Skip to content


Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?


Failed to load latest commit information.
Latest commit message
Commit time

Hazel Build Status

Hazel Mascot

Hazel is a live functional-programming environment rooted in the principles of type theory. You can find the relevant papers and more motivation at the Hazel website.

You can try Hazel online with either the trunk or dev version. Note that the trunk branch is updated infrequently and is currently almost two years behind!

Building and Running Hazel

Short version

If you already have ocaml version 4.13.1 and least version 2.0 of opam installed, you can build Hazel by running the following commands.

  • git clone
  • cd hazel
  • make deps
  • make dev

To run Hazel, run the command make echo-html, which will print a filename. Then use your preferred browser to open that file. For convenience, the following make targets open the corresponding browser or invoke the corresponding command immediately (see

  • make firefox
  • make chrome
  • make chrome-browser
  • make chromium
  • make chromium-browser
  • make win-chrome
  • make win-firefox
  • make xdg-open
  • make open

Long Version

If you are unfamiliar with ocaml or opam, do not have them installed, or just get stuck, we recommend you follow the step-by-step installation instructions contained in


From OCaml to ReasonML

This link lets you type OCaml and see what the corresponding ReasonML syntax is:

This is useful if you are trying to figure out the ReasonML syntax for something that you know the OCaml syntax for.

Suggested Extensions for VS Code

Most of our team uses VisualStudio Code to write code. If you use VS Code, here are a few extensions that might be helpful.

In addition to these extensions, enabling the breadcrumbs bar can make navigating a large code base easier. There are multiple ways to make the breadcrumbs bar visible:

  • Click View / Show Breadcrumbs from the menu bar.
  • Press Ctrl+Shift+P (macOS: Cmd+Shift+P), start typing breadcrumbs, and select View: Toggle Breadcrumbs from the dropdown menu to toggle breadcrumbs on and off.
  • Press Ctrl+Shift+. to start breadcrumbs navigation.

Suggested Setup for NeoVim

The recommended setup is to use Neovim's built-in LSP support. We recommend using the nvim-lspconfig plugin. See the plugin's main page for recommended setup instructions.

To set-up Reason ML support, enable ocamllsp in your configuration:

local lspconfig = require('lspconfig')
lspconfig.ocamllsp.setup { on_attach = on_attach }

Build System Details

Hazel is implemented in Reason (a dialect of OCaml) and is compiled to Javascript for the web browser via the js_of_ocaml compiler.

Though make targets are provided as a convenience, they mostly translate to dune commands.

Invoking make by itself is equivalent to invoking make dev. With these commands we pass additional flags to js_of_ocaml that cause the insertion of comments that map locations in the generated JS to locations in the source files. This is useful for debugging purposes.

make dev also auto-formats Reason source files using refmt (this is what the @src/fmt alias is for). This ensures code from all contributors follows the same style.

The make dev and make release commands do three things:

  1. Generate some internal parsers using menhir.
  2. Compile the Reason code to OCaml bytecode using the OCaml compiler.
  3. Compile the OCaml bytecode to JavaScript (_build/default/src/hazelweb/www/hazel.js) using js_of_ocaml.

For a smoother dev experience, use make watch to automatically watch for file changes. This will require installing fswatch (see


You can print to the browser console using the standard print_endline function. This is probably the easiest method right now.

js_of_ocaml does support source maps and has some other flags that might be useful. If you experiment with those and get them to work, please update this README with some notes.


You can run all of the unit tests located in src/hazelcore/test by running make test.

Unit tests are written using ppx_expect and ppx_inline_tests. If you would like to adjust your expect tests to assert for the output that was last printed, run make fix-test-answers.

If the inline test runner causes problems for you, you can likely resolve the issue by running opam update then opam upgrade.

Continuous Integration

When you push your branch to the main hazelgrove/hazel repository, we have a GitHub Action setup (see .github/workflows/deploy_branches.yml) that will build that branch (in release mode) and deploy it to the URL<branch name>, assuming the build succeeds.

It usually takes about 2 minutes if the build environment cache hits, or 20+ minutes if not. You can view the status of the build in the Actions tab on Github.