspend function and vice versa

CLOVIS · Answer

You cannot call `suspend` functions from regular functions Imagine if you could do what you wanted ```fun main non suspend main val a BaseClass = ChildClass we re calling the a function of the base class which is not suspend that s OK a foo ``` What would happen It can t call the overriden function because it s `suspend` and we re in a function where `suspend` is not allowed

Joffrey · Answer

To put it more plainly a `suspend` function technically has a different signature in bytecode so it s not a valid override for a non suspend function

Joffrey · Answer

gt then every child class will override suspend fun funtions < chanjungskim> What is the problem with this

CLOVIS · Answer

< Joffrey> that s true but it makes it seem like it s a technical limitation of coroutines whereas it s on purpose because it wouldn t make sense even if it was possible to make it compatible in bytecode

Joffrey · Answer

Right It was not my intention to convey that idea so you re right that it might not be the best angle to explain this sweat smile

Joffrey · Answer

A more correct explanation is that a `suspend` function has a different contract from a regular function

CLOVIS · Answer

more generally this concept of contract is called code coloring if anyone is interested by the broader theory

krzysztof · Answer

For anyone interested <https journal stuffwithstuff com 2015 02 01 what color is your function >

chanjungskim · Answer

Well still I don t understand why it s not possible I am actually implementing logger with Slack webhook and Datadog I wanted to create a common parent class for making them interchangeable Datadog uses async call under the hood But my custom webhook doesn t using suspend function has no problem with calling normal function like Datadog but that can be confusing if Datadog use supend function because it doesn t need suspend actually

Casey Brooks · Answer

It s not usually confusing to override a `suspend` function and then never suspend within it any more than overriding a function and ignoring parameters passed to it that you don t need But it communicates that the code is intended to run async may be running on background threads rather than the main thread and helps with handling errors thrown within it So even if you re not strictly calling any `suspend` functions within the Datadog implementation there is still usefulness to having the method marked suspend

CLOVIS · Answer

`suspend` functions are colored differently than regular functions they can never be compatible You want to create a common ancestor but synchronous and asynchronous are very different things

uli · Answer

< Casey Brooks> even though it is fine for a suspend function not to suspend it is not ok to block in a suspend function The implicit contract of a suspend function is to not block the current thread but to suspend and potentially schedule to another thread for blocking

Joffrey · Answer

< uli> true but that is not very confusing for implementers of the interface with `suspend` methods Technically they are aware of the `suspend` keyword and should implement the function accordingly at least with some `withContext ` to dispatch to another thread