# haskell-template

Haskell project template optimized for a fully reproducible and friendly development environment. Based on:

- [Nix](http://www.srid.ca/haskell-nix) + [Flakes](https://serokell.io/blog/practical-nix-flakes) + GHC 9
- VSCode + [HLS](https://github.com/haskell/haskell-language-server)
- [fourmolu](https://github.com/fourmolu/fourmolu) autoformatting 
- [Relude](https://github.com/kowainik/relude#relude) as Prelude.
  - `.hlint.yaml` is [from relude](https://github.com/kowainik/relude/blob/main/.hlint.yaml)

## Getting Started

First-time setup:

- [Install Nix](https://nixos.org/download.html) & [enable Flakes](https://nixos.wiki/wiki/Flakes)
- Run `nix develop -c haskell-language-server` to sanity check your environment 
- [Open as single-folder workspace](https://code.visualstudio.com/docs/editor/workspaces#_singlefolder-workspaces) in Visual Studio Code
    - When prompted by VSCode, install the [workspace recommended](https://code.visualstudio.com/docs/editor/extension-marketplace#_workspace-recommended-extensions) extensions
    - <kbd>Ctrl+Shift+P</kbd> 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 <kbd>Ctrl+Shift+B</kbd> 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:

```sh
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. nixpkgs is pinned to a specific a rev in `flake.nix`, which you can advance to the latest rev reported in [status.nixos.org](https://status.nixos.org).
- Run `treefmt` in nix shell to autoformat the project. This uses [treefmt](https://github.com/numtide/treefmt), which uses `./treefmt.toml` (where fourmolu and nixpkgs-fmt are specified).

## Alternatives

- haskell.nix: [Getting started with Flakes](https://input-output-hk.github.io/haskell.nix/tutorials/getting-started-flakes.html)
- [Serokell's Flake template](https://github.com/serokell/templates/tree/master/haskell-cabal2nix)
  - [Same, but using haskell.nix](https://github.com/serokell/templates/pull/2)