PureScript FFI & Aff Monad: Why does the effect never run?

Question

I'm a total PureScript newbie and need a bit of help figuring out why a FFI function modeled with the Aff monad doesn't seem to be working for me.

The expected behavior is to log a message "keyMessage" to the console after 1000ms.

Instead, the program just hangs indefinitely after the following output:

Compiling Main
* Build successful.
Waiting for message...

Main.purs:

module Main where

import Prelude
import Control.Monad.Aff (Aff, Fiber, launchAff)
import Control.Monad.Aff.Console (log)
import Control.Monad.Eff (Eff, kind Effect)
import Control.Monad.Eff.Console (CONSOLE)

main :: forall e. Eff (console :: CONSOLE, to :: TIMEOUT | e) (Fiber (console :: CONSOLE, to :: TIMEOUT | e) Unit)
main = launchAff do
  log "Waiting for message..."
  m <- message "key"
  log m

foreign import data TIMEOUT :: Effect

foreign import message :: forall e. String -> Aff (to :: TIMEOUT | e) String

Main.js:

'use strict';

exports.message = function(key) {
  return function(errback, callback) {
    var timeout = setTimeout(function() {
      callback(key + 'Message');
    }, 1000);

    return function() {
      return function (cancelErrback, cancelCallback) {
        clearTimeout(timeout);
        return cancelCallback();
      };
    };
  };
};

Thanks in advance for your insights!

Answer 1

If you're using the latest major version of purescript-aff (v4 or later) then the runtime representation of Aff has changed, and you can't create it directly using the errback/callback function style anymore.

Take a look at the https://pursuit.purescript.org/packages/purescript-aff/4.0.2/docs/Control.Monad.Aff.Compat module, in particular the EffFnAff type / fromEffFnAff functions for an explanation of how the equivalent thing works now.

Alternatively you can also construct Affs with makeAff, but that would require reformulating your FFI code a bit.