-- | The message passing effect.
--
-- This module describes an abstract message passing effect, and a process
-- effect, mimicking Erlang's process and message semantics.
--
-- An implementation of a handler for these effects can be found in
-- 'Control.Eff.Concurrent.Process.ForkIOScheduler'.
{-# LANGUAGE DeriveTraversable #-}
{-# LANGUAGE DeriveFoldable #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE GADTs #-}
module Control.Eff.Concurrent.Process
( ProcessId(..)
, fromProcessId
, Process(..)
, ConsProcess
, ResumeProcess(..)
, SchedulerProxy(..)
, thisSchedulerProxy
, yieldAndCatchProcess
, sendMessage
, spawn
, receiveMessage
, receiveMessageAs
, receiveLoop
, self
, sendShutdown
, exitWithError
, exitNormally
, raiseError
, catchRaisedError
, ignoreProcessError
)
where
import GHC.Stack
import Control.Eff
import Control.Lens
import Control.Monad (forever)
import Data.Dynamic
import Data.Kind
import Text.Printf
-- * Process Effects
-- | The process effect is the basis for message passing concurrency. This
-- effect describes an interface for concurrent, communicating isolated
-- processes identified uniquely by a process-id.
--
-- Processes can raise exceptions that can be caught, exit gracefully or with an
-- error, or be killed by other processes, with the option of ignoring the
-- shutdown request.
--
-- Process Scheduling is implemented in different modules. All scheduler
-- implementations should follow some basic rules:
--
-- * fair scheduling
--
-- * sending a message does not block
--
-- * receiving a message does block
--
-- * spawning a child blocks only a very moment
--
-- * a newly spawned process shall be scheduled before the parent process after
-- * the spawn
--
-- * when the first process exists, all process should be killed immediately
data Process (r :: [Type -> Type]) b where
-- | Return the current 'ProcessId'
SelfPid :: Process r (ResumeProcess ProcessId)
-- | Start a new process, the new process will execute an effect, the function
-- will return immediately with a 'ProcessId'.
Spawn :: Eff (Process r ': r) () -> Process r (ResumeProcess ProcessId)
-- | Process exit, this is the same as if the function that was applied to a
-- spawn function returned.
Shutdown :: Process r a
-- | Exit the process due to an error, this cannot be caught.
ExitWithError :: String -> Process r b
-- | Raise an error, that can be handled.
RaiseError :: String -> Process r b
-- | Request that another a process exits. The targeted process is interrupted
-- and gets a 'ShutdownRequested', the target process may decide to ignore the shutdown
-- requests.
SendShutdown :: ProcessId -> Process r (ResumeProcess Bool)
-- LinkProcesses :: ProcessId -> ProcessId -> Process ()
-- | Send a message to a process addressed by the 'ProcessId'. Sending a
-- message should **always succeed** and return **immediately**, even if the
-- destination process does not exist, or does not accept messages of the
-- given type.
SendMessage :: ProcessId -> Dynamic -> Process r (ResumeProcess Bool)
-- | Receive a message. This should block until an a message was received. The
-- pure function may convert the incoming message into something, and the
-- result is returned as 'ProcessMessage' value. Another reason why this function
-- returns, is if a process control message was sent to the process. This can
-- only occur from inside the runtime system, aka the effect handler
-- implementation. (Currently there is one in 'Control.Eff.Concurrent.Process.ForkIOScheduler'.)
ReceiveMessage :: Process r (ResumeProcess Dynamic)
-- | Every 'Process' action returns it's actual result wrapped in this type. It
-- will allow to signal errors as well as pass on normal results such as
-- incoming messages.
data ResumeProcess v where
-- | The process is required to exit.
ShutdownRequested :: ResumeProcess v
-- | The process is required to exit from an error condition, that cannot be
-- recovered from.
OnError :: String -> ResumeProcess v
-- | The process may resume to do work, using the given result.
ResumeWith :: a -> ResumeProcess a
-- | This indicates that the action did not complete, and maybe retried
RetryLastAction :: ResumeProcess v
deriving (Typeable, Foldable, Functor, Show, Eq, Ord, Traversable)
-- | /Cons/ 'Process' onto a list of effects.
type ConsProcess r = Process r ': r
-- | Execute a 'Process' action and resume the process, retry the action or exit
-- the process when a shutdown was requested.
yieldProcess :: forall r q v .
( SetMember Process (Process q) r
, HasCallStack)
=> Process q (ResumeProcess v)
-> Eff r v
yieldProcess processAction =
do result <- send processAction
case result of
ResumeWith value -> return value
RetryLastAction -> yieldProcess processAction
ShutdownRequested -> send (Shutdown @q)
OnError e -> send (ExitWithError @q e)
-- | Execute a and action and resume the process, retry the action, shutdown the process or return an error.
yieldAndCatchProcess :: forall q r v.
(SetMember Process (Process q) r, HasCallStack)
=> SchedulerProxy q
-> Eff r (ResumeProcess v)
-> Eff r (Either String v)
yieldAndCatchProcess px processAction =
do result <- processAction
case result of
ResumeWith value -> return (Right value)
RetryLastAction -> yieldAndCatchProcess px processAction
ShutdownRequested -> send (Shutdown @q)
OnError e -> return (Left e)
-- | Every function for 'Process' things needs proxy for the low-level
-- effect list depending on the scheduler implementation.
-- I don't know a smarter way yet to do this.
data SchedulerProxy :: [Type -> Type] -> Type where
SchedulerProxy :: SchedulerProxy q
-- | Return a 'SchedulerProxy' for a 'Process' effect.
thisSchedulerProxy :: Eff (Process r ': r) (SchedulerProxy r)
thisSchedulerProxy = return SchedulerProxy
-- | Send a message to a process addressed by the 'ProcessId'.
-- See 'SendMessage'.
sendMessage
:: forall r q
. (HasCallStack, SetMember Process (Process q) r)
=> SchedulerProxy q
-> ProcessId
-> Dynamic
-> Eff r Bool
sendMessage _ pid message =
yieldProcess (SendMessage pid message)
-- | Send a message to a process addressed by the 'ProcessId'.
-- See 'SendMessage'.
sendShutdown
:: forall r q
. (HasCallStack, SetMember Process (Process q) r)
=> SchedulerProxy q
-> ProcessId
-> Eff r Bool
sendShutdown _ pid =
yieldProcess (SendShutdown pid)
-- | Start a new process, the new process will execute an effect, the function
-- will return immediately with a 'ProcessId'.
spawn :: forall r q .
(HasCallStack, SetMember Process (Process q) r)
=> Eff (Process q ': q) ()
-> Eff r ProcessId
spawn child =
yieldProcess (Spawn @q child)
-- | Block until a message was received.
receiveMessage
:: forall r q
. (HasCallStack, SetMember Process (Process q) r)
=> SchedulerProxy q -> Eff r Dynamic
receiveMessage _ = do
yieldProcess ReceiveMessage
-- | Receive and cast the message to some 'Typeable' instance.
receiveMessageAs
:: forall a r q .
(HasCallStack, Typeable a, SetMember Process (Process q) r)
=> SchedulerProxy q -> Eff r a
receiveMessageAs px =
do messageDynamic <- receiveMessage px
let castAndCheck dm =
case fromDynamic dm of
Nothing ->
Left ("Invalid message type received: " ++ show dm)
Just m ->
Right m
maybeMessage = castAndCheck messageDynamic
either (raiseError px) return maybeMessage
-- | Enter a loop to receive messages and pass them to a callback, until the
-- function returns 'False'.
receiveLoop
:: forall r q
. ( SetMember Process (Process q) r
, HasCallStack)
=> SchedulerProxy q
-> (Either (Maybe String) Dynamic -> Eff r ())
-> Eff r ()
receiveLoop _ handlers =
forever $
do mReq <- send (ReceiveMessage @q)
case mReq of
RetryLastAction ->
return ()
ShutdownRequested ->
handlers (Left Nothing)
OnError reason ->
handlers (Left (Just reason))
ResumeWith message ->
handlers (Right message)
-- | Returns the 'ProcessId' of the current process.
self :: (HasCallStack, SetMember Process (Process q) r) => SchedulerProxy q -> Eff r ProcessId
self _px = yieldProcess SelfPid
-- | Exit the process.
exitNormally :: forall r q a. (HasCallStack, SetMember Process (Process q) r) => SchedulerProxy q -> Eff r a
exitNormally _ = send (Shutdown @q)
-- | Exit the process with an error.
exitWithError :: forall r q a. (HasCallStack, SetMember Process (Process q) r) => SchedulerProxy q -> String -> Eff r a
exitWithError _ = send . ExitWithError @q
-- | Thrown an error, can be caught by 'catchRaisedError'.
raiseError :: forall r q b. (HasCallStack, SetMember Process (Process q) r) => SchedulerProxy q -> String -> Eff r b
raiseError _ = send . RaiseError @q
-- | Catch and handle an error raised by 'raiseError'. Works independent of the
-- handler implementation.
catchRaisedError
:: forall r q w . (HasCallStack, SetMember Process (Process q) r) => SchedulerProxy q -> (String -> Eff r w) -> Eff r w -> Eff r w
catchRaisedError _ onErr = interpose return go
where
go :: forall b . Process q b -> (b -> Eff r w) -> Eff r w
go (RaiseError emsg) _k = onErr emsg
go s k = send s >>= k
-- | Like 'catchRaisedError' it catches 'raiseError', but instead of invoking a
-- user provided handler, the result is wrapped into an 'Either'.
ignoreProcessError
:: (HasCallStack, SetMember Process (Process q) r) => SchedulerProxy q -> Eff r a -> Eff r (Either String a)
ignoreProcessError px = catchRaisedError px (return . Left) . fmap Right
-- | Each process is identified by a single process id, that stays constant
-- throughout the life cycle of a process. Also, message sending relies on these
-- values to address messages to processes.
newtype ProcessId = ProcessId { _fromProcessId :: Int }
deriving (Eq,Ord,Typeable,Bounded,Num, Enum, Integral, Real)
instance Read ProcessId where
readsPrec _ ('<':'0':'.':rest1) =
case reads rest1 of
[(c, '.':'0':'>':rest2)] -> [(ProcessId c, rest2)]
_ -> []
readsPrec _ _ = []
instance Show ProcessId where
show (ProcessId c) =
printf "<0.%d.0>" c
makeLenses ''ProcessId