Quick and dirty setup for Haskell on NixOS

Usecase

I have a bunch of free time and want to pick up haskellbook again. Unfortunately, three years have passed since my last attempt, so I have little recollection of my previous progress. I’m prompted to restart from the very beginning.

In the meantime, I’ve switched my operating system from Ubuntu and PopOS to NixOS, so I need to figure out how to set up a Haskell development environment on NixOS. This includes IDE support for my current editor of choice, VS Code.

I’m both a Nix noob and a Haskell noob, so I don’t want to mess around too long with this. Anything quick and dirty that works will fit the bill. I don’t have any strong opinions on the “correct” setup either.

Stack

The first thing I need is a ghc / ghci installation. I know one way to do this is obtain it via a build tool, stack. That’s also what the book recommends, so I’m going to go with the flow here.

stack

The program 'stack' is not in your PATH. It is provided by several packages.
You can make it available in an ephemeral shell by typing one of the following:
  nix-shell -p haskellPackages.stack
  nix-shell -p stack

Okay.

As we said, we don’t want to mess around with nix expressions right now. Let’s go with an ephemeral shell.

nix-shell -p stack
stack

stack ghci

Note: No local targets specified, so a plain ghci will be started with no package hiding or package options.
      
      You are using snapshot: lts-21.1
      
      If you want to use package hiding and options, then you can try one of the following:
      
      * If you want to start a different project configuration than /home/fmap/.stack/global-project/stack.yaml, then you can use stack init to create a new stack.yaml for
        the packages in the current directory. 
        
      * If you want to use the project configuration at /home/fmap/.stack/global-project/stack.yaml, then you can add to its 'packages' field.
      
Configuring GHCi with the following packages: 
GHCi, version 9.4.5: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /run/user/1000/haskell-stack-ghci/2a3bbd58/ghci-script
ghci> 

Cool.

Note that under my current setup (nixos-unstable, nix-channel managed nixpkgs) and at the time of writing, stack points to

• stack 2.9.1
• lts-21.1 resolver
• 9.4.5 ghc

This might vary for you depending on your specific channel state or flake input.

Create project

Let’s create a new project that’s going to be our workspace for code snippets and exercises while working through the book.

stack new haskbook

Once this finishes, let’s browse around to see what’s been created.

cd haskbook
code .

There’s an exampe haskell file created in the project, Lib.hs. It is recognised as a haskell source file, but on a plain VSCode installation, it doesn’t even have syntax highlighting.

Let’s work on IDE setup, then.

code extension

A google search for “vscode haskell” yields https://github.com/haskell/vscode-haskell. The extension name is haskell.haskell. We add it to our IDE setup. In my case, this is managed by home-manager:

programs.vscode = {
  enable = true;
  package = pkgs.vscode;
  extensions = [
    pkgs.vscode-extensions.bbenoist.nix
    pkgs.vscode-extensions.scalameta.metals
    pkgs.vscode-extensions.scala-lang.scala
    pkgs.vscode-extensions.haskell.haskell
  ];
};

Activate that change, and let’s relaunch code again. Open a haskell file, and the extension greets us with

Project requires HLS but it isn't installed

Haskell Language Server

HLS stands for haskell-language-server This is a (LSP) backend that the IDE extension needs.

If I tried typing that in a console, I get

haskell-language-server
The program 'haskell-language-server' is not in your PATH. You can make it available in an
ephemeral shell by typing:
  nix-shell -p haskellPackages.haskell-language-server

Okay. Let’s launch a shell with stack and haskell-language-server, and relaunch code.

nix-shell -p stack haskellPackages.haskell-language-server

code .

I fire up the IDE and open the Lib.hs file. I get nothing on hover, so something must be wrong. Select “Output” in VSCode, and select “Haskell” - the standard output of the extension might point us to an error.

2023-07-06 10:36:55.2430000 [client] INFO Activating the language server in working dir: /home/fmap/work/own/haskbook (the workspace folder)
2023-07-06 10:36:55.2450000 [client] INFO run command: haskell-language-server-wrapper --lsp
2023-07-06 10:36:55.2450000 [client] INFO debug command: haskell-language-server-wrapper --lsp
2023-07-06 10:36:55.2450000 [client] INFO server environment variables:
2023-07-06 10:36:55.2660000 [client] INFO Starting language server
No 'hie.yaml' found. Try to discover the project type!
Run entered for haskell-language-server-wrapper(haskell-language-server-wrapper) Version 2.0.0.0 x86_64 ghc-9.2.8
Current directory: /home/fmap/work/own/haskbook
Operating system: linux
Arguments: ["--lsp"]
Cradle directory: /home/fmap/work/own/haskbook
Cradle type: Stack

Tool versions found on the $PATH
cabal:          Not found
stack:          2.9.1
ghc:            9.2.8


Consulting the cradle to get project GHC version...
2023-07-06T10:36:56.298573Z | Debug | executing command: stack setup --silent
2023-07-06T10:36:58.571617Z | Debug | executing command: stack exec ghc -- --numeric-version
Project GHC version: 9.4.5
haskell-language-server exe candidates: ["haskell-language-server-9.4.5","haskell-language-server"]
Launching haskell-language-server exe at:/nix/store/dbyjzdqcvn2q1wha2i1p4bf9jdwm23k2-haskell-language-server-2.0.0.0/bin/haskell-language-server
2023-07-06T10:37:00.589523Z | Debug | executing command: stack setup --silent
2023-07-06T10:37:02.379620Z | Debug | executing command: stack exec ghc -- -v0 -package-env=- -ignore-dot-ghci -e Control.Monad.join (Control.Monad.fmap System.IO.putStr System.Environment.getExecutablePath)
2023-07-06T10:37:04.753173Z | Debug | executing command: stack setup --silent
2023-07-06T10:37:06.926440Z | Debug | executing command: stack exec ghc -- --print-libdir
GHC versions don't match!

Expected: 9.2.8
Got:      9.4.5
[Error - 13:37:08] Connection to server is erroring. Shutting down server.
[Error - 13:37:08] Connection to server is erroring. Shutting down server.

Right. Apparently ghc and haskell-language-server must agree on the version, and they don’t. We previously launched a nix-shell, let’s see what haskell-language-server version that brought us.

haskell-language-server --version
2023-07-06T10:42:08.734221Z | Info | No log file specified; using stderr.
haskell-language-server version: 2.0.0.0 (GHC: 9.2.8) (PATH: /nix/store/dbyjzdqcvn2q1wha2i1p4bf9jdwm23k2-haskell-language-server-2.0.0.0/bin/.haskell-language-server-9.2.8-unwrapped)

Okay. In our case and at the time of writing, that’s haskell-language-server-9.2.8. But GHC is 9.4.5, so that’s no good.

Overriding HLS version

A quick web search and we hopefully end up here. This tells us there’s an attribute supportedGhcVersions which will let us override the version.

Summon the incantation:

nix-shell -p stack 'pkgs.haskell-language-server.override { supportedGhcVersions = [ "945" ]; }'

code .

HLS will take a bunch of time to fire up, and afterwards we get a working IDE integration:

Happy hacking!