mirror of
https://github.com/simplex-chat/haskell.nix.git
synced 2026-06-03 09:17:32 +00:00
Deniz Dogan
127e423397
When I followed this tutorial, my default package ended up as `packages.aarch64-darwin.aarch64-darwin.default`. Removing this "extra" `${system}` solves the problem, but admittedly I'm not 100% sure what's going on here.
143 lines
4.6 KiB
Markdown
143 lines
4.6 KiB
Markdown
# Getting started with flakes
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This version of the getting started guide is for users who
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are using [Nix Flakes](https://nixos.wiki/wiki/Flakes).
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The non flakes version of the guide is [here](getting-started.md).
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`haskell.nix` can automatically translate your
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[Cabal](https://cabal.readthedocs.io/en/latest/cabal-project.html)
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or [Stack](https://docs.haskellstack.org/en/stable/README/#quick-start-guide)
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project and its dependencies into Nix code.
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Assuming you have [Nix](https://nixos.org/download.html) installed, you can
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start setting up your project.
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## Setting up the binary cache
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IMPORTANT: you *must* do this or you *will* build several copies of GHC!
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You can configure Nix to use our binary cache, which is pushed to by CI, so should contain the artifacts that you need.
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You need to add the following sections to `/etc/nix/nix.conf` or, if you are a trusted user, `~/.config/nix/nix.conf` (if you don't know what a "trusted user" is, you probably want to do the former).
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```
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trusted-public-keys = [...] hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= [...]
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substituters = [...] https://cache.iog.io [...]
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```
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If you're running NixOS, you need to add/update the following in your `/etc/nixos/configuration.nix` files instead.
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```
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# Binary Cache for Haskell.nix
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nix.settings.trusted-public-keys = [
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"hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ="
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];
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nix.settings.substituters = [
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"https://cache.iog.io"
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];
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```
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NixOS-21.11 and older use slightly different settings.
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```
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# Binary Cache for Haskell.nix
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nix.binaryCachePublicKeys = [
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"hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ="
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];
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nix.binaryCaches = [
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"https://cache.iog.io"
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];
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```
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This can be tricky to get setup properly. If you're still having trouble getting cache hits, consult the corresponding [troubleshooting section](../troubleshooting.md#why-am-i-building-ghc).
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## Scaffolding
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The following work with `stack.yaml` and `cabal.project` based
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projects.
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Add `flake.nix`:
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```nix
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{
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description = "A very basic flake";
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inputs.haskellNix.url = "github:input-output-hk/haskell.nix";
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inputs.nixpkgs.follows = "haskellNix/nixpkgs-unstable";
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inputs.flake-utils.url = "github:numtide/flake-utils";
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outputs = { self, nixpkgs, flake-utils, haskellNix }:
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flake-utils.lib.eachSystem [ "x86_64-linux" "x86_64-darwin" ] (system:
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let
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overlays = [ haskellNix.overlay
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(final: prev: {
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# This overlay adds our project to pkgs
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helloProject =
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final.haskell-nix.project' {
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src = ./.;
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compiler-nix-name = "ghc8104";
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# This is used by `nix develop .` to open a shell for use with
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# `cabal`, `hlint` and `haskell-language-server`
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shell.tools = {
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cabal = {};
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hlint = {};
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haskell-language-server = {};
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};
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# Non-Haskell shell tools go here
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shell.buildInputs = with pkgs; [
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nixpkgs-fmt
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];
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# This adds `js-unknown-ghcjs-cabal` to the shell.
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# shell.crossPlatforms = p: [p.ghcjs];
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};
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})
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];
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pkgs = import nixpkgs { inherit system overlays; inherit (haskellNix) config; };
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flake = pkgs.helloProject.flake {
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# This adds support for `nix build .#js-unknown-ghcjs:hello:exe:hello`
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# crossPlatforms = p: [p.ghcjs];
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};
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in flake // {
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# Built by `nix build .`
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packages.default = flake.packages."hello:exe:hello";
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});
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}
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```
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!!! note "git dependencies"
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If you have git dependencies in your project, you'll need
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to [calculate sha256 hashes for them](./source-repository-hashes.md).
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### Working with a project
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Top-level attributes are Haskell packages (incl. dependencies) part of your project.
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To build the library component of a package in the project run:
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```shell
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nix build .#your-package-name:lib:your-package-name
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```
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There are also other components such as `exe`, `test` and `benchmark`.
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To build an executable:
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```shell
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nix build .#your-package-name:exe:your-exe-name
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```
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To use the `devShell` provided by the flake run:
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```shell
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nix develop .
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cabal repl your-package-name:lib:your-package-name
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cabal build your-package-name
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```
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To open a shell for use with `stack` see [the following issue](https://github.com/input-output-hk/haskell.nix/issues/689#issuecomment-643832619).
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## Going forward
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Read through [project](../reference/library.md#project) function reference to see how the API works.
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There are a number of things to explore further in the tutorials section.
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[haskell.nix]: https://github.com/input-output-hk/haskell.nix
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