Build System Overview

ReScript comes with a build system, bsb, that's meant to be fast, lean and used as the authoritative build system of the community.

Bsb provides a few templates to quickly start a new project:

bsb -init my-directory-name

Feel free to inspect the various files in the newly generated directory. To see all the templates available, do:

bsb -themes

The build description file is called bsconfig.json. Every ReScript project needs one.

To build a project, run:

bsb -make-world

Add -w to keep the built-in watcher running. Any new file change will be picked up and the build will re-run.

Note: third-party libraries (in node_modules) aren't watched, as doing so may exceed the node.js watcher count limit. If you're doing quick and dirty modifications inside node_modules, you have to do bsb -clean-world -make-world to rebuild them.

Note 3: If you are developing across multiple devices, you may find the -ws configuration useful in order to have live-reloading across the network. Possible configurations are:

  • bsb -make-world -w -ws _ (default)

  • bsb -make-world -w -ws

  • bsb -make-world -w -ws 5000

To build only yourself, use bsb.

bsb -help to see all the available options.

Artifacts Cleaning

If you ever get into a stable build for edge-case reasons, use:

bsb -clean-world

Or bsb -clean to clean only your own artifacts.

Editor Support

Bsb generates a .merlin file, used by various editor plugins under the hood to power e.g. autocomplete, type hint, diagnosis, etc.

Tips & Tricks

A typical problem with traditional build systems is that they're not resilient against the user moving/deleting source files. Most don't clean up the old artifacts correctly after such user action*. Bsb is unfortunately no different, unless you turn on "suffix": ".bs.js" in bsconfig.json, in which case we can track which JS artifact belongs to which source file correctly, even against source file moving/deletion.

Design Decisions

* One such build system that tracks these correctly & efficiently is Tup. See the (rather accessible!) paper here. Unfortunately, Tup's implementation uses FUSE and other systems, which we can't safely use on every platform.