Haskell for Visual Studio Code

This extension adds language support for Haskell, powered by the Haskell Language Server.

Features

• Warning and error diagnostics from GHC

• Type information and documentation on hover

• Jump to definition

• Document symbols

• Highlight references in document

• Code completion

• Formatting via Brittany, Floskell, Ormolu or Stylish Haskell

• Multi-root workspace support

• Code evaluation (Haskell Language Server)

  

Requirements

• For standalone .hs/.lhs files, ghc must be installed and on the PATH. The easiest way to install it is with ghcup or Chocolatey on Windows.
• For Cabal based projects, both ghc and cabal-install must be installed and on the PATH. It can also be installed with ghcup or Chocolatey on Windows.
• For Stack based projects, stack must be installed and on the PATH.

Language Servers

Whilst this extension is powered by the Haskell Language Server by default, it also supports several others which can be manually installed:

• Haskell Language Server: This is the default language server which will automatically be downloaded, so it does not need manual installation. It builds upon ghcide by providing extra plugins and features.
• ghcide: A fast and reliable LSP server with support for basic features.
• Haskell IDE Engine: A stable and mature language server, but note that development has moved from this to the Haskell Language Server.

You can choose which language server to use from the "Haskell > Language Server Variant" configuration option.

Configuration options

Enable/disable server

You can enable or disable the chosen haskell language server via configuration. This is sometimes useful at workspace level, because multi-root workspaces do not yet allow you to manage extensions at the folder level, which can be necessary.

"haskell.enable": true

Path to server executable executable

If your server is manually installed and not on your path, you can also manually set the path to the executable.

"haskell.serverExecutablePath": "~/.local/bin/hie"

There are a few placeholders which will be expanded:

• ~, ${HOME} and ${home} will be expanded into your users' home folder.
• ${workspaceFolder} and ${workspaceRoot} will expand into your current project root.

Haskell Language Server specifics

Local documentation

Haskell Language Server can display Haddock documentation on hover and completions if the project and its dependencies have been built with the -haddock GHC flag.

• For cabal:

  • Add to your global config file (e.g. ~/.cabal/config):

    program-default-options
      ghc-options: -haddock
    

  • Or, for a single project, run cabal configure --ghc-options=-haddock

• For stack, add to global $STACK_ROOT\config.yaml, or project's stack.yaml:

  ghc-options:
    "$everything": -haddock
  

  Note that this flag will cause compilation errors if a dependency contains invalid Haddock markup, until GHC 8.12 which will report warnings instead.

Downloaded binaries

This extension will download haskell-language-server binaries to a specific location depending on your system. If you find yourself running out of disk space, you can try deleting old versions of language servers in this directory. The extension will redownload them, no strings attached.

Platform	Path
macOS	~/Library/Application\ Support/Code/User/globalStorage/haskell.haskell/
Windows	%APPDATA%\Code\User\globalStorage\haskell.haskell
Linux	$HOME/.config/Code/User/globalStorage/haskell.haskell

Note that if haskell-language-server-wrapper/haskell-language-server is already on the PATH, then the extension will launch it directly instead of downloading binaries.

Supported GHC versions

These are the versions of GHC that there are binaries of haskell-language-server for. Building from source may support more versions!

GHC	Linux	macOS	Windows
8.10.1	✓	✓	✓
8.8.3	✓	✓
8.8.2	✓	✓
8.6.5	✓	✓	✓
8.6.4	✓	✓	✓

Haskell IDE Engine specifics

If you are using Haskell IDE Engine as your language server, there are a number of additional configuration options.

Local documentation

Haskell Language Server can display Haddock documentation on hover and in code completion for your code if you have built your project with haddock enabled.

For Stack projects, in your project directory run

$ stack haddock --keep-going

For Cabal projects, run

$ cabal build --haddock

Or alternatively add the following to your ~/.cabal/config or cabal.config[.local]

documentation: True

Liquid Haskell

If Liquid Haskell is installed, haskell-ide-engine can be configured to run the liquidhaskell executable on save and display diagnostics:

"haskell.liquidOn": true,

Hoogle

HIE pulls in documentation via Hoogle. After installing Hoogle via cabal install hoogle or stack install hoogle, generate the database with:

$ hoogle generate

Using multi-root workspaces

First, check out what multi-root workspaces are. The idea of using multi-root workspaces, is to be able to work on several different Haskell projects, where the GHC version or stackage LTS could differ, and have it work smoothly.

The language server is now started for each workspace folder you have in your multi-root workspace, and several configurations are on a resource (i.e. folder) scope, instead of window (i.e. global) scope.

Investigating and reporting problems

1. Go to extensions and right click Haskell and choose Configure Extensions Settings
2. Scroll down to Language Server Haskell › Trace: Server and set it to verbose
3. Restart vscode and reproduce your problem
4. Go to the main menu and choose View -> Output (Ctrl + Shift + U)
5. On the new Output panel that opens on the right side in the drop down menu choose Haskell

Please include the output when filing any issues on the relevant language server's issue tracker.

Troubleshooting

• Usually the error or unexpected behaviour is already reported in the haskell language server used by the extension. Finding the issue in its issue tracker could be useful to help resolve it. Sometimes even it includes a workaround for the issue.
• Haskell language servers issue trackers:
  • haskell-ide-engine (the default haskell language server): https://github.com/haskell/haskell-ide-engine/issues
  • haskell-language-server: https://github.com/haskell/haskell-language-server/issues
• Common issues:
  • For now, the extension is not able to open a single haskell source file. You need to open a workspace or folder, configured to be built with cabal, stack or other hie-bios compatible program.
  • Check you don't have other haskell extensions active, they can interfere with each other.

Contributing

If you want to help, get started by reading Contributing for more details.

Release Notes

See the Changelog for more details.