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ReScript: The Module or File Can't Be Found, Unabridged

webbureaucrat
functional programming enthusiast and civic tech nerd.
Originally published at webbureaucrat.gitlab.io on ・2 min read

If you work with BuckleScript, now ReScript, you'll likely come across the common build error message, "The module or file $name can't be found" followed by a few helpful suggestions for how to properly install a ReScript module.It's a good error message, but I've found it underestimates me in my ability to get things wrong, especially if I'm the one who wrote the missing module to begin with. I'm going to write a guide while one of these problems is still fresh in my mind so that I have a checklist to go through the next time I get frustrated.

I'm going to start with the supply side of the module. If you're referencing someone else's third party and can assume that their code works on a basic level, feel free to skip ahead to "Consuming your ReScript package."

If you wrote the ReScript module you're having trouble adding

Check the name property in package.json

Make sure your package is what you expected it to be in npm.

Check the namespace property in bsconfig.json

The naming convention for BuckleScript is to begin the package name with bs-, and the emerging naming convention for ReScript is to begin the package name with res-. Meanwhile, the naming convention for modules is to drop the prefix. (E. g., bs-service-worker and then open ServiceWorker;.) However,when scaffolding the namespace attribute of bsconfig.json often defaults to true, which strips special characters like "-" and Pascal-cases the name, but otherwise leaves the prefix attached.

The solution is to set the namespace explicitly. Instead of true, use the module name that fits the module, like "ServiceWorker" forbs-service-worker.

Make sure you've run npm publish recently.

Consider setting up CI/CD on git push.

Consuming your ReScript package

Make sure your package is properly installed, both places.

Make sure the package is installed by npm into your node_modules, and make sure the package is listed in your bsconfig.json as a dependency.

Check the namespace of your package

Your package's namespace is determined by the namespace attribute of the bsconfig.json (as described above). You need to know it to open it.

Make sure you haven't already opened the package at the top of the file

Yes, this is also something I have done.

This closes out the list of possible causes I'm aware of so far, but I believe that if I work hard and believe in myself I'm bound to find more ways into this compiler error, and when I do, I'll revisit this post.

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