Could you had "type": "module" to the package.json

[ruifortes]

I always have to change it manually after the "Cannot use import statement outside a module" error

[ryyppy]

did you configure bsconfig to have mjs as a file extension?

[ruifortes]

I don't like using .mjs although I forgot why... should I ?
Also I don't have much experience with npm publishing but I'm curious about the downside to adding type:module to packege.json.

[ryyppy]

type:module only makes sense when the package is guaranteed to be compiled to ES6 modules. A library doesn't know how a user configures ReScript, so what if someone compiles to commonjs and the package.json still instructs node to parse them as es6 modules?

So you either use the mjs extension if possible, or you adjust your tooling (esbuild / babel / etc) to read certain directories in a different mode? Not sure what setup you are using.

[ruifortes]

I'm using esbuild but... I have to dig deeper into this, sorry
I'll get back when I have the answer to your question ;-)

[dusty-phillips]

I think this is the same issue; here are some more details:

Repro here: https://github.com/dusty-phillips/rescript_promise_repro

Summary

I cannot build a call to Promise.catch if I have package-specs.module set to es6 and `"type": "module" in package.json:

exception MyError(string)

Promise.reject(MyError("test"))

// When this section is commented, the program behaves as expected
// and UnhandledPromiseRejection is thrown.

// If you uncomment this section, it fails to compile
// with a syntax error on attempting to import Curry
->Promise.catch(_ => {
  "ok"->Promise.resolve
})
->ignore

error message:

(node:22455) Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
(Use `node --trace-warnings ...` to show where the warning was created)
/Users/dustyphillips/Desktop/Code/promise_repro/node_modules/@ryyppy/rescript-promise/src/Promise.js:3
import * as Curry from "rescript/lib/es6/curry.js";
^^^^^^

SyntaxError: Cannot use import statement outside a module
    at Object.compileFunction (node:vm:355:18)
    at wrapSafe (node:internal/modules/cjs/loader:1039:15)
    at Module._compile (node:internal/modules/cjs/loader:1073:27)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1138:10)
    at Module.load (node:internal/modules/cjs/loader:989:32)
    at Function.Module._load (node:internal/modules/cjs/loader:829:14)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/translators:201:29)
    at ModuleJob.run (node:internal/modules/esm/module_job:175:25)
    at async Loader.import (node:internal/modules/esm/loader:178:24)
    at async Object.loadESM (node:internal/process/esm_loader:68:5)
╭─dustyphillips at imac in ~/Desktop/Code/promise_repro 10:54:26

As suggested, if I change "suffix" to ".mjs" everything works as expected. I'm comfortable doing that for my project, but wanted to add the repro in case this is something that is intended to be supported (e.g: is bs.js still required for gentype support?).

Version info:
rescript 9.1.4
rescript-promise: 2.1.0

[kevanstannard]

@dusty-phillips @ruifortes some extra info that may help.

If you configure your project with:

• "type": "module" in package.json, and
• "module": "es6" in bsconfig.json, and
• .js or .bs.js file extensions

then it introduces a problem.

Keeping in mind that ReScript compiles your project files and the Promise lib files using these settings, and they are all compiled as es6 with a .js extension.

Here's what happens when you run the code:

[1] In your project, node will look at your package.json file, see the "type": "module" and then interpret .js as es6. This is fine.

[2] However in the Promise lib, node will look at its package.json file, find it has no type property, and then interpret .js as commonjs. This triggers the error, because the file has been compiled as es6.

If you set your file extensions to .mjs then node doesn't need to try determine the type of the file, it immediately knows it's an es6 file, which is why everything then works.

This problem is not related to the ReScript promise lib, it's about ensuring the compiler settings are compatible with the node settings.

Here's the docs on how module files are interpreted:

https://nodejs.org/api/packages.html#packages_determining_module_system

Hope that helps.

[dusty-phillips]

Hm, so to solve it Rescript would need to take its module setting from the package.json instead of bsconfig.json and compile files differently depending on the underlying project. ie: build --with-deps would need to be aware of cjs or mjs format depending on the dependent library. And that would probably completely fall over if building for the browser with webpack. I wonder if Rescript has enough information to issue a warning. Considering that the error is coming from node, I don't think Rescript can have a say in it.

[impvd]

I just spent two hours to try to make the demo run, it's kind of horrible experience.

According to rescript official tutorial:

git clone https://github.com/rescript-lang/rescript-project-template
cd rescript-project-template
npm install
npm run build
node src/Demo.bs.js

Then for the promise part:

npm install @ryyppy/rescript-promise --save
npm install node-fetch --save

Add @ryyppy/rescript-promise as a dependency in bsconfig.json.

Copy the FetchExample.res to src/Demo.res.

And here's what the error looks like:

$ npm run build

> rescript-project-template@0.0.1 build
> rescript

Dependency on @ryyppy/rescript-promise
rescript: [5/5] src/Promise.cmj
rescript: [7/7] install.stamp
Dependency Finished
rescript: [3/3] src/Demo.cmj

$ node src/Demo.bs.js
/Users/impvd/Downloads/todel/rescript-project-template/src/Demo.bs.js:6
var NodeFetch = require("node-fetch");
                ^

Error [ERR_REQUIRE_ESM]: require() of ES Module /Users/impvd/Downloads/todel/rescript-project-template/node_modules/node-fetch/src/index.js from /Users/impvd/Downloads/todel/rescript-project-template/src/Demo.bs.js not supported.
Instead change the require of index.js in /Users/impvd/Downloads/todel/rescript-project-template/src/Demo.bs.js to a dynamic import() which is available in all CommonJS modules.
    at Object.<anonymous> (/Users/impvd/Downloads/todel/rescript-project-template/src/Demo.bs.js:6:17) {
  code: 'ERR_REQUIRE_ESM'
}

I had tried the combinations of:

• type to module
• setting the suffix to .mjs
• run npx rescript build -with-deps
• "module": "commonjs" to "module": "es6" off and on.

I'm new to NodeJS, just wondering if there's anything that I'd missed?