Effectful Channel Management

Posted on October 5, 2016

Categories: idris, effects, sessions, tdvcs

Communicating systems require channels to communicate, in many of the programming languages that provide communication channels the management of these channels at the application layer is left to the programmer to get right. That is, the programmer is responsible for ensuring that the channel is established, connected to, and then destroyed.

In languages that support dependent types we can encoded, at the type level, extra properties about our software program. Further, the idea of algebraic effects supports the separation of a program's description from its realisation in different contexts. Edwin Brady's EFFECTS library has shown how this is possible using dependent types [1]. The implementation of EFFECTS allows for efficient management of a program's side-effects and dealings with the outside world. See the Effects Tutorial for more information.

The FILE Effect

An interesting aspect of EFFECTS is that each effect is associated with a resource. For the FILE effect it is a file handler, for the LOG and PERF effects that I contributed to EFFECTS it is logging levels and captured metrics. With resource based algebraic effects we can capture (at the type level) the allowed (state) changes to this resource. If we examine the current FILE effect from Idris' effects library you can see this for file access. The resource for the FILE effect is the file descriptor and its relation to the mode of operation.

||| The file handle associated with the effect.
||| @m The `Mode` that the handle was generated under.
data FileHandle : (m : Mode) -> Type where
    FH : File -> FileHandle m

The FILE effect is a dependent effect in that the result of the effectful operation is used to calculate the value of the resource being computed over. Given the function calcResourceTy:

calcResourceTy : (m  : Mode)
              -> (ty : FileOpReturnTy fOpTy retTy)
              -> Type
calcResourceTy _ (FError e) = ()
calcResourceTy m _          = FileHandle m

calcResourceTy computes the type of the resource as being unit if there is a failure, and if not the resource is the FileHandle parameterised by the mode of operation. When combined with the mode of operation we can have a more informed means to calculate the next value of the resource.

  ||| An effect to describe operations on a file.
  data FileE : Effect where

    -- Open/Close

    Open : (fname : String)
        -> (m : Mode)
        -> sig FileE
               (FileOpReturnTy SUCCESS ())
               (\res => calcResourceTy m res)

Communication is Communication

But why talk about the FILE effect? Well if we think about it, reading and writing with a file is analogous to communicating over a channel. We have to:

  1. establish access to the file (set up a means to communication);
  2. read or write to the file (receive or send messages); and
  3. shutdown access to the file (shut down the means of communication).

So why not apply creation of a file effect to management of a communication channel?

Fortunatly, Idris comes with an unsafe means to perform concurrent communication between different Idris processes (and IPC too). Edwin has shown how we can reason about communication in Idris at Lambda World, and spoiler alert in the TDD book too.

However, over the summer I took a stab at using effects to manage channels. This was inspired by earlier work by Simon Fowler.

For the remainder of this post, I shall describe my implementation of a resource based algebraic effect for channels.

The Resource

Like the FILE effect we are going to model an active session using a simple resource OpenSesh, and parameterise it by the state of the channel represented by the enumerated type SSTATE.

data SSTATE =  Unconnected | Spawned | Active

Channels are either: Unconnected---literally unconnected; Spawned---alive but not communicating; or Active---are communicating with another entity. OpenSesh is defined as follows:

data OpenSesh  : (st : SSTATE) -> Type where
   NoSesh      : OpenSesh Unconnected
   SpawnedSesh : (pid : PID) -> OpenSesh Spawned
   ActiveSesh  : (pid : Maybe PID) -> (sesh : Channel) -> OpenSesh Active

Here an unconnected channel is specified using NoSesh and contains no information. SpawnedSesh represents a spawned process, so we must capture the processes PID. ActiveSesh represents either a client channel or server channel. If the channel is connecting to another process, not only do we need the channel to communication on, but also the PID of the process. If we are the server process, we need to keep track of the communication channel only.


Before we define the effect we first define some helper code.

Type Synonyms

First, type synonyms.

INIT : Type
INIT = OpenSesh Unconnected

WAIT : Type
WAIT = OpenSesh Spawned

ACTIVE = OpenSesh Active


The allowed state transitions between resource types. We have decided that we shall use a Bool to describe success or not.

Connecting to a Process

  1. As a Client

    If connection was successful then move to active, otherwise go back to wait.

    connectedOK : Bool -> Type
    connectedOK False = OpenSesh Spawned
    connectedOK True  = OpenSesh Active
  2. As a Server

    If attempt to listen was successful then move to active, otherwise go back to unconnected.

    listenedOK : Bool -> Type
    listenedOK False = OpenSesh Unconnected
    listenedOK True  = OpenSesh Active

Sending Messages

If message was sent then stay active, otherwise there was a failure and move to unconnected.

sentOK : Bool -> Type
sentOK True  = OpenSesh Active
sentOK False = OpenSesh Unconnected

Receiving Messages

If message was received then stay active, otherwise there was a failure and move to unconnected.

receivedOK : Maybe a -> Type
receivedOK (Just v) = OpenSesh Active
receivedOK Nothing  = OpenSesh Unconnected

The CHANNEL Effect

With the helper code defined, we can now describe an effectful means to manage processes. To do so we have to use a mutual block to allow an effectful process to spawn a second effectful process.

  ProcessE : (st      : Type)
          -> (rTy     : Type)
          -> (inEffs  : List EFFECT)
          -> (outEffs : List EFFECT)
          -> Type
  ProcessE st ty inEffs outEffs = Eff ty
                                      (CHANNEL st :: inEffs)
                                      (\_ => (CHANNEL st :: outEffs))

  ||| The effectful description for `Session`.
  data ChannelE : Effect where

    ||| Spawn an effectful process to communicate with, transitioning
    ||| from `INIT` to `WAIT`.
    Spawn : Env IO esi
         -> ProcessE INIT () esi eso
         -> sig ChannelE (PID) (INIT) (WAIT)

    ||| Connect to a spawned process, transitioning from `INIT` to
    ||| result of `connectedOK`.
    Connect : sig ChannelE (Bool) (WAIT) (\res => connectedOK res)

    ||| Listen for a connection, traisitioning from `INIT` to result
    ||| of `listenedOK`.
    Listen : (tout : Int)
          -> sig ChannelE (Bool) (INIT) (\res => listenedOK  res)

    ||| Send a message of type `a`, transitioning from `ACTIVE` to
    ||| result of `sentOK`.
    Send : a -> sig ChannelE (Bool) (ACTIVE) (\res => sentOK res)

    ||| Receive a message of type `a`, trasitioning from `ACTIVE` to
    ||| result of `receivedOK`.
    Recv : (a : Type)
        -> sig ChannelE (Maybe a) (ACTIVE) (\res => receivedOK res)

    ||| End a session, trasitioning back to `INIT`.
    End : sig ChannelE () (ty) (INIT)

  ||| An effectful API for `System.Concurrency.Sessions`.
  ||| @st The state the session is in.
  CHANNEL : (st : Type) -> EFFECT
  CHANNEL st = MkEff st ChannelE

If we examine the description of the effectful functions, we can see the allowed state transitions between different API calls. For example, spawning a process will transition the effect from being unconnected to waiting. Then we can only progress to an active session if connecting to the spawned resource was successful. Similar transitions are encoded in the effect between the different operations.

Implementation for IO.

With the effect specified, we can now define an implementation for this effect for the IO context. This handler will describe how to realise the management tasks described in the effect. For concurrent processes this will require spawning processes, connecting to processes, listening for processes, sending and receding messages, and closing sessions down.

Handler ChannelE IO where

  handle NoSesh (Spawn env proc) k = do
      pid <- spawn $ runInit (NoSesh::env) (proc)
      k (pid) (SpawnedSesh pid)

  handle (SpawnedSesh pid) Connect k = do
      msesh <- connect pid

      case msesh of
          Nothing      => k False (SpawnedSesh pid)
          r@(Just res) => k True  (ActiveSesh (Just pid) res)

  handle (NoSesh) (Listen tout) k = do
      msesh <- listen tout

      case msesh of
          Nothing      => k False NoSesh
          r@(Just res) => k True  (ActiveSesh Nothing res)

  handle st@(ActiveSesh pid sesh) (Send m) k = do
      sent <- unsafeSend sesh m
      if sent
        then k True st
        else k False NoSesh

  handle st@(ActiveSesh pid sesh) (Recv ty) k = do
      res <- unsafeRecv ty sesh
      case res of
        Nothing    => k Nothing    NoSesh
        (Just val) => k (Just val) st

  handle st End k = k () NoSesh

Effectful API

With the effect and handler defined, we can now present the API that the developer can use to program with.

Describing a process.

A Process is a function that can communicate using the session effect. We use this alias to make describing communicating processes that little bit easier.

Note that the definition of Process already specifies the session required for communicating.

Process : (st  : Type)
       -> (rTy : Type)
       -> (es  : List EFFECT)
       -> Type
Process st ty inEffs = ProcessE st ty inEffs inEffs

Spawning, Connecting, and Listening

Attempt to spawn an effectful process that communicates using a Sessions. Returning the PID of the spawned process.

spawn : (env  : Env IO es)
     -> (proc : Process INIT () es)
     -> Eff (PID)
            [CHANNEL INIT]
            [CHANNEL WAIT]
spawn env proc = call $ Spawn env proc

Attempt to establish a connection to the spawned process. The function can only be used once a session has been spawned.

connect : Eff (Bool)
              [CHANNEL WAIT]
              (\res => [CHANNEL (connectedOK res)])
connect = call $ Connect

Listen for a fixed amount of time for a connection from an external process. The function is used by the spawned process to accept incomming requests.

listen : (timeout : Int)
      -> Eff (Bool)
             [CHANNEL INIT]
             (\res => [CHANNEL (listenedOK res)])
listen t = call $ Listen t

Message Passing

Send a message of type ty. Returning a Bool describing success.

send : (msg : ty)
    -> Eff Bool
           [CHANNEL ACTIVE]
           (\res => [CHANNEL (sentOK res)])
send msg = call $ Send msg

Expect to receive a message of the specifyed type. This is unsafe as receive expects the incomming message to be of the specified type.

recv : (ty : Type)
    -> Eff (Maybe ty)
           [CHANNEL ACTIVE]
           (\res => [CHANNEL (receivedOK res)])
recv ty = call $ Recv ty

Stopping the Session

stop : Eff () [CHANNEL ty] [CHANNEL INIT]
stop = call End


We finish of this post by showing the code for a game of ping pong.

First we describe the called process ping.

ping : Process INIT () [STDIO]
ping = do
  True <- listen 10
          | False => stop
  True <- send "ping"
          | False => stop

  msg <- recv String
  case msg of
    Nothing => pure ()
    (Just v) => do
        putStrLn "Ping received"
        printLn v

Notice, that in the code we have to explicitly stop and terminate the process upon failure. If we remove these statements, the program will not compile as we have violated the transitions described in the effect. Further, if we try to send a message before we have connected, Idris will complain further.

Below pong is the client process that initiates the game of ping pong.

pong : Process INIT () [STDIO]
pong = do
  pid <- spawn [()] ping
  True <- connect | False => stop
  msg <- recv String
  case msg of
    Nothing => stop
    (Just v) => do
        putStrLn "pong received"
        printLn v
        True <- send "pong" | False => pure ()

Again we must call spawn before we call connect and both before we can send or receive messages.

We can play the game of ping pong by running the effectful program specifying that we expect two sessions to be used.

play : IO ()
play = runInit [NoSesh,NoSesh] pong

The End.

This post describes how effects can be used to manage our communication channels. A full code listing is available online.

However, the communication over these channels is not type safe. Edwin demonstrated one means of ensuring such type safe communication at Lambda World 2016. However, there are some other things we can do that are more advantageous and cooler. /Yes, there is more cooler stuff that what Edwin demonstrated at Lambda World./ My research is currently focused on not only how to manage channels using dependent types but how to type check the communication itself.

Hopefully I will blog more about what I am up to, I do have some things in the pipeline that I won't share just yet as they are not complete.


[1] E. Brady, “Resource-dependent algebraic effects,” in Trends in functional programming: 15th international symposium, tFP 2014, soesterberg, the netherlands, may 26-28, 2014. revised selected papers, J. Hage and J. McCarthy, Eds. Cham: Springer International Publishing, 2015, pp. 18–33.