Asynchronous Programming Primitives

This module provides a layered approach to asynchronous programming, combining monadic types, type classes, and concrete task types that work together in a cohesive system.

• Monadic Types: These types provide a good way to to chain and manipulate context. These can contain a Task, enabling manipulation of both asynchronous and synchronous code.
• Concrete Task Types: Concrete units of work that can be executed within these contexts.

Monadic Types

These types provide a good way to to chain and manipulate context. These can contain a Task, enabling manipulation of both asynchronous and synchronous code.

• BaseAsync: A monadic type for infallible asynchronous computations
• EAsync: A monadic type for asynchronous computations that may fail with an error of type ε
• Async: A monadic type for IO-based asynchronous computations that may fail with IO.Error (alias for EAsync IO.Error)

Concurrent Units of Work

These are the concrete computational units that exist within the monadic contexts. These types should not be created directly.

• Task: A computation that will resolve to a value of type α,
• ETask: A task that may fail with an error of type ε.
• AsyncTask: A task that may fail with an IO.Error (alias for ETask IO.Error).

Relation

These types are related by two functions in the type classes MonadAsync and MonadAwait: async and await. The async function extracts a concrete asynchronous task from a computation within the monadic context. In effect, it runs the computation in the background and returns a task handle that can be awaited later. On the other hand, the await function takes a task and re-inserts it into the monadic context, allowing its result to be composed using monadic bind and also pausing to wait for that result. This relationship between async and await enables precise control over when a computation begins and when its result is used. You can spawn multiple asynchronous tasks using async, perform other operations in the meantime, and later rejoin the computation flow by awaiting their results.

These functions should not be used directly. Instead, prefer higher-level combinators such as race, raceAll, concurrently, background and concurrentlyAll. The best way to think about how to write your async code, it to avoid using the concurrent units of work, and only use it when integrating with non async code that uses them.

class Std.Internal.IO.Async.MonadAwait (t m : Type → Type) extends Monad m : Type 1

Typeclass for monads that can "await" a computation of type t α in a monad m until the result is available.

• map {α β : Type} : (α → β) → m α → m β
• mapConst {α β : Type} : α → m β → m α
• pure {α : Type} : α → m α
• seq {α β : Type} : m (α → β) → (Unit → m α) → m β
• seqLeft {α β : Type} : m α → (Unit → m β) → m α
• seqRight {α β : Type} : m α → (Unit → m β) → m β
• bind {α β : Type} : m α → (α → m β) → m β
• await {α : Type} : t α → m α

  Awaits the result of t α and returns it inside the m monad.

class Std.Internal.IO.Async.MonadAsync (t m : Type → Type) extends Monad m : Type 1

Represents monads that can launch computations asynchronously of type t in a monad m.

• map {α β : Type} : (α → β) → m α → m β
• mapConst {α β : Type} : α → m β → m α
• pure {α : Type} : α → m α
• seq {α β : Type} : m (α → β) → (Unit → m α) → m β
• seqLeft {α β : Type} : m α → (Unit → m β) → m α
• seqRight {α β : Type} : m α → (Unit → m β) → m β
• bind {α β : Type} : m α → (α → m β) → m β
• async {α : Type} (x : m α) (prio : Task.Priority := Task.Priority.default) : m (t α)

  Starts an asynchronous computation in another monad.

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAwaitStateTOfMonad {m t : Type → Type} {n : Type} [Monad m] [MonadAwait t m] : MonadAwait t (StateT n m)

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAwaitExceptTOfMonad {m t : Type → Type} {n : Type} [Monad m] [MonadAwait t m] : MonadAwait t (ExceptT n m)

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAwaitReaderTOfMonad {m t : Type → Type} {n : Type} [Monad m] [MonadAwait t m] : MonadAwait t (ReaderT n m)

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAwaitStateRefT' {t m : Type → Type} {s n : Type} [MonadAwait t m] : MonadAwait t (StateRefT' s n m)

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAwaitStateT {t m : Type → Type} {s : Type} [MonadAwait t m] : MonadAwait t (StateT s m)

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAsyncReaderT {t m : Type → Type} {n : Type} [MonadAsync t m] : MonadAsync t (ReaderT n m)

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAsyncStateRefT' {t m : Type → Type} {s n : Type} [MonadAsync t m] : MonadAsync t (StateRefT' s n m)

@[defaultInstance 1000]

instance Std.Internal.IO.Async.instMonadAsyncStateTOfFunctor {t m : Type → Type} {s : Type} [Functor t] [inst : MonadAsync t m] : MonadAsync t (StateT s m)

@[reducible, inline]

abbrev Std.Internal.IO.Async.ETask (ε α : Type) : Type

A Task that may resolve to either a value of type α or an error value of type ε.

@[inline]

def Std.Internal.IO.Async.ETask.pure {α ε : Type} (x : α) : ETask ε α

Construct an ETask that is already resolved with value x.

@[inline]

def Std.Internal.IO.Async.ETask.map {α β ε : Type} (f : α → β) (x : ETask ε α) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : ETask ε β

Creates a new ETask that will run after x has finished. If x:

• errors, return an ETask that resolves to the error.
• succeeds, return an ETask that resolves to f x.

@[inline]

def Std.Internal.IO.Async.ETask.bind {ε α β : Type} (x : ETask ε α) (f : α → ETask ε β) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : ETask ε β

Creates a new ETask that will run after x has completed. If x:

• errors, return an ETask that resolves to the error.
• succeeds, run f on the result of x and return the ETask produced by f.

@[inline]

def Std.Internal.IO.Async.ETask.bindEIO {ε α β : Type} (x : ETask ε α) (f : α → EIO ε (ETask ε β)) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : EIO ε (ETask ε β)

Similar to bind, however f has access to the EIO monad. If f throws an error, the returned ETask resolves to that error.

@[inline]

def Std.Internal.IO.Async.ETask.mapEIO {α ε β : Type} (f : α → EIO ε β) (x : ETask ε α) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : BaseIO (ETask ε β)

Similar to bind, however f has access to the EIO monad. If f throws an error, the returned ETask resolves to that error.

@[inline]

def Std.Internal.IO.Async.ETask.block {ε α : Type} (x : ETask ε α) : EIO ε α

Block until the ETask in x finishes and returns its value. Propagates any error encountered during execution.

@[inline]

def Std.Internal.IO.Async.ETask.ofPromise {ε α : Type} (x : IO.Promise (Except ε α)) : ETask ε α

Create an ETask that resolves to the value of the promise x.

@[inline]

def Std.Internal.IO.Async.ETask.ofPurePromise {α ε : Type} (x : IO.Promise α) : ETask ε α

Create an ETask that resolves to the pure value of the promise x.

@[inline]

def Std.Internal.IO.Async.ETask.getState {ε α : Type} (x : ETask ε α) : BaseIO IO.TaskState

Obtain the IO.TaskState of x.

instance Std.Internal.IO.Async.ETask.instFunctor {ε : Type} : Functor (ETask ε)

instance Std.Internal.IO.Async.ETask.instMonad {ε : Type} : Monad (ETask ε)

@[reducible, inline]

abbrev Std.Internal.IO.Async.AsyncTask (α : Type) : Type

A Task that may resolve to a value or an error value of type IO.Error. Alias for ETask IO.Error.

@[inline]

def Std.Internal.IO.Async.AsyncTask.mapIO {α β : Type} (f : α → IO β) (x : AsyncTask α) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : BaseIO (AsyncTask β)

Similar to map, however f has access to the IO monad. If f throws an error, the returned AsyncTask resolves to that error.

@[inline]

def Std.Internal.IO.Async.AsyncTask.pure {α : Type} (x : α) : AsyncTask α

Construct an AsyncTask that is already resolved with value x.

@[inline]

def Std.Internal.IO.Async.AsyncTask.bind {α β : Type} (x : AsyncTask α) (f : α → AsyncTask β) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : AsyncTask β

Create a new AsyncTask that will run after x has finished. If x:

• errors, return an AsyncTask that resolves to the error.
• succeeds, run f on the result of x and return the AsyncTask produced by f.

@[inline]

def Std.Internal.IO.Async.AsyncTask.map {α β : Type} (f : α → β) (x : AsyncTask α) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : AsyncTask β

Create a new AsyncTask that will run after x has finished. If x:

• errors, return an AsyncTask that resolves to the error.
• succeeds, return an AsyncTask that resolves to f x.

@[inline]

def Std.Internal.IO.Async.AsyncTask.bindIO {α β : Type} (x : AsyncTask α) (f : α → IO (AsyncTask β)) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : BaseIO (AsyncTask β)

Similar to bind, however f has access to the IO monad. If f throws an error, the returned AsyncTask resolves to that error.

@[inline]

def Std.Internal.IO.Async.AsyncTask.mapTaskIO {α β : Type} (f : α → IO β) (x : AsyncTask α) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : BaseIO (AsyncTask β)

Similar to map, however f has access to the IO monad. If f throws an error, the returned AsyncTask resolves to that error.

def Std.Internal.IO.Async.AsyncTask.block {α : Type} (x : AsyncTask α) : IO α

Block until the AsyncTask in x finishes.

@[inline]

def Std.Internal.IO.Async.AsyncTask.ofPromise {α : Type} (x : IO.Promise (Except IO.Error α)) : AsyncTask α

Create an AsyncTask that resolves to the value of x.

@[inline]

def Std.Internal.IO.Async.AsyncTask.ofPurePromise {α : Type} (x : IO.Promise α) : AsyncTask α

Create an AsyncTask that resolves to the value of x.

@[inline]

def Std.Internal.IO.Async.AsyncTask.getState {α : Type} (x : AsyncTask α) : BaseIO IO.TaskState

Obtain the IO.TaskState of x.

inductive Std.Internal.IO.Async.MaybeTask (α : Type) : Type

A MaybeTask α represents a computation that either:

• Is immediately available as an α value, or
• Is an asynchronous computation that will eventually produce an α value.

• pure {α : Type} : α → MaybeTask α
• ofTask {α : Type} : Task α → MaybeTask α

@[inline]

def Std.Internal.IO.Async.MaybeTask.toTask {α : Type} : MaybeTask α → Task α

Constructs an Task from a MaybeTask.

@[inline]

def Std.Internal.IO.Async.MaybeTask.get {α : Type} : MaybeTask α → α

Gets the value of the MaybeTask by blocking.

@[inline]

def Std.Internal.IO.Async.MaybeTask.map {α β : Type} (f : α → β) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : MaybeTask α → MaybeTask β

Maps a function over a MaybeTask.

@[inline]

def Std.Internal.IO.Async.MaybeTask.bind {α β : Type} (t : MaybeTask α) (f : α → MaybeTask β) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : MaybeTask β

Sequences two computations, allowing the second to depend on the value computed by the first.

@[inline]

def Std.Internal.IO.Async.MaybeTask.joinTask {α : Type} (t : Task (MaybeTask α)) : Task α

Join the MaybeTask to an Task.

instance Std.Internal.IO.Async.MaybeTask.instFunctor : Functor MaybeTask

instance Std.Internal.IO.Async.MaybeTask.instMonad : Monad MaybeTask

def Std.Internal.IO.Async.BaseAsync (α : Type) : Type

An asynchronous computation that never fails.

@[inline]

def Std.Internal.IO.Async.BaseAsync.mk {α : Type} (x : BaseIO (MaybeTask α)) : BaseAsync α

Converts a BaseIO into a BaseAsync

@[inline]

def Std.Internal.IO.Async.BaseAsync.toRawBaseIO {α : Type} (x : BaseAsync α) : BaseIO (MaybeTask α)

Converts a BaseAsync into a BaseIO

@[inline]

def Std.Internal.IO.Async.BaseAsync.toBaseIO {α : Type} (x : BaseAsync α) : BaseIO (Task α)

Converts a BaseAsync to a BaseIO Task.

@[inline]

def Std.Internal.IO.Async.BaseAsync.ofTask {α : Type} (x : Task α) : BaseAsync α

Creates a new BaseAsync out of a Task.

@[inline]

def Std.Internal.IO.Async.BaseAsync.pure {α : Type} (a : α) : BaseAsync α

Creates a BaseAsync computation that immediately returns the given value.

@[inline]

def Std.Internal.IO.Async.BaseAsync.map {α β : Type} (f : α → β) (self : BaseAsync α) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : BaseAsync β

Maps the result of a BaseAsync computation with a function.

@[inline]

def Std.Internal.IO.Async.BaseAsync.bind {α β : Type} (self : BaseAsync α) (f : α → BaseAsync β) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : BaseAsync β

Sequences two computations, allowing the second to depend on the value computed by the first.

def Std.Internal.IO.Async.BaseAsync.bind.bindAsyncTask {α β : Type} (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) (t : MaybeTask α) (f : α → BaseAsync β) : BaseAsync β

@[inline]

def Std.Internal.IO.Async.BaseAsync.lift {α : Type} (x : BaseIO α) : BaseAsync α

Lifts a BaseIO action into a BaseAsync computation.

@[inline]

def Std.Internal.IO.Async.BaseAsync.wait {α : Type} (self : BaseAsync α) : BaseIO α

Waits for the result of the BaseAsync computation, blocking if necessary.

@[inline]

def Std.Internal.IO.Async.BaseAsync.asTask {α : Type} (x : BaseAsync α) (prio : Task.Priority := Task.Priority.default) : BaseIO (Task α)

Lifts a BaseAsync computation into a Task that can be awaited and joined.

@[inline]

def Std.Internal.IO.Async.BaseAsync.await {α : Type} (t : Task α) : BaseAsync α

Creates a BaseAsync that awaits the completion of the given Task α.

@[inline]

def Std.Internal.IO.Async.BaseAsync.async {α : Type} (self : BaseAsync α) (prio : Task.Priority := Task.Priority.default) : BaseAsync (Task α)

Returns the BaseAsync computation inside a Task α, so it can be awaited.

instance Std.Internal.IO.Async.BaseAsync.instFunctor : Functor BaseAsync

instance Std.Internal.IO.Async.BaseAsync.instMonad : Monad BaseAsync

instance Std.Internal.IO.Async.BaseAsync.instMonadLiftBaseIO : MonadLift BaseIO BaseAsync

instance Std.Internal.IO.Async.BaseAsync.instMonadAwaitTask : MonadAwait Task BaseAsync

instance Std.Internal.IO.Async.BaseAsync.instMonadAsyncTask : MonadAsync Task BaseAsync

instance Std.Internal.IO.Async.BaseAsync.instInhabited {α : Type} [Inhabited α] : Inhabited (BaseAsync α)

def Std.Internal.IO.Async.EAsync (ε α : Type) : Type

An asynchronous computation that may produce an error of type ε.

@[inline]

def Std.Internal.IO.Async.EAsync.toBaseIO {ε α : Type} (x : EAsync ε α) : BaseIO (ETask ε α)

Converts a EAsync to a ETask.

@[inline]

def Std.Internal.IO.Async.EAsync.ofTask {ε α : Type} (x : ETask ε α) : EAsync ε α

Creates a new EAsync out of a RTask.

@[inline]

def Std.Internal.IO.Async.EAsync.toEIO {ε α : Type} (x : EAsync ε α) : EIO ε (ETask ε α)

Converts a BaseAsync to a EIO ETask.

@[inline]

def Std.Internal.IO.Async.EAsync.ofETask {ε α : Type} (x : ETask ε α) : EAsync ε α

Creates a new EAsync out of a ETask.

@[inline]

def Std.Internal.IO.Async.EAsync.pure {α ε : Type} (a : α) : EAsync ε α

Creates an EAsync computation that immediately returns the given value.

@[inline]

def Std.Internal.IO.Async.EAsync.map {α β ε : Type} (f : α → β) (self : EAsync ε α) : EAsync ε β

Maps the result of an EAsync computation with a pure function.

@[inline]

def Std.Internal.IO.Async.EAsync.bind {ε α β : Type} (self : EAsync ε α) (f : α → EAsync ε β) : EAsync ε β

Sequences two computations, allowing the second to depend on the value computed by the first.

@[inline]

def Std.Internal.IO.Async.EAsync.lift {ε α : Type} (x : EIO ε α) : EAsync ε α

Lifts an EIO action into an EAsync computation.

@[inline]

def Std.Internal.IO.Async.EAsync.wait {ε α : Type} (self : EAsync ε α) : EIO ε α

Waits for the result of the EAsync computation, blocking if necessary.

@[inline]

def Std.Internal.IO.Async.EAsync.asTask {ε α : Type} (x : EAsync ε α) (prio : Task.Priority := Task.Priority.default) : EIO ε (ETask ε α)

Lifts an EAsync computation into an ETask that can be awaited and joined.

@[inline]

def Std.Internal.IO.Async.EAsync.throw {ε α : Type} (e : ε) : EAsync ε α

Raises an error of type ε within the EAsync monad.

@[inline]

def Std.Internal.IO.Async.EAsync.tryCatch {ε α : Type} (x : EAsync ε α) (f : ε → EAsync ε α) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : EAsync ε α

Handles errors in an EAsync computation by running a handler if one occurs.

def Std.Internal.IO.Async.EAsync.tryFinally' {ε α β : Type} (x : EAsync ε α) (f : Option α → EAsync ε β) (prio : Task.Priority := Task.Priority.default) (sync : Bool := false) : EAsync ε (α × β)

Runs an action, ensuring that some other action always happens afterward.

@[inline]

def Std.Internal.IO.Async.EAsync.await {ε α : Type} (x : ETask ε α) : EAsync ε α

Creates an EAsync computation that awaits the completion of the given ETask ε α.

@[inline]

def Std.Internal.IO.Async.EAsync.async {ε α : Type} (self : EAsync ε α) (prio : Task.Priority := Task.Priority.default) : EAsync ε (ETask ε α)

Returns the EAsync computation inside an ETask ε α, so it can be awaited.

instance Std.Internal.IO.Async.EAsync.instFunctor {ε : Type} : Functor (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonad {ε : Type} : Monad (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadLiftEIO {ε : Type} : MonadLift (EIO ε) (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadExcept {ε : Type} : MonadExcept ε (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadExceptOf {ε : Type} : MonadExceptOf ε (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadFinally {ε : Type} : MonadFinally (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instOrElse {ε α : Type} : OrElse (EAsync ε α)

instance Std.Internal.IO.Async.EAsync.instInhabited {ε α : Type} [Inhabited ε] : Inhabited (EAsync ε α)

instance Std.Internal.IO.Async.EAsync.instMonadAwaitETask {ε : Type} : MonadAwait (ETask ε) (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadAwaitTask {ε : Type} : MonadAwait Task (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadAwaitAsyncTaskError : MonadAwait AsyncTask (EAsync IO.Error)

instance Std.Internal.IO.Async.EAsync.instMonadAwaitPromise {ε : Type} : MonadAwait IO.Promise (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadAsyncETask {ε : Type} : MonadAsync (ETask ε) (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadAsyncAsyncTaskError : MonadAsync AsyncTask (EAsync IO.Error)

instance Std.Internal.IO.Async.EAsync.instMonadLiftBaseIO {ε : Type} : MonadLift BaseIO (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadLiftEIO_1 {ε : Type} : MonadLift (EIO ε) (EAsync ε)

instance Std.Internal.IO.Async.EAsync.instMonadLiftBaseAsync {ε : Type} : MonadLift BaseAsync (EAsync ε)

@[inline]

def Std.Internal.IO.Async.EAsync.forIn {ε β : Type} [i : Inhabited ε] (init : β) (f : Unit → β → EAsync ε (ForInStep β)) (prio : Task.Priority := Task.Priority.default) : EAsync ε β

@[specialize #[]]

partial def Std.Internal.IO.Async.EAsync.forIn.loop {ε β : Type} (f : Unit → β → EAsync ε (ForInStep β)) (prio : Task.Priority := Task.Priority.default) (promise : IO.Promise (Except ε β)) (b : β) : EAsync ε (ETask ε Unit)

instance Std.Internal.IO.Async.EAsync.instForInLoopUnitOfInhabited {ε : Type} [Inhabited ε] : ForIn (EAsync ε) Lean.Loop Unit

@[reducible, inline]

abbrev Std.Internal.IO.Async.Async (α : Type) : Type

An asynchronous computation that may produce an error of type IO.Error..

@[inline]

def Std.Internal.IO.Async.Async.toIO {α : Type} (x : Async α) : IO (AsyncTask α)

Converts a Async to a AsyncTask.

@[defaultInstance 1000]

instance Std.Internal.IO.Async.Async.instMonadAsyncAsyncTask : MonadAsync AsyncTask Async

instance Std.Internal.IO.Async.Async.instMonadAwaitAsyncTask : MonadAwait AsyncTask Async

instance Std.Internal.IO.Async.Async.instMonadAwaitPromise : MonadAwait IO.Promise Async

@[inline, specialize #[]]

def Std.Internal.IO.Async.background {m t : Type → Type} {α : Type} [Monad m] [MonadAsync t m] (prio : Task.Priority := Task.Priority.default) : m α → m Unit

This function transforms the operation inside the monad m into a task and let it run in the background.

@[inline, specialize #[]]

def Std.Internal.IO.Async.concurrently {m t : Type → Type} {α β : Type} [Monad m] [MonadAwait t m] [MonadAsync t m] (x : m α) (y : m β) (prio : Task.Priority := Task.Priority.default) : m (α × β)

Runs two computations concurrently and returns both results as a pair.

@[inline, specialize #[]]

def Std.Internal.IO.Async.race {m t : Type → Type} {α : Type} [MonadLiftT BaseIO m] [MonadAwait Task m] [MonadAsync t m] [MonadAwait t m] [Monad m] [Inhabited α] (x y : m α) (prio : Task.Priority := Task.Priority.default) : m α

Runs two computations concurrently and returns the result of the one that finishes first. The other result is lost and the other task is not cancelled, so the task will continue the execution until the end.

@[inline, specialize #[]]

def Std.Internal.IO.Async.concurrentlyAll {m t : Type → Type} {α : Type} [Monad m] [MonadAwait t m] [MonadAsync t m] (xs : Array (m α)) (prio : Task.Priority := Task.Priority.default) : m (Array α)

Runs all computations in an Array concurrently and returns all results as an array.

@[inline, specialize #[]]

def Std.Internal.IO.Async.raceAll {m : Type → Type} {c : Type u_1} {α : Type} {t : Type → Type} [ForM m c (m α)] [MonadLiftT BaseIO m] [MonadAwait Task m] [MonadAsync t m] [MonadAwait t m] [Monad m] [Inhabited α] (xs : c) (prio : Task.Priority := Task.Priority.default) : m α

Runs all computations concurrently and returns the result of the first one to finish. All other results are lost, and the tasks are not cancelled, so they'll continue their executing until the end.