Shestakov Alex Blog Re-using common parts of the Cabal configuration

When a your haskell repository consists of several Cabal libraries, it is very handy (from nix perspective) to have folders isolated from each other. Each sub-folder contains own .cabal file inside. That allows to declare derivations in very clean way:

haskellOverlay = hfinal: hprev:
  let
    build = name: source: hfinal.callCabal2nix "project-${name}" (gitignore-nix-src.gitignoreSource source) { };
    project = {
      project-types = build "types" ./src/types;
      project-core = build "core" ./src/core;
      project-tests = build "tests" ./tests;
    };
  in project // {
    project = builtins.attrValues project;
  };

That project attrSet is useful inside shellFor to discover dependencies from all of the libraries without the need to list all names again in packages.

haskellPkgs.shellFor {
  withHoogle = true;
  packages = p: p.project;
  genericBuilderArgsModifier = pkgs.lib.mapAttrs (_n: v: if pkgs.lib.isList v then builtins.filter (a: a == null || !(pkgs.lib.hasPrefix "project-" a.pname)) v else v);
}

Having multiple .cabal files is sure handy for nix, but it certainly hurts feelings of minimalist like me, because there has to be lots of repetitions. Even if you use hpack for generating .cabal files, you’ll have to repeat things like:

default extensions
common dependencies (each of the libraries use)
ghc-options, etc.

The solution I found is to use !include directive, which hpack natively supports. Put package.common.yaml file in the root of the repository, create a symlink to it in each of the sub-folders.

# package.common.yaml
_all: &all
  version: "0.0.0.0"
  author: Alexander Shestakov
  license: AllRightsReserved

  language: GHC2024
  default-extensions:
    - DefaultSignatures
    - DeriveAnyClass
    - DerivingVia
    - NoImplicitPrelude
    - OverloadedLabels
    - OverloadedStrings
    - QuantifiedConstraints
    - TypeFamilies
    - UndecidableInstances

  dependencies:
    - base
    - deriving-aeson

  ghc-options: -Wall
  source-dirs: .

_executable: &executable
  ghc-options: -Wall -rtsopts "-with-rtsopts=-N" -threaded
  main: Main.hs

hpack will happily apply YAML merge keys defined in a package.common.yaml file. So each package.yaml becomes trivially short.

# package.yaml from types folder
_common: !include "package.common.yaml"
<<: *all
name: project-types
library:
  dependencies:
    - aeson
    - ordered-containers
    - reflection
    - relude
    - scientific

To re-generate all the .cabal files, there is a short entry in project’s Justfile.

hpack:
  find -iname package.yaml -exec hpack {} \;

build: hpack
  cabal build all