public interface Continuation<in T> {
    public val context: CoroutineContext
    public fun resumeWith(result: Result<T>)
}

suspend fun executeNetworkRequest(): String = 
    suspendCoroutine { continuation ->
        thread {
            continuation.resumeWith(someBlockingRequest())
        }
    }

suspend fun executeNetworkRequest(): String =
    suspendCoroutine { continuation ->
        apiService.getSomething()
            // onSuccess & onFailure is a callback
            .onSuccess(continuation::resume)
            .onFailure(continuation::resumeWithException)
            .executeAsync()
    }

import kotlin.coroutines.*

fun main() {
    val operation: suspend () -> Unit = ::a
    operation.startCoroutine(
        object : Continuation<Unit> {
            // we will talk about it lower
            override val coroutineContext = EmptyCoroutineContext

            // called when coroutine finished its work
            override fun resumeWith(result: Result<Unit>) {
                println(
                    if(result.isSuccess) 
                        "Successfully done" 
                    else "Error happenned: ${result.exceptionOrNull()}"
                )
            }
        }
    )
}

suspend fun a() {...}

import kotlin.coroutines.*

// define our container for data we need inside coroutine
// it should always inherit 'CoroutineContext.Element'
data class ExecutionContext(val someInt: Int) : CoroutineContext.Element {
    override val key = EXECUTION_CONTEXT_KEY
}

// define type-safe key using which we will get our value
val EXECUTION_CONTEXT_KEY = object : CoroutineContext.Key<ExecutionContext> {}

// define coroutine context that we will pass into Continuation
private val myCoroutineContext = object : CoroutineContext {
    private val values = mapOf<CoroutineContext.Key<*>, CoroutineContext.Element>(
        EXECUTION_CONTEXT_KEY to ExecutionContext(10000)
    )

    override operator fun <E : CoroutineContext.Element> get(key: CoroutineContext.Key<E>): E? {
        return values[key] as? E
    }

    // .. we omit other functions for simplicity
}

suspend fun a() {
    // here we retrieve value from coroutine context
    // coroutineContext is compiler intrinsic, can be called
    // only from suspend world
    val executionContext = coroutineContext[EXECUTION_CONTEXT_KEY]!!
    println(executionContext.someInt!!)
}

fun main() {
    val operation: suspend () -> Unit = ::a
    operation.startCoroutine(
        object : Continuation<Unit> {
            override val context: CoroutineContext = myCoroutineContext

            override fun resumeWith(result: Result<Unit>) {
                println(
                    if(result.isSuccess) 
                        "Successfully done" 
                    else "Error happenned: ${result.exceptionOrNull()}"
                )
            }
        }
    )
}

// let's define an element that will persist in `CoroutineContext`
data class AuthorizationContext(
    val accessHash: String?,
    val provider: AuthorizationProvider,
) : CoroutineContext.Element {
    companion object Key : CoroutineContext.Key<AuthorizationContext>

    override val key: CoroutineContext.Key<*> = Key
}

// now we define our interceptor
class AuthorizationInterceptor(
    private val authorizationProvider: AuthorizationProvider,
) : CoroutineContextServerInterceptor() {
    companion object {
        private val ACCESS_TOKEN_METADATA_KEY: Metadata.Key<String> =
            Metadata.Key.of("access-token", Metadata.ASCII_STRING_MARSHALLER)
    }

    override fun coroutineContext(call: ServerCall<*, *>, headers: Metadata): CoroutineContext {
        return AuthorizationContext(
            accessHash = headers.get(ACCESS_TOKEN_METADATA_KEY),
            provider = authorizationProvider,
        )
    }
}

suspend inline fun provideAuthorization(block: (UserId) -> Unit) {
    val authContext = coroutineContext[AuthorizationContext]
    authContext.accessHash ?: throw StatusException(Status.UNAUTHORIZED)

    val userId = authContext.provider.provide(authContext.accessHash)
    return block(userId)
}

@RestrictSuspension
public abstract class SequenceScope<in T> internal constructor() {
    /**
     * Yields a value to the [Iterator] being built and suspends
     * until the next value is requested.
     */
    public abstract suspend fun yield(value: T)

    // ... other
}

private typealias State = Int  

private const val State_NotReady: State = 0  
private const val State_ManyNotReady: State = 1  
private const val State_ManyReady: State = 2  
private const val State_Ready: State = 3  
private const val State_Done: State = 4  
private const val State_Failed: State = 5  

private class SequenceBuilderIterator<T> : SequenceScope<T>(), Iterator<T>, Continuation<Unit> {  
    private var state = State_NotReady  
    private var nextValue: T? = null  
    private var nextIterator: Iterator<T>? = null  
    var nextStep: Continuation<Unit>? = null  

    override fun hasNext(): Boolean {  
        while (true) {  
            when (state) {  
                State_NotReady -> {}  
                State_ManyNotReady ->  
                if (nextIterator!!.hasNext()) {  
                    state = State_ManyReady  
                    return true  
                } else {  
                    nextIterator = null  
                }  
                State_Done -> return false  
                State_Ready, State_ManyReady -> return true  
                else -> throw exceptionalState()  
            }  

            state = State_Failed  
            val step = nextStep!!  
            nextStep = null  
            step.resume(Unit)  
        }  
    }  

    override fun next(): T {  
        when (state) {  
            State_NotReady, State_ManyNotReady -> return nextNotReady()  
            State_ManyReady -> {  
                state = State_ManyNotReady  
                return nextIterator!!.next()  
            }  
            State_Ready -> {  
                state = State_NotReady  
                @Suppress("UNCHECKED_CAST")  
                val result = nextValue as T  
                nextValue = null  
                return result  
            }  
            else -> throw exceptionalState()  
        }  
    }  

    private fun nextNotReady(): T {  
        if (!hasNext()) throw NoSuchElementException() else return next()  
    }  

    private fun exceptionalState(): Throwable = when (state) {  
        State_Done -> NoSuchElementException()  
        State_Failed -> IllegalStateException("Iterator has failed.")  
        else -> IllegalStateException("Unexpected state of the iterator: $state")  
    }  


    override suspend fun yield(value: T) {  
        nextValue = value  
        state = State_Ready  
        return suspendCoroutineUninterceptedOrReturn { c ->  
            nextStep = c  
            COROUTINE_SUSPENDED  
        }  
    }    
    override fun resumeWith(result: Result<Unit>) {  
        result.getOrThrow() // just rethrow exception if it is there  
        state = State_Done  
    }  

    override val context: CoroutineContext  
        get() = EmptyCoroutineContext  
}

   fun foo(callback: (Result<Foo>) -> Unit) {
       // pseudo-code
       executor.execute { result -> callback(result) }
   }

   suspend fun bar(): Result<Foo> {
       // same if was like that
       val result = executor.execute() // suspend point
       return result
   }

@RestrictsSuspension // <---
@JvmDefaultWithCompatibility
interface AwaitPointerEventScope : Density {
    // ...

    suspend fun awaitPointerEvent(
        pass: PointerEventPass = PointerEventPass.Main
    ): PointerEvent

    suspend fun <T> withTimeoutOrNull(
        timeMillis: Long,
        block: suspend AwaitPointerEventScope.() -> T
    ): T? = block()

    suspend fun <T> withTimeout(
        timeMillis: Long,
        block: suspend AwaitPointerEventScope.() -> T
    ): T = block()

    // ...
}

This is a restricted suspension scope. Code in this scope is
always called un-dispatched and may only suspend for calls 
to [awaitPointerEvent]. These functions resume synchronously and the caller may mutate the result
**before** the next await call to affect the next stage of the input processing pipeline.

val result = Warden
    .with(this)
    .requestPermission(Manifest.permission.CALL_PHONE)

when (result) {
    is PermissionState.Denied -> dialNumber(phoneNumber)
    PermissionState.Granted -> startCall(phoneNumber)
}