This page describes how to easily setup Visual Studio Code (VSCode) for GHC development. This includes Language Server Protocol (LSP) support for Haskell and C files. The former is accomplished by using ghcide, the latter by ccls.

The whole procedure is very similar to that described for Spacemacs. If you miss somethings here, it might be on that other page. However, this page should be self-contained (with some links to other pages to reduce duplication in the wiki).

Some animations to demonstrate what you can expect:

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⚠ Currently ghcide (ghc.nix argument: withIde = true;) is not supported by ghc.nix: https://github.com/alpmestan/ghc.nix#using-ghcide-%EF%B8%8F-currently-unavailable . Most of this page still applies, but you have to provide your own ghcide and bear binaries. ghcide must have been built with exactly the same GHC version you use to build your GHC project.

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Table of contents

• Table of contents
• Install ghc.nix
  • Configure ghc.nix with a shell.nix file
  • Build GHC with ghc.nix
    • What is bear?
• Start and configure VSCode
  • Start VSCode in a ghc.nix/nix-shell environment
  • Install ghcide and ccls plugins
  • Create workspaces
  • Plugin configuration
    • ccls
    • ghcide
• Troubleshooting

Install ghc.nix

If you haven't already installed ghc.nix, please refer to the corresponding section on the Spacemacs page.

Configure ghc.nix with a shell.nix file

Create a shell.nix file your ghc source folder, that looks like this:

import ./ghc.nix/default.nix {
  bootghc = "ghc865";
  withIde = true;
  withHadrianDeps = true;
  cores = 8;
  withDocs = false;
}

For the context of this page, only the argument withIde = true; matters. It makes sure that the resulting Nix environments contain the right ghcide executable.

Build GHC with ghc.nix

❗ The first time you call nix-shell in the ghc source folder, all dependencies are downloaded and (if needed) built. Building ghcide takes much time. Consider to enable cachix, a binary cache for Nix packages: spacemacs/cachix

Some source code files are generated during build.

Build GHC in the context defined by ghc.nix and shell.nix, e.g.

nix-shell --command 'bear hadrian/build.sh -c -j12 --docs=none --flavour=Devel2'

What is bear?

ccls needs a compile_commands.json (JSON Compilcation Database) file as configuration. This file is mostly a mapping of *.c filenames to C compiler arguments.

compile_commands.json is generated by bear, a tool that intercepts all calls to the C compiler. That means, you have to trigger a full (re-)build with bear, even if you already got a fully compiled GHC project.

Start and configure VSCode

Start VSCode in a ghc.nix/nix-shell environment

nix-shell -p vscode ccls

Install ghcide and ccls plugins

Install the ghcide plugin in VSCode. It's listed as an extension in the VSCode Market Place: https://marketplace.visualstudio.com/items?itemName=DigitalAssetHoldingsLLC.ghcide

Some goes for the ccls plugin: https://marketplace.visualstudio.com/items?itemName=ccls-project.ccls

Create workspaces

Create two distinct workspaces for the GHC compiler and Hadrian.

The simplest way is to:

• File -> New Window
• File -> Open Folder... (in the new window)
• File -> Save Workspace As... (in the new window)

The folder of your GHC compiler workspace is the source root of the cloned GHC repository. The Hadrian folder, is hadrian/ in that previously described folder.

So, e.g. if your GHC source folder has the path /home/sven/src/ghc, then the two folders are:

• /home/sven/src/ghc for GHC
• /home/sven/src/ghc/hadrian for Hadrian

Plugin configuration

ccls

Works out of the box! It automatically finds the compile_commands.json file and uses it as configuration.

ghcide

The are the arguments to the ghcide plugin: Hic: Arguments (hic.arguments) and Hic: Executable Path (hic.executablePath). You have to set them per workspace, i.e. hic.arguments differs for the GHC compiler and the Hadrian workspace.

Workspace	hic.arguments	hic.executablePath
GHC Compiler	--lsp compiler	ghcide
Hadrian	--lsp .	ghcide

Troubleshooting

Please have a look at spacemacs/Troubleshooting. The first analysis steps are editor-agnostic.