While haskell-flake is generally used to develop and build individual Haskell projects, you can also use it to create a custom Haskell package set that you can use in other projects. This is useful if you want to create a common package set to be shared across multiple projects.
A “project” in haskell-flake primarily serves the purpose of developing Haskell projects. Additionally, a project also exposes the final package set via the readonly option outputs.finalPackages. This package set includes the base packages (basePackages), the local packages as well as any dependency overrides you set. Since we are are only interested in creating a new package set, we can use empty local packages and disable the dev shell:
{
haskellProjects.ghc810 = {
defaults.packages = {}; # Disable scanning for local package
devShell.enable = false; # Disable devShells
autoWire = [ ]; # Don't wire any flake outputs
# Start from nixpkgs's ghc8107 package set
basePackages = pkgs.haskell.packages.ghc8107;
};
}
You can access this package set as config.haskellProjects.ghc810.outputs.finalPackages. But this is not terribly interesting, because it is the exact same as the package set pkgs.haskell.packages.ghc8107 from nixpkgs. So let’s add and override some packages in this set:
{
haskellProjects.ghc810 = {
defaults.packages = {}; # No local packages
devShell.enable = false;
basePackages = pkgs.haskell.packages.ghc8107;
packages = {
# New packages from flake inputs
mylib.source = inputs.mylib;
# Dependencies from Hackage
aeson.source = "1.5.6.0";
dhall.source = "1.35.0";
};
settings = {
aeson.jailbreak = true;
};
};
}
This will create a package set that overrides the aeson and dhall packages using the specified versions from Hackage, but with the aeson package having the jailbreak flag set (which relaxes its Cabal constraints). It also adds the mylib package which exists neither in nixpkgs nor in Hackage, but comes from somewhere arbitrary and specified as flake input.
In your actual haskell project, you can use this package set (config.haskellProjects.ghc810.outputs.finalPackages) as its base package set:
{
haskellProjects.myproject = {
packages.mypackage.source = ./.;
basePackages = config.haskellProjects.ghc810.outputs.finalPackages;
};
}
Finally, you can externalize this ghc810 package set as either a flake-parts module or as a haskell-flake module, and thereon import it from multiple repositories.