suspend

Learn suspendable functions with suspend fun.

suspend functions

A suspend function can suspend and resume its work. The definition suspend fun compute(): Int { delay(100); return 42 } computes a value but releases the thread while waiting.

Suspend functions can be called only from a coroutine or another suspend function. Calling them from a regular function is a compile error. This constraint ensures waiting always happens in a coroutine context. They can be called safely from runBlocking or launch.

The suspend keyword

The suspend keyword goes at the start of a function declaration. Example: suspend fun compute(): Int. The body can call other suspend functions like delay.

The return value of a suspend function is used like a normal function. runBlocking { println(compute()) } waits for compute to finish and prints the result. Waiting does not block the thread; during delay other work can proceed. This lets concurrent code look synchronous.

Exercise Define and call a suspend function

Add import kotlinx.coroutines.*. Define suspend fun compute(): Int { delay(100); return 42 }. Write fun main() = runBlocking { println(compute()) }.

Show example solution

import kotlinx.coroutines.*

suspend fun compute(): Int {
    delay(100)
    return 42
}

fun main() = runBlocking {
    println(compute())
}

Test yourself

Match the suspend concepts

suspend fun

Can pause
delay(100)

Wait
return 42

Value
runBlocking

Bridge
Where can a suspend function be called?
- From anywhere
- Only from main
- From a coroutine or suspend function
- Only from a class
Show answer

From a coroutine or suspend function — A suspend function is called from a coroutine or suspend context.

Complete the compute body

suspend fun compute(): Int {
    delay(100)
    return ____
}

Show answer

suspend fun compute(): Int {
    delay(100)
    return 42
}

Next lesson: runBlocking

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