Can't find variable: EMPTY

[dgendill]

I'm not sure if this issue should go into purescript-aff or this repo, but when I use purescript-aff#4.0.0 and use the makeVar function, the Control.Monad.Eff.AVar module is missing the EMPTY variable in the compiled JavaScript. I think these steps can reproduce the issue. Here's a sample module...

module Test.Main where

import Prelude

import Control.Monad.Aff (message, runAff_)
import Control.Monad.Aff.AVar (makeEmptyVar, makeVar)
import Control.Monad.Eff.Console (log)
import Control.Monad.Eff.Exception (stack)
import Data.Either (Either(..))

main = runAff_ (case _ of
    Left err -> do
      log $ "ERROR: " <> message err
      log $ show (stack err)
    Right r -> log "OK"
  ) $ do
    a <- makeVar "s"
    pure unit

If you compile using pulp build --main Test.Main --to test.js, the resulting test.js has this compiled Control.Monad.Eff.AVar module.

(function(exports) {
  /* globals exports, setTimeout */
  /* jshint -W097 */
  "use strict";

  // ...

  function drainVar (util, avar) {

    // ...

    if (value === EMPTY && (p = takeHead(ps))) {
      avar.value = value = p.value;
    }
  }
  // ...
})(PS["Control.Monad.Eff.AVar"] = PS["Control.Monad.Eff.AVar"] || {});

However, if I substitute makeEmptyVar for makeVar, then I get this output...

(function(exports) {
  /* globals exports, setTimeout */
  /* jshint -W097 */

  "use strict";

  var EMPTY = {};
  // ...

I'm using compile 0.11.6.

[coot]

I was hit by this too. That's an issue with purs bundle --main Test.Main --module Test.Main. The DCE performed removed the EMPTY variable. zephyr does not suffer from this.

[natefaubion]

purs bundle is extremely brittle when it comes to DCE on FFI code 😞. It's designed to perform DCE on PureScript code which has very specific patterns, completely ignoring other common JS things (like switches). I had tested the bundler with Test.Main and everything worked, so I assumed it was OK, but clearly there are bad cases where DCE will remove the var even though it's in use. I fixed this for Aff by putting everything important in a big IIFE, which purs bundle seems to leave alone.

[natefaubion]

Can you try out the changes in #6 and confirm it fixes your problem?

[coot]

The EMPTY is now preserved. With zephyr we'll stop having this kind of problems, since DCE in foreign modules can be done by javascript tooling, while DCE on corefn is much easier to do.

[dgendill]

#6 fixed it for me. Thanks, Nate!