Haskell is a great language, but compared with other languages, the IDE ecosystem leaves much to be desired. This page gives a good breakdown of the various options and their capabilities. After trying many of the available tools, I have finally found this setup which works with relatively little configuration. There are other tools out there that have more features, but I have not been able to get them working.
The Visual Studio Code extension I am using, vscode-ghc-simple, is easy to get started with because it only depends on ghci. If you are not using Nix to provision your Haskell development environment, vscode-ghc-simple will most likely work for you without any configuration. However, if you use Nix, vscode-ghc-simple may not be running in a Nix environment, and therefore will not have ghci in its path. This tutorial will show you how to configure vscode-ghc-simple so that it can run in the Nix environment for your project.
Install Nix
If you’re reading this, you probably already have Nix installed. If not, the official docs will do a better job explaining how to install than I can.
Install vscode-ghc-simple
The extension can be found in the Visual Studio Marketplace.
Setup Nix Haskell Environment
I use the patterns described in Soares Chen’s great article. I’ll describe my setup in brief, but I recommend reading the referenced article for a deeper understanding.
Globally Install Cabal
nix-env -i cabal-installInstalling cabal globally introduces some impurity, but it makes writing the Nix package for your project much easier, so I use this method.
Project Instantiation
First use cabal to create the skeleton of your project:
nix-shell --pure -p ghc cabal-install --run "cabal init"With the skeleton in place, we can use cabal2nix to generate a default Nix package:
nix-shell --pure -p cabal2nix --run "cabal2nix --shell ./." > default.nixNote: In the referenced article, Chen creates separate default.nix and shell.nix files. His approach is more flexible. I mostly use the --shell approach because I can get started on a new project more quickly.
Configure vscode-ghc-simple
Finally, we need to configure vscode-ghc-simple to run ghci in a Nix environment. This is done by setting the Repl Command field to:
nix-shell --command "cabal new-repl all"It should look something like
That’s It
After completing these steps, vscode-ghc-simple will automatically start in the background whenever you open a Haskell project.
Alternatives
Instead of running the cabal process in a nix-shell directly, it is possible to run the VSCode process in a nix-shell. Then the cabal process spawned by VSCode will inherit its environment including ghci and all other dependencies. I did not go down this route because I want to be able to open a project in VSCode without having to remember to restart my editor with the correct environment for each project I’m working on.