haskell-multi-nix

Just a simple demo of Nixifying a multi-package Haskell project.

Packages

This project has two local Haskell packages:

1. foo: a Haskell library exporting Foo.fooFunc.
2. bar: a Haskell executable that depends on foo

To build the foo library:

nix build .#foo

To build the bar executable:

nix build

To run the executable:

nix run

Explanation

The Haskell infrastructure in nixpkgs provides a package set (an attrset) called pkgs.haskellPackages¹. We add two more packages -- foo and bar (the local packages) -- to this package set. We do this by using the standard nixpkgs overlay API (specifically extend, which was created by the implicit makeExtensible) defined in fixed-points.nix. After having added the local packages, the result is a new package set, which is no different in essense to the original package set (we can also put our dependency overrides in the same, or different, overlay). Note that any package in a package set can depend on any other packages; thus, it becomes possible to make bar depend on foo (see "build-depends" in ./bar/bar.cabal) even though they come from the same overlay.

Footnotes

1. The package set pkgs.haskellPackages corresponds to the default GHC version. Non-default GHC versions have their own package sets, for e.g.: pkgs.haskell.packages.ghc924 is the package set for GHC 9.2.4. ↩