Docs / BuckleScript / UpgradeToV7

Upgrade Guide to v7

If you have been using BuckleScript for the last couple of years, you probably noticed that upgrading to the newest version has always been a seamless experience without any breaking changes or performance loss. That's because the BuckleScript team puts a lot of effort in making sure users can upgrade as soon as possible without any changes in their workflow.

That said, this time with the v7 release, there have been some fundamental compiler changes, which means that upgrades will involve a few more installation steps than usual (depending on your project setup). Luckily, the upgrade should still be straight forward and easy to do.

This guide will highlight some of the changes and give you a list of instructions to upgrade your BuckleScript projects.

Note: You probably noticed that the version number jumped from v5 to v7. The v6 version was used for the transition phase, so if you are using v6 right now, just upgrade to bs-platform@7 and you are good to go.

A Quick Look under the Hood

BuckleScript v7 (and v6) changed the underlying OCaml version from 4.02.3 (BS v5) to 4.06.1. Usually this shouldn't impact any existing codebases, unless they are using PPXes or other meta programming facilities. Simply explained, PPXes rely on the data representation of the OCaml / Reason syntax (AST), so if you are using a tool such as graphql_ppx (which has usually been compiled natively with a 4.02.3 OCaml version) you will get a compatibility error when used with BuckleScript v7.

Luckily during the transition phase between v5 and v7 (which has been an ongoing process for for several months), the community has been coordinating efforts to upgrade and release commonly used tools to BS v7 compatible compile artifacts. You will find all installation instructions in this guide as well.

BS Version Overview / Features

  • BS 5.x and below: based on OCaml 4.02.3

  • BS 6.x and above: based on OCaml 4.06.1 (transition version: if in use, upgrade to v7 now)

  • BS 7.x and above: based on OCaml 4.06.1 (stops 4.02.3 support)

    • Adds Inline Records support (introduced with 4.03)

    • Adds the Records-as-Objects feature, which makes BS compile record type values into JS objects instead of nested arrays (see blog post 1 and post 2)

    • Ships with refmt version 3.5.1 with greatly improved syntax error messages (codename reerror)

The biggest upside of upgrading to BS v7, beside enjoying newer language features, is that we will finally be able to deprecate underlying OCaml 4.02.3 code, which will make maintenance way easier (lots of code / processes can be removed and simplified). Also the OCaml community will greatly benefit from this change, since a lot of OCaml libraries still have to support a minimum OCaml version of 4.02.3 just for BuckleScript <= v5 compatibility.

We recommend upgrading to BS v7 as soon as possible. Your Reason code will still work as expected, you'll only need to upgrade your native tools to make everything behave correctly.

Please support us moving forward and help us by providing bug reports in case you have any issues.

How to upgrade to v7


Note: During the migration phase, some projects / repositories have changed owners or Github organizations to make it easier for us to upgrade build artifacts in the future. If you are using a PPX we are not aware of, please let us know in this issue.

Upgrade your bs-platform dependency:

  • npm install bs-platform@7 --dev

Upgrade any PPX dependencies you're using:

graphql_ppx -> graphql_ppx_re:

  • Remove the deprecated graphql_ppx (by mhallin): npm remove graphql_ppx

  • Add the new one: npm install @baransu/graphql_ppx_re --save-dev

  • Update your ppx-flags: "ppx-flags": ["@baransu/graphql_ppx_re/ppx6"]


  • npm install gentype@3 --save-dev


  • npm install decco@1 --save-dev


  • npm install --save-dev bs-let

  • Update your ppx-flags: "ppx-flags": ["bs-let/ppx"]


  • npm install bs-emotion-ppx@1 --save-dev

  • Update your ppx-flags: "ppx-flags": ["@ahrefs/bs-emotion-ppx/bin/bs-emotion-ppx"]



  • No upgrade activity yet, please let us know if you are using it!

Clean & Rebuild your project:

  • npx bsb -clean-world

  • npx bsb -make-world

  • Your project should now be built and working correctly

Fix specific warnings:

  • BuckleScript warning: isWebUri : the external name is inferred from val name is unsafe from refactoring when changing value name

    • This means that you have external bindings ending with = ""; (which will automatically take the name of the binding itself), you now need to be explicit with the name. Example:

      /* Before (warning) */ [@bs.module "valid-url"] external isWebUri: string => option(string) = ""; /* After (fixed the warning) */ [@bs.module "valid-url"] external isWebUri: string => option(string) = "isWebUri";
  • Don't forget to rebuild your project again

Commit your changes:

  • Your .bs.js files will very likely change, make sure to commit everything to have a clean BS upgrade commit

Note: Since this upgrade will change the internal data representation of records to JS objects, make sure you are not having any Reason code relying on the previous record-as-array representation. This is a quick reminder that it is generally discouraged to rely on internal data structures.

Editor extensions

How to upgrade PPXes (for tool maintainers)

Step 1: Upgrade build dependencies

We recommend using esy for building your native project, so all instructions are based on the esy workflows. Mostly it's about setting the OCaml version to 4.06 and rebuild the project accordingly (e.g. in your package.json):

"devDependencies": { "ocaml": "~4.6.1000", "@opam/dune": "*", }

If you are using migrate-parse-tree, you might want to drop any AST upgrades from 4.02 -> 4.06 etc, making the native binary more lightweight.

Tip: If you still want to maintain a 4.02.3 based version (for legacy reasons), you can alternatively create a esy.json / esy-406.json file and use esy's sandboxing feature.

We know that these instructions are very rough, many projects use different configurations. To give you some inspiration on how to tackle an upgrade, here is a list of migrations / project setups you might find useful:

Step 2: Release artifacts and add a proper Note in the README

Make sure to release the newly built binary on npm and make clear which PPX versions are compatible with BS6 / BS5 etc. For example, this is how gentype does it:

bs-platform 7.0.0 or higher: use genType 3.2.0 or higher. bs-platform 6.2.0 or higher: use genType 3.0.0 or higher. bs-platform 5.2.0 or higher: use genType 2.40.0 or higher. bs-platform 5.0.x and 5.1.x: use genType 2.17.0 or higher.


We are really excited for the upcoming future of BuckleScript. The v7 release will make JS interop considerably easier while being completely compatible with existing codebases.

If you are experiencing any installation / upgrade issues, please be sure to either check the BuckleScript issue tracker, or drop us a message in Discord.