EscapedSkolem error implementing websocket reconnection in PureScript and Halogen

Question

I'm trying to implement Websocket reconnection in PureScript and am at a complete loss at how to proceed. I've added the reconnection code at the top level due to the use of Aff ; I think this is the correct place but I'm not sure.

I've tried to implement it as I might in Haskell but I can't make it typecheck due to an EscapedSkolem error in runWs . I get the impression I can fix this by adding a type signature but I can't for the life of me work out what the signature might be!

So I have three questions:

• Is this the correct way of implementing reconnection?
• What is the type of runWs (any hints on how I might work this out for myself would be fantastic)?
• If adding a type signature doesn't fix the EscapedSkolem error how would I go about fixing it?

And finally, I'm a complete newb when it comes to PureScript so if anything's unclear please point that out and I'll try and clarify.

EDIT: Added the error compiler output and changed the title slightly.

module Main where

import Prelude

import Control.Coroutine (Producer, Consumer, runProcess, consumer, ($$))
import Control.Coroutine.Aff (produce)

import Control.Monad.Aff (Aff, delay)
import Control.Monad.Aff.AVar (AVAR)
import Control.Monad.Eff (Eff)
import Control.Monad.Eff.Console (CONSOLE, log)
import Control.Monad.Eff.Exception (EXCEPTION)
import Control.Monad.Eff.Ref (REF)
import Control.Monad.Eff.Var (($=), get)

import DOM (DOM)
import DOM.Websocket.Event.CloseEvent (reason)


import Data.Either (Either(..))
import Data.Maybe (Maybe(..))
import Data.Time.Duration (Milliseconds(..))
import Halogen as H
import Halogen.Aff (HalogenEffects, awaitBody, runHalogenAff)
import Halogen.VDom.Driver (runUI)
import Log (Query(..), component)
import WebSocket (Connection(..), URL(..), WEBSOCKET, newWebSocket, runMessage, runURL, runMessageEvent)


wsURI :: URL
wsURI = URL "ws://localhost:6385"


reconnectionDelay :: Milliseconds
reconnectionDelay = Milliseconds 10000.0


main :: forall eff. Eff (HalogenEffects (console :: CONSOLE, err :: EXCEPTION , avar :: AVAR , dom :: DOM , exception :: EXCEPTION , ref :: REF , ws :: WEBSOCKET | eff)) Unit
main = do
  runHalogenAff do
    body <- awaitBody
    driver <- runUI component unit body

    ---> Replace this: <---
    runProcess (wsProducer $$ wsConsumer driver.query)
    ---> with this: <---
    --   runWs driver

-- -------------------------------------------------
-- -------------------------------------------------
--
-- Reconnection function
-- runWs :: ????????
runWs p = go
  where
    go = do
      runProcess (wsProducer $$ wsConsumer p)
      delay reconnectionDelay
      go

-- -------------------------------------------------
-- -------------------------------------------------


wsProducer :: forall eff. Producer String (Aff (console :: CONSOLE, err :: EXCEPTION , ws :: WEBSOCKET , avar :: AVAR | eff)) Unit
wsProducer = produce \emit -> do

  Connection socket <- newWebSocket wsURI []

  socket.onopen $= \event -> do
    log "onopen: Connection opened"
    log <<< runURL =<< get socket.url


  socket.onmessage $= \event -> do
    emit $ Left $ runMessage (runMessageEvent event)


  socket.onclose $= \event -> do
    log $ "Socket Closed, returning to runHalogenAff: "  <> reason event
    emit $ Right unit


  socket.onerror $= \event -> do
    log "Error."
    emit $ Right unit


wsConsumer :: forall eff . (Query ~> Aff (HalogenEffects eff)) -> Consumer String (Aff (HalogenEffects eff)) Unit
wsConsumer driver = consumer \msg -> do
  driver $ H.action $ AddMessage msg
  pure Nothing

And the compiler output is:

Compiling Main
[1/1 MissingTypeDeclaration] src/Main.purs:54:1

      v
  54  runWs p = go
  55    where
  56      go = do
  57        runProcess (wsProducer $$ wsConsumer p)
  58        delay reconnectionDelay
  59        go
            ^

  No type declaration was provided for the top-level declaration of runWs.
  It is good practice to provide type declarations as a form of documentation.
  The inferred type of runWs was:

    forall t110 t120.
      (Query a0
      -> Aff
            ( avar :: AVAR
            , ref :: REF
            , exception :: EXCEPTION
            , dom :: DOM
            , console :: CONSOLE
            , err :: EXCEPTION
            , ws :: WEBSOCKET
            | t120
            )
            a0
      )
      -> Aff
          ( console :: CONSOLE
          , err :: EXCEPTION
          , ws :: WEBSOCKET
          , avar :: AVAR
          , dom :: DOM
          , exception :: EXCEPTION
          , ref :: REF
          | t120
          )
          t110

  where a0 is a rigid type variable
          bound at line 57, column 44 - line 57, column 45

[1/1 EscapedSkolem] src/Main.purs:54:1

      v
  54  runWs p = go
  55    where
  56      go = do
  57        runProcess (wsProducer $$ wsConsumer p)
  58        delay reconnectionDelay
  59        go
            ^

  The type variable a, bound at

    /home/rgh/dev/purescript/translate/sidebar/src/Main.purs line 57, column 44 - line 57, column 45

  has escaped its scope, appearing in the type

    (Query a2
    -> Aff
          ( avar :: AVAR
          , ref :: REF
          , exception :: EXCEPTION
          , dom :: DOM
          , console :: CONSOLE
          , err :: EXCEPTION
          , ws :: WEBSOCKET
          | t120
          )
          a2
    )
    -> Aff
        ( console :: CONSOLE
        , err :: EXCEPTION
        , ws :: WEBSOCKET
        , avar :: AVAR
        , dom :: DOM
        , exception :: EXCEPTION
        , ref :: REF
        | t120
        )
        t110

  in the expression \p ->
                      let
                        go = ...
                      in go
  in value declaration runWs

          Src   Lib   All
Warnings   1     0     1
Errors     1     0     1
* Failed to rebuild; try to fix the compile errors

Answer 1

Compiler error messages may be hard to decrypt sometimes, but in this case it turns out to be the answer you're looking for. Let's look at your do block here:

do
  runHalogenAff do
    body <- awaitBody
    driver <- runUI component unit body
    runWs driver.query -- < assuming you made a small mistake here

I usually start by desugaring, I find it makes it easier to follow the types, but ymmv:

runHalogenAff $
  awaitBody >>= \body ->
    runUI component unit body >>= \driver ->
      runWs driver.query

Looking at the signature of runHalogenAff , we can see that it accepts an argument of type Aff (HalogenEffects eff) x , meaning the following term must evaluate to a value of that type. It must be then that runWs returns a value of that type.

Now let's turn to runWs . Its argument is a natural transformation f ~> m which in your example takes your query algebra into the Aff monad . We can write this down and ask the compiler to figure out the rest for us:

runWs :: (Query ~> Aff _) -> Aff _ Unit

That will build successfully and give you what you can fill these holes with. Here is the final signature:

runWs :: forall eff. 
      (Query ~> Aff (HalogenEffects 
        ( console :: CONSOLE
        , err :: EXCEPTION
        , ws :: WEBSOCKET
        | eff
        )))
      -> Aff (HalogenEffects
        ( console :: CONSOLE
        , err :: EXCEPTION
        , ws :: WEBSOCKET
        | eff
        )) Unit

Indeed that is exactly what the compiler output gives you. I am assuming the error message "the type variable a has escaped its scope" is because of the universal quantifier in the definition of a natural transformation .