Is the .mjs suffix required? What is Spago doing for its script?

Well, here's our options for this.

1. We call node using the --eval arg and it evaluates the string you give as a script. To make that work on ES modules, we include the --input-type module flag, so that the whole code looks like this: node --input-type module --eval "import { main } from 'file:///path/to/rootDir/output/Module.Name/index.js'; main();". Unfortunately, this is problematic as described in an earlier comment
2. We use .js and have to also add a package.json file that has {"type": "module"} as its contents in the directory containing it, so that Node interprets that JS file as ES modules rather than CommonJS files. AFAIK, this is likely fine since purs creates that 'package.json` file
3. We use .mjs because Node will immediately understand that it's ES modules, regardless of whether the package.json file is there.

I went with 3 mainly because it removed an additional problem that could arise. Now that CI builds, I think it's worth it to try the .js version.

Also, spago stopped using files because I thought using node --eval would work until I came across the CLI args issue. See purescript/spago#865 for more info.

If we can do just the .js version I'd prefer that.

Welp. Looks like I might be wrong about 3. https://github.com/purescript-contrib/pulp/runs/5488535135?check_suite_focus=true#step:8:583

Yup, the package.json file is there, but perhaps node isn't treating it as ES modules because the root dir's package.json file doesn't have type: module? And it can't yet because pulp is being compiled with v0.14.5, not v0.15.0.
I'm going to revert this back to .mjs because that at least builds.

I took a look at this again. I think the issue is that we're not passing in the --experimental-modules flag to Node when Node is < 13.0.0. See working-group-purescript-es/purescript#11 where this same issue arose.

So, I installed Node 12.6.0 and ran a pulp run test locally. However, I still got an error. So, I tried out a simple example to see if there was something else going on:

$ cat psOutput.js 
export const main = function () {
  console.log("this works");
}
$ cat entryPoint.js 
import { main } from '/home/jordan/Programming/Projects/pulp/psOutput.js';

main();
$ node --version
v12.6.0
$ node --experimental-modules entryPoint.js
(node:28551) ExperimentalWarning: The ESM module loader is experimental.
/home/jordan/Programming/Projects/pulp/entryPoint.js:1
import { main } from 'file:///home/jordan/Programming/Projects/pulp/entryPoint.js';
       ^

SyntaxError: Unexpected token {
    at Module._compile (internal/modules/cjs/loader.js:720:23)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:787:10)
    at Module.load (internal/modules/cjs/loader.js:643:32)
    at Function.Module._load (internal/modules/cjs/loader.js:556:12)
    at internal/modules/esm/translators.js:87:15
    at Object.meta.done (internal/modules/esm/create_dynamic_module.js:48:9)
    at file:///home/jordan/Programming/Projects/pulp/entryPoint.js:9:13
    at ModuleJob.run (internal/modules/esm/module_job.js:111:37)
    at processTicksAndRejections (internal/process/task_queues.js:85:5)
    at async Loader.import (internal/modules/esm/loader.js:134:24)

I tried import Main ... and import * as Main ..., and still got the same error.

I shifted the files around a bit

$ ls z/
entryPoint.js  package.json  psOut.js
$ cat z/package.json 
{"type": "module"}
$ node --experimental-modules z/entryPoint.js 
(node:29562) ExperimentalWarning: The ESM module loader is experimental.
this works

So, using .js can work. The issue is the package.json file's location.

OK! Figured it out. The issue is that the script generated by pulp is stored in the /tmp/ directory. Since that directory doesn't contain a package.json file with {"type": "module"} as its content, Node throws the above error. Once I create a package.json file there, Node runs fine.

What should happen once we no longer have #es-modules-libraries branches?

It's an unfortunate a cyclical dependency. When bower install is run by pulp init, it downloads the repos and stores their bower.json file as .bower.json. At some point, this file is parsed as a bower file. However, that parsing only works if the version for a dependency follows the typical semver variant (e.g. "purescript-prelude": "^7.0.0"), not the repo-branch variant (e.g. "purescript-prelude": "working-group-purescript-es/purescript-prelude#es-modules-libraries").

So, we'd need to make a release for prelude, effect, console, and psci-support where their bower.json file's dependencies versions point to actual versions (e.g. ^7.0.0), update Init.purs to install those versions rather than the es-modules-libraries branch, and then drop this code.

OK. Then when merging this we should create an issue to come back and fix this, because it'll break later.

I opened #402