Contributing guidelines

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Clone the repository:

$ git clone

The project can then be built with both cabal build and stack build.

Using Cabal

# If you have not run `cabal update` in a while
$ cabal update
# Then
$ cabal build

Using Stack

$ stack build

Using Nix

The instructions below show how to set up a Cachix binary cache and open a nix shell for local development.

$ cachix use haskell-language-server
$ nix-shell
$ cabal update
$ cabal build

Flakes support

If you are using nix 2.4 style command (enabled by experimental-features = nix-command), you can use nix develop instead of nix-shell to enter the development shell. To enter the shell with specific GHC versions:

  • nix develop - default GHC version

  • nix develop .#shell-ghc90 - GHC 9.0.1 (substitute GHC version as appropriate)

If you are looking for a Nix expression to create haskell-language-server binaries, see


The tests make use of the Tasty test framework.

There are two test suites in the main haskell-language-server package, functional tests, and wrapper tests. Some of the wrapper tests expect stack to be present on the system, or else they fail. Other project packages, like the core library or plugins, can have their own test suite.

Testing with Cabal

Running all the tests

$ cabal test

Running just the functional tests

$ cabal test func-test

Running just the wrapper tests

$ cabal test wrapper-test

Running a subset of tests

Tasty supports providing Patterns as command line arguments, to select the specific tests to run.

$ cabal test func-test --test-option "-p hlint"

The above recompiles everything every time you use a different test option though.

An alternative, which only recompiles when tests (or dependencies) change:

$ cabal run haskell-language-server:func-test -- -p "hlint enables"

Using HLS on HLS code

Refer to the HLS project configuration guidelines as they also apply to the HLS project itself.

Note: HLS implicitly detects the HLS codebase as a Stack project (since there is a stack.yaml file). If you want HLS to use Cabal, create this hie.yaml file at the root of the project:


Manually testing your hacked HLS

If you want to test HLS while hacking on it (you can even test it on HLS codebase itself, see previous section), you need to:

  1. (Once) Find the path to the hacked HLS you build

  2. (Once) Configure your editor to use it

  3. (Every time you change the HLS code) Rebuild HLS

  4. (Every time you change the HLS code) Restart the LSP workspace

Find the path to the hacked HLS you build

Note that unless you change the GHC version or the HLS version between builds, the path should remain the same, this is why you need to set it only once.

Using Cabal


$ cabal build exe:haskell-language-server && cabal list-bin exe:haskell-language-server
<some long path>/haskell-language-server

Using Stack


$ echo $(pwd)/$(stack path --dist-dir)/build/haskell-language-server/haskell-language-server
<some long path>/haskell-language-server

Configure your editor to use it

VS Code

When using VS Code you can set up each project to use a specific HLS executable:

  • If it doesn’t already exist in your project directory, create a directory called .vscode.

  • In the .vscode directory create a file called settings.json with the below contents.

    "haskell.serverExecutablePath": "/path/to/your/hacked/haskell-language-server"


There are several ways to configure the HLS server path:

  • M-x customize-group<RET>lsp-haskell<RET>Lsp Haskell Server Path

  • Evaluate (setq lsp-haskell-server-path "/path/to/your/hacked/haskell-language-server")

  • Create a file .dir-locals.el with the following content:

((haskell-mode . ((lsp-haskell-server-path . "/path/to/your/hacked/haskell-language-server"))))

Rebuild HLS

  • With Stack: stack build haskell-language-server:exe:haskell-language-server

  • With Cabal: cabal build exe:haskell-language-server

Restart the LSP workspace

  • With VS Code: Press Ctrl + Shift + p and type Haskell: Restart Haskell LSP Server

  • With Emacs: M-x lsp-workspace-restart

Style guidelines

The project includes a .editorconfig file with the editor basic settings used by the project. However, most editors will need some action to honour those settings automatically. For example vscode needs to have installed a specific extension. Please, try to follow those basic settings to keep the codebase as uniform as possible.

Formatter pre-commit hook

We are using pre-commit to configure git pre-commit hook for formatting. Although it is possible to run formatting manually, we recommend you to use it to set pre-commit hook as our CI checks pre-commit hook is applied or not.

If you are using Nix or Gitpod, pre-commit hook is automatically installed. Otherwise, follow instructions on to install the pre-commit tool, then run the following command:

pre-commit install

Why some components are excluded from automatic formatting?

  • test/testdata and test/data are there as we want to test formatting plugins.

  • hie-compat is there as we want to keep its code as close to GHC as possible.

Introduction tutorial

See the tutorial on writing a plugin in HLS.

Measuring, benchmarking and tracing


If you are touching performance sensitive code, take the time to run a differential benchmark between HEAD and master using the benchHist script. This assumes that “master” points to the upstream master.

Run the benchmarks with cabal bench.

It should take around 25 minutes and the results will be stored in the bench-results folder. To interpret the results, see the comments in the bench/Main.hs module.

More details in bench/README


HLS records opentelemetry eventlog traces via opentelemetry. To generate the traces, build with -eventlog and run with +RTS -l. To visualize the traces, install Tracy and use eventlog-to-tracy to open the generated eventlog.

Adding support for a new editor

Adding support for new editors is fairly easy if the editor already has good support for generic LSP-based extensions. In that case, there will likely be an editor-specific support system for this (like lsp-mode for Emacs). This will typically provide instructions for how to support new languages.

In some cases you may need to write a small bit of additional client support, or expose a way for the user to set the server’s configuration options and for them to configure how the server is started.

Building the docs

The docs are built with Sphinx and ReadTheDocs, the documentation for both is helpful.

To build the docs you need to install some Python prerequisites. You can either pip install -r docs/requirements.txt, or simply enter a nix-shell.

Then to build and preview the docs:

cd docs
make html
firefox _build/html/index.html

Alternatively, you can build the entire thing as a Nix derivation from the flake with nix build .#docs.

The docs are also built and previewed on every PR, so you can check them from the PR status.

Working on code actions

To make HLS easier to maintain, please follow these design guidelines when adding or modifying code actions:

  1. Prefer ghc-exactprint to manual text parsing.

  2. Prefer ghc-exactprint to manual code generation.

  3. Code generating actions should not try to format the generated code. Assume that the user is also leveraging HLS for automated code formatting.

  4. Put new code actions in their own plugin unless they are very closely aligned with an existing ghcide code action.


If you want to contribute financially you can do so via open-collective. In the past the funding has been used to sponsor summer student projects.