haskell-language-server: LSP server for GHC

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Please see the README on GitHub at https://github.com/haskell/haskell-language-server#readme

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Change log ChangeLog.md
Dependencies aeson, base (>=4.12 && <5), binary, brittany (>=, bytestring, containers, data-default, deepseq, directory, extra, filepath, floskell (>=0.10 && <0.11), fourmolu (>=0.3 && <0.4), fuzzy, ghc, ghc-boot-th, ghc-paths, ghcide (>=0.7), gitrev, hashable, haskell-language-server, haskell-lsp (>=0.23 && <0.24), hie-bios, hls-class-plugin, hls-eval-plugin, hls-explicit-imports-plugin, hls-haddock-comments-plugin, hls-hlint-plugin, hls-plugin-api (>=0.6), hls-retrie-plugin, hls-splice-plugin, hls-tactics-plugin, hslogger, lens, mtl, optparse-applicative, optparse-simple, ormolu (>=0.1.2 && <0.2), process, regex-tdfa, safe-exceptions, shake (>=0.17.5), stylish-haskell (>=0.12 && <0.13), temporary, text, transformers, unordered-containers, with-utf8 [details]
License Apache-2.0
Copyright The Haskell IDE Team
Author The Haskell IDE Team
Maintainer alan.zimm@gmail.com
Category Development
Home page https://github.com/haskell/haskell-language-server#readme
Bug tracker https://github.com/haskell/haskell-language-server/issues
Source repo head: git clone https://github.com/haskell/haskell-language-server
Uploaded by PepeIborra at 2021-02-07T18:23:17Z


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Integration point for ghcide and haskell-ide-engine. One IDE to rule them all. Read the project's background.





If you are using ghcup to manage your installations, you can install the latest version of haskell-language-server with

ghcup install hls

Visual Studio Code

If you are using Visual Studio Code, the Haskell extension will automatically download and install haskell-language-server for you.

Pre-built binaries

There are pre-built binaries available from the releases page for Linux, Windows and macOS. To install, download the haskell-language-server-wrapper executable for your platform as well as any haskell-language-server executables for the GHC versions you plan on working with, and either put them on your PATH or point your client to them.

Installation from source

Common pre-requirements

Tip: you can quickly check if some command is in your path by running the command. If you receive some meaningful output instead of "command not found"-like message then it means you have the command in PATH.

Linux-specific pre-requirements

On Linux you will need install a couple of extra libraries:

Debian 9/Ubuntu 18.04 or earlier:

sudo apt install libicu-dev libtinfo-dev libgmp-dev zlib1g-dev

Debian 10/Ubuntu 18.10 or later:

sudo apt install libicu-dev libncurses-dev libgmp-dev zlib1g-dev


sudo dnf install libicu-devel ncurses-devel zlib-devel

Windows-specific pre-requirements

In order to avoid problems with long paths on Windows you can do either one of the following:

  1. Clone the haskell-language-server to a short path, for example the root of your logical drive (e.g. to C:\hls). Even if you choose C:\haskell-language-server you could hit the problem. If this doesn't work or you want to use a longer path, try the second option.

  2. If the Local Group Policy Editor is available on your system, go to: Local Computer Policy -> Computer Configuration -> Administrative Templates -> System -> Filesystem set Enable Win32 long paths to Enabled. If you don't have the policy editor you can use regedit by using the following instructions here. You also need to configure git to allow longer paths by using unicode paths. To set this for all your git repositories use git config --system core.longpaths true (you probably need an administrative shell for this) or for just this one repository use git config core.longpaths true.

In addition make sure haskell-language-server.exe is not running by closing your editor, otherwise in case of an upgrade the executable can not be installed.

Download the source code

git clone https://github.com/haskell/haskell-language-server --recurse-submodules
cd haskell-language-server


Note, on first invocation of the build script with stack, a GHC is being installed for execution. The GHC used for the install.hs can be adjusted in ./install/stack.yaml by using a different resolver.

Available commands can be seen with:

stack ./install.hs help

Remember, this will take time to download a Stackage-LTS and an appropriate GHC for build haskell-language-server the first time.

Install via cabal

The install-script can be invoked via cabal instead of stack with the command

cabal v2-run ./install.hs --project-file install/shake.project <target>

or using the existing alias script

./cabal-hls-install <target>

Running the script with cabal on windows requires a cabal version greater or equal to

For brevity, only the stack-based commands are presented in the following sections.

Install specific GHC Version

The script will install the executables haskell-language-server-wrapper and haskell-language-server.

It will copy the latter appending the used ghc version, needed by the wrapper to choose the suitable version for the project at hand.

So installing the executables directly with stack install or cabal v2-install may not be enough for it to work properly.

Install haskell-language-server for the latest available and supported GHC version (and hoogle docs):

stack ./install.hs hls

Install haskell-language-server for a specific GHC version (and hoogle docs):

stack ./install.hs hls-8.8.3

hls-8.8.3 target will build the project and install haskell-language-server-wrapper, haskell-language-server, haskell-language-server-8.8.3 and haskell-language-server-8.8 executables.

The Haskell Language Server can also be built with cabal v2-build instead of stack build. This has the advantage that you can decide how the GHC versions have been installed. To see what GHC versions are available, the command cabal-hls-install ghcs can be used. It will list all supported GHC versions that are on the path for build with their respective installation directory. If you think, this list is incomplete, you can try to modify the PATH variable, such that the executables can be found. Note, that the targets hls and data depend on the found GHC versions.

An example output is:

> ./cabal-hls-install ghcs
Found the following GHC paths:
ghc-8.6.5: /opt/bin/ghc-8.6.5
ghc-8.8.3: /opt/bin/ghc-8.8.3


If your desired ghc has been found, you use it to install haskell-language-server.

./cabal-hls-install hls-8.6.5

Installation from Hackage

Direct installation from Hackage, while possible via cabal install haskell-language-server, is not recommended for most people. Said command builds the haskell-language-server binary and installs it in the default Cabal binaries folder, but the binary will only work with projects that use the same GHC version that built it.

Configuring haskell-language-server

Language servers like haskell-language-server expose most of their configuration via the client (i.e. the editor). That means that the way in which you configure the settings will depend on the client.

Most clients (editors) already have an opinion about how settings should be configured! For example, in VS Code you use the graphical Settings tab or settings.json, whereas in Emacs you use customization variables. In the editor configuration section we give some pointers for popular editors, but you should consult the documentation for your specific editor if you have trouble.

However, we can say some high-level things about the kinds of configuration haskell-language-server uses, and how to use them. This can sound a bit confusing, but ultimately the client should present you with these options in a user-friendly way that makes sense for that editor.

Generic server options

The LSP protocol is designed to support many useful server configuration options generically. These are sent to the server by the client, and can be controlled without reference to a specific language.

For example, there are protocol methods for highlighting matching identifiers throughout a document. This is a capability which any server can implement, so the client can decide generically whether to ask the server to do it or not. So your editor can provide a setting to turn this on or off globally, for any language server you might use.

Settings like this are typically provided by the generic LSP client support for your editor, for example in Emacs by lsp-mode.

Generic editor options

Your editor may provide some settings that affect how the information from the language server is used. For example, whether popups are shown, or whether code lenses appear by default.

Settings like this are typically provided by the generic LSP client support for your editor, for example in Emacs by lsp-mode.

Language-specific server options

A specific language server can also have its own configuration options. These are still sent to the server by the client, but they can only be controlled by a specific client that knows about those options.

For example, haskell-language-server allows you to choose the formatting provider which will be used for formatting Haskell source. This option obviously would not make sense for language servers for other languages, or even for other Haskell language servers (which need not even support formatting).

Here is a list of the additional settings currently supported by haskell-language-server, along with their setting key (you may not need to know this) and default:

Settings like this are typically provided by the language-specific LSP client support for your editor, for example in Emacs by lsp-haskell.

Client options

A particular client might also have some options of its own, for example to control how the server executable is started.

Settings like this are typically be provided by the language-specific LSP client support for your editor, for example in Emacs by lsp-haskell.

Configuring your project build

haskell-language-server has to compile your project in order to give you diagnostics, which means that it needs to know how to do so. This is handled by the hie-bios project.

For a full explanation of how hie-bios determines the project build configuration, and how to configure it manually, refer to the hie-bios README.

At the moment, haskell-language-server has some limited support to automatically detect your project build configuration. The plan is to improve it to handle most use cases.

However, for now, the most reliable way is to manually configure hie-bios using a hie.yaml file in the root of the workspace. A hie.yaml file explicitly describes how to setup the environment to compile the various parts of your project. For that you need to know what components your project has, and the path associated with each one. So you will need some knowledge about stack or cabal components.

You also can use this utility to automatically generate hie.yaml files for the most common stack and cabal configurations

For example, to state that you want to use stack then the configuration file would look like:

    component: "haskell-language-server:lib"

If you use cabal then you probably need to specify which component you want to use.

    component: "lib:haskell-language-server"

If you have a project with multiple components, you can use a cabal-multi cradle:

    - path: "./test/functional/"
      component: "haskell-language-server:func-test"
    - path: "./test/utils/"
      component: "haskell-language-server:hls-test-utils"
    - path: "./exe/Main.hs"
      component: "haskell-language-server:exe:haskell-language-server"
    - path: "./exe/Wrapper.hs"
      component: "haskell-language-server:exe:haskell-language-server-wrapper"
    - path: "./src"
      component: "lib:haskell-language-server"
    - path: "./ghcide/src"
      component: "ghcide:lib:ghcide"
    - path: "./ghcide/exe"
      component: "ghcide:exe:ghcide"

Equivalently, you can use stack:

    - path: "./test/functional/"
      component: "haskell-language-server:func-test"
    - path: "./exe/Main.hs"
      component: "haskell-language-server:exe:haskell-language-server"
    - path: "./exe/Wrapper.hs"
      component: "haskell-language-server:exe:haskell-language-server-wrapper"
    - path: "./src"
      component: "haskell-language-server:lib"
    - path: "./ghcide/src"
      component: "ghcide:lib:ghcide"
    - path: "./ghcide/exe"
      component: "ghcide:exe:ghcide"

Or you can explicitly state the program which should be used to collect the options by supplying the path to the program. It is interpreted relative to the current working directory if it is not an absolute path.

    program: ".hie-bios"

The complete configuration is a subset of

    component: "optional component name"
    component: "optional component name"
    program: "program to run"
    dependency-program: "optional program to run"
    arguments: ["list","of","ghc","arguments"]

  - someDep

Configuring your editor

Most editors provide a Haskell-specific extension that provides support for launching haskell-language-server and talking to it, as well as exposing configuration options.

Editors typically assume that you have already installed haskell-language-server (see above) and that the installation script put the haskell-language-server and haskell-language-server-wrapper binaries in your PATH (usually ~/.local/bin or ~/.cabal/bin on Linux and macOS, %APPDATA%\local\bin or %APPDATA%\cabal\bin on Windows). The exception is VS Code, which can automatically install the binaries if they are not installed already.

Using Haskell Language Server with VS Code

Install from the VSCode marketplace, or manually from the repository vscode-haskell. The haskell-language-server and haskell-language-server-wrapper binaries will be automatically downloaded on an ad-hoc basis, but if you have them already installed on your PATH then it will just use them instead.

Configuration is done via the "Haskell" section of "Settings".

Using Haskell Language Server with Sublime Text

  "clients": {
    "haskell-language-server": {
      "command": ["haskell-language-server-wrapper", "--lsp"],
      "scopes": ["source.haskell"],
      "syntaxes": ["Packages/Haskell/Haskell.sublime-syntax"],
      "languageId": "haskell"

Now open a Haskell project with Sublime Text and enable Language Server in the project. You should have these features available:

  1. Errors are underlined in red
  2. LSP: Show Diagnostics will show a list of hints and errors
  3. LSP: Format Document will prettify the file

Using Haskell Language Server with Vim or Neovim

You can use Coc, LanguageClient-neovim or any other Vim Language server protocol client. Coc is recommend since it is the only complete LSP implementation for Vim and Neovim and offers snippets and floating documentation out of the box.


Follow Coc's installation instructions. Then issue :CocConfig and add the following to your Coc config file.

  "languageserver": {
    "haskell": {
      "command": "haskell-language-server-wrapper",
      "args": ["--lsp"],
      "rootPatterns": ["*.cabal", "stack.yaml", "cabal.project", "package.yaml", "hie.yaml"],
      "filetypes": ["haskell", "lhaskell"]



If you use vim-plug, then you can do this by e.g., including the following line in the Plug section of your init.vim or ~/.vimrc:

Plug 'autozimu/LanguageClient-neovim', {
    \ 'branch': 'next',
    \ 'do': 'bash install.sh'
    \ }

and issuing a :PlugInstall command within Neovim or Vim.

Clone the LanguageClient-neovim repo

As an alternative to using vim-plug shown above, clone LanguageClient-neovim into ~/.vim/pack/XXX/start/, where XXX is just a name for your "plugin suite".

Configuration and sample ~/.vimrc sections
set rtp+=~/.vim/pack/XXX/start/LanguageClient-neovim
let g:LanguageClient_serverCommands = { 'haskell': ['haskell-language-server-wrapper', '--lsp'] }

You'll probably want to add some mappings for common commands:

nnoremap <F5> :call LanguageClient_contextMenu()<CR>
map <Leader>lk :call LanguageClient#textDocument_hover()<CR>
map <Leader>lg :call LanguageClient#textDocument_definition()<CR>
map <Leader>lr :call LanguageClient#textDocument_rename()<CR>
map <Leader>lf :call LanguageClient#textDocument_formatting()<CR>
map <Leader>lb :call LanguageClient#textDocument_references()<CR>
map <Leader>la :call LanguageClient#textDocument_codeAction()<CR>
map <Leader>ls :call LanguageClient#textDocument_documentSymbol()<CR>

Use Ctrl+xCtrl+o (<C-x><C-o>) to open up the auto-complete menu, or for asynchronous auto-completion, follow the setup instructions on LanguageClient.

If you'd like diagnostics to be highlighted, add a highlight group for ALEError/ALEWarning/ALEInfo, or customize g:LanguageClient_diagnosticsDisplay:

hi link ALEError Error
hi Warning term=underline cterm=underline ctermfg=Yellow gui=undercurl guisp=Gold
hi link ALEWarning Warning
hi link ALEInfo SpellCap

If you're finding that the server isn't starting at the correct project root, it may also be helpful to also specify root markers:

let g:LanguageClient_rootMarkers = ['*.cabal', 'stack.yaml']

Further configuration can be done by pointing the g:LanguageClient_settingsPath variable to the file in which you want to keep your LSP settings.

Using Haskell Language Server with Atom

Install the two Atom packages atom-ide-ui and haskell,

$ apm install language-haskell atom-ide-ui haskell

Using haskell-language-server with Emacs

Emacs support is provided by a combination of the following packages:

lsp-mode lsp-ui lsp-haskell

You can install these manually if you are using plain Emacs; instructions for some specific flavours are included below.

Make sure to check the READMEs of each of these packages, which explain how to configure the various parts of the Emacs integration. In particular, lsp-haskell provides customization options for the haskell-language-server-specific parts, such as the path to the server executable.

Using haskell-language-server with doom-emacs

Manual installation of packages is not required. Enable the lsp module and the haskell lang module with lsp flag in .doom.d/init.el:

;; ...
(haskell +lsp)

then do $HOME/.emacs.d/bin/doom sync

Using haskell-language-server with Spacemacs

Manual installation of packages is not required. Enable the haskell layer and the lsp layer in your Spacemacs config file:

    ;; ...

Using haskell-language-server with Kakoune

  1. Grab a copy of kak-lsp, and follow the setup instructions.
  2. Point your kak-lsp.toml to haskell-language-server-wrapper.
filetypes = ["haskell"]
roots = ["Setup.hs", "stack.yaml", "*.cabal"]
command = "haskell-language-server-wrapper"
args = ["--lsp"]

Known limitations


HLS is not yet able to find project preprocessors, which may result in could not execute: <preprocessor> errors. This problem is tracked in https://github.com/haskell/haskell-language-server/issues/176 and originally comes from https://github.com/mpickering/hie-bios/issues/125

As a workaround, you need to ensure the preprocessor is available in the path (install globally with Stack or Cabal, provide in shell.nix, etc.).

Example with tasty-discover:

{-# OPTIONS_GHC -F -pgmF tasty-discover #-}

This returns an error in HLS if 'tasty-discover' is not in the path: could not execute: tasty-discover.


Common issues

Difficulties with Stack and Paths_ modules

These are known to be somewhat buggy at the moment: https://github.com/haskell/haskell-language-server/issues/478. This issue should be fixed in Stack versions >= 2.5.

Problems with dynamic linking

As haskell-language-server prebuilt binaries are statically linked, they don't play well with projects using dynamic linking. An usual symptom is the presence of errors containing unknown symbol and it is typical in arch linux, where a dynamically linked version of ghc is used.

The workaround is to use a version of haskell-language-server compiled from source with -dynamic enabled`. See more details here.

Troubleshooting the server

Diagnostic mode

The haskell-language-server executable can be run in diagnostic mode, where it will just try to load modules from your project, printing all of its output to stdout. This makes it much easier to see what's going on and to diagnose build-related problems.

To do this, simply run the executable directly from your shell in the project root. You can either run it without an argument, in which case it will load random modules, or with a path, in which case it will load modules in that file or directory.

Examining the log

Most clients will launch haskell-language-server with --logfile to make it write a log file. Please consult the documentation for your client to find out where this is (or how to set it).

The log will contain all the messages that are sent to the server and its responses. This is helpful for low-level debugging: if you expect a certain action to happen, you can look in the log to see if the corresponding messages are sent, or if there are any errors.

To get a more verbose, also pass --debug to the executable.

Troubleshooting the client

Many clients provide diagnostic information about a LSP session. In particular, look for a way to get the status of the server, the server stderr, or a log of the messages that the client has sent to the server. For example, lsp-mode provides all of these (see its troubleshooting page for details).

The most common client-related problem is the client simply not finding the server executable, so make sure that you have the right PATH and you have configured it to look for the right executable.


❤️ The Haskell tooling dream is near, we need your help! ❤️

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.

Building haskell-language-server

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

haskell-language-server can also be used with itself. We provide preset samples of hie.yaml for Cabal and Stack.

Note: the ./install/ folder is not directly tied to the project so it has dedicated ./install/hie.yaml.[cbl|stack] templates.

Using Cabal

$ cp hie-cabal.yaml hie.yaml
$ cp install/hie-cabal.yaml install/hie.yaml

Using Stack

$ cp hie-stack.yaml hie.yaml
$ cp install/hie-stack.yaml install/hie.yaml
$ cp ghcide/hie-stack.yaml ghcide/hie.yaml
$ stack build --test --no-run-tests
$ cd install
$ 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

If you are looking for a Nix expression to create haskell-language-server binaries, see https://github.com/haskell/haskell-language-server/issues/122

Introduction tutorial

Pepeiborra wrote an tutorial on writing a plugin in HLS.

Test your hacked HLS in your editor

If you want to test HLS while hacking on it, follow the steps below.

To do once:

To do every time you changed code and want to test it:

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.