haskell-template

Get a Haskell development environment up and running quickly, as long as Nix is available on your system¹.

This repository is a Haskell project template that is optimized for a fully reproducible and friendly development environment. It is based on:

• Nix + Flakes (via github:srid/haskell-flake) + GHC 9
• VSCode + HLS
• fourmolu autoformatting
• Relude as Prelude.
  • .hlint.yaml is from relude

Getting Started

First-time setup:

• Install Nix (>= 2.8) & enable Flakes (Using Windows? See footnote¹)
• Run nix develop -i -c haskell-language-server to sanity check your environment
• Open as single-folder workspace in Visual Studio Code
  • When prompted by VSCode, install the workspace recommended extensions
  • Ctrl+Shift+P to run command "Nix-Env: Select Environment" and then select shell.nix.
    • The extension will ask you to reload VSCode at the end. Do it.

To run the program with auto-recompile:

• Press Ctrl+Shift+B in VSCode, or run bin/run in terminal, to launch Ghcid running your program.

Open Main.hs, and expect all HLS IDE features like hover-over tooltip to work out of the box. Try changing the source, and expect Ghcid to re-compile and re-run the app in the terminal below.

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Renaming the project:

# First, click the green "Use this template" button on GitHub to create your copy.
git clone <your-clone-url>
cd your-project
NAME=myproject

git mv haskell-template.cabal ${NAME}.cabal
nix run nixpkgs#sd -- haskell-template ${NAME} * */*
git add . && git commit -m rename

Tips

• Run nix flake update to update all flake inputs.
• Run treefmt in nix shell to autoformat the project. This uses treefmt, which uses ./treefmt.toml (where fourmolu and nixpkgs-fmt are specified).
• Run bin/hoogle to start Hoogle with packages in your cabal file.
• Run bin/test to run the test suite.
• Run the application without installing: nix run github:srid/haskell-template (or nix run . from checkout)

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1. On Windows, you may install Nix under WSL2 and use the Remote - WSL extension to connect from the native VSCode. This runs your project under Linux while providing a near-native development experience on Windows. ↩︎