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.

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 nix --option sandbox false build .#check -L to run the flake checks.
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)

Common workflows

Adding tests

Split any logic code out of Main.hs into, say, a Lib.hs.
Correspondingly, add other-modules: Lib to the "shared" section of your cabal file.

Add tests/Spec.hs (example below):

module Main where

import Lib qualified
import Test.Hspec (describe, hspec, it, shouldContain)

main :: IO ()
main = hspec $ do
  describe "Lib.hello" $ do
    it "contains the world emoji" $ do
      toString Lib.hello `shouldContain` "🌎"

Add the tests stanza to the cabal file:

test-suite tests
import:         shared
main-is:        Spec.hs
type:           exitcode-stdio-1.0
hs-source-dirs: tests
build-depends:  hspec

Update hie.yaml accordingly; for example, by adding,

  - path: "tests"
    component: "test:tests"

Add bin/test and chmod a+x it:

#!/usr/bin/env bash
set -xe

exec nix develop -i -c ghcid -c "cabal repl test:tests" -T :main

Commit your changes to Git, and test it out by running bin/test.

Discussions

Got questions? Ideas? Suggestions? Post them here: https://github.com/srid/haskell-template/discussions

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. ↩︎

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