Capnp.Rpc.Promise

This module defines a Promise type, represents a value which is not yet available, and related utilities.

Documentation

data Promise a

A promise is a value that may not be ready yet.

Instances

Eq (Promise a)

Defined in Capnp.Rpc.Promise

Methods

(==) :: Promise a -> Promise a -> Bool

(/=) :: Promise a -> Promise a -> Bool

data Fulfiller a

A Fulfiller is used to fulfill a promise.

Creating promises

newPromise :: MonadSTM m => m (Promise a, Fulfiller a)

Create a new promise and an associated fulfiller.

newPromiseWithCallback :: MonadSTM m => (Either Exception a -> STM ()) -> m (Promise a, Fulfiller a)

Create a new promise which also excecutes an STM action when it is resolved.

newCallback :: MonadSTM m => (Either Exception a -> STM ()) -> m (Fulfiller a)

Like newPromiseWithCallback, but doesn't return the promise.

Fulfilling or breaking promises

fulfill :: MonadSTM m => Fulfiller a -> a -> m ()

Fulfill a promise by supplying the specified value. It is an error to call fulfill if the promise has already been fulfilled (or broken).

breakPromise :: MonadSTM m => Fulfiller a -> Exception -> m ()

Break a promise. When the user of the promise executes wait, the specified exception will be raised. It is an error to call breakPromise if the promise has already been fulfilled (or broken).

data ErrAlreadyResolved

An exception thrown if breakPromise or fulfill is called on an already-resolved fulfiller.

Constructors

ErrAlreadyResolved

Instances

Show ErrAlreadyResolved

Defined in Capnp.Rpc.Promise

Methods

showsPrec :: Int -> ErrAlreadyResolved -> ShowS

show :: ErrAlreadyResolved -> String

showList :: [ErrAlreadyResolved] -> ShowS

Exception ErrAlreadyResolved

Defined in Capnp.Rpc.Promise

Methods

toException :: ErrAlreadyResolved -> SomeException

fromException :: SomeException -> Maybe ErrAlreadyResolved

displayException :: ErrAlreadyResolved -> String

Getting the value of a promise

wait :: MonadSTM m => Promise a -> m a

Wait for a promise to resolve, and return the result. If the promise is broken, this raises an exception instead (see breakPromise).