Nested arrow async functions

[zhanzhenzhen]

var a = async () => {
  return await (async () => {
    return await b();
  })();
};

It can't parse this. It seems the syntax is OK.

[benjamn]

What's the stack trace? Sorry not at my computer at the moment.

[zhanzhenzhen]

Error while reading module a:
Error: unknown Expression of type "ArrowFunctionExpression"
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1209:11)
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1043:24)
    at explodeViaTempVar (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:954:23)
    at NodePath.<anonymous> (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1062:20)
    at NodePath.each (/home/zzz/async-test/node_modules/ast-types/lib/path.js:99:22)
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1061:14)
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1184:37)
    at Emitter.Ep.explodeStatement (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:672:19)
    at NodePath.each (/home/zzz/async-test/node_modules/ast-types/lib/path.js:99:22)
    at Emitter.Ep.explodeStatement (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:399:29)
From previous event:
    at buildP (/home/zzz/async-test/node_modules/commoner/lib/reader.js:167:16)
    at Object.ModuleReader.processOutputP (/home/zzz/async-test/node_modules/commoner/lib/reader.js:221:16)
    at Object.ModuleReader.buildModuleP (/home/zzz/async-test/node_modules/commoner/lib/reader.js:132:23)
    at Object.wrapper [as buildModuleP] (/home/zzz/async-test/node_modules/commoner/lib/util.js:53:31)
    at /home/zzz/async-test/node_modules/commoner/lib/reader.js:126:27
Error: unknown Expression of type "ArrowFunctionExpression"
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1209:11)
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1043:24)
    at explodeViaTempVar (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:954:23)
    at NodePath.<anonymous> (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1062:20)
    at NodePath.each (/home/zzz/async-test/node_modules/ast-types/lib/path.js:99:22)
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1061:14)
    at Emitter.Ep.explodeExpression (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:1184:37)
    at Emitter.Ep.explodeStatement (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:672:19)
    at NodePath.each (/home/zzz/async-test/node_modules/ast-types/lib/path.js:99:22)
    at Emitter.Ep.explodeStatement (/home/zzz/async-test/node_modules/regenerator/lib/emit.js:399:29)
From previous event:
    at buildP (/home/zzz/async-test/node_modules/commoner/lib/reader.js:167:16)
    at Object.ModuleReader.processOutputP (/home/zzz/async-test/node_modules/commoner/lib/reader.js:221:16)
    at Object.ModuleReader.buildModuleP (/home/zzz/async-test/node_modules/commoner/lib/reader.js:132:23)
    at Object.wrapper [as buildModuleP] (/home/zzz/async-test/node_modules/commoner/lib/util.js:53:31)
    at /home/zzz/async-test/node_modules/commoner/lib/reader.js:126:27

[insidewhy]

I've seen the same thing via babel, the error message you get back in this case is mega deceptive.

[insidewhy]

I get the same unknown expression of type... error even when not using nested async functions, sometimes just having an arrow function in an async function is enough to give that same error from babel. It also doesn't give you a line number, just the file. Pretty serious bug.

[insidewhy]

async function registerApps(app, r) {
  console.log('server listening, registering apps')

  let appModulePaths = await glob('./api/*/app.js', { cwd: 'server' })

  // regenerator forces me to use non-arrow function... stupid bug
  return Promise.map(appModulePaths, function(appModulePath) {
    let appName = path.dirname(path.relative('api', appModulePath))
    let ApiApp = require(appModulePath)
    console.info('loading app', appName)
    return buildApp(ApiApp, r, app).then(router => {
      app.use(`/${NAMESPACE}/${appName}`, router)
    })
  })

This here it won't accept in arrow function in the Promise.map callback.

[kangax]

I ran into this at FB as well when trying an experiment couple weeks ago. Was just about to report and here it is, fixed. @benjamn thank you!

[kangax]

@benjamn it seems that regenerator always transpiles to arrow functions now (due to transform-es2015-arrow-functions)? Is it possible to have it working with nested arrows while still outputting them? Should I open a new issue?

[benjamn]

@kangax I agree that forcing all arrow functions to be transpiled is overkill. Can you open a new issue to handle arrows in the transform (rather than assuming they've all been transpiled already)?

[rjdestigter]

I'm having this issue at the moment using:
node: 6.9.3
npm: 4.1.1
webpack: 2 + babel-preset-env + stage-0

presets: [
      ['env', {
        targets: {
          browsers: [
            '>1%',
            'last 2 versions',
            'not ie <= 11',
          ],
        },
        modules: false,
        useBuiltIns: false,
        debug: true,
      }],
      'react',
      'stage-0',
    ],

But only on a windows machine. On a linux machine no problems compiling.

Edit: I had everything installed using npm install however yarn was also available and after running yarn the issue was resolved. I'm assuming an older version of one of the dependencies was still present.