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

Dev Shell

The Nix development shell (nix develop) allows you to run the various cabal commands on the local packages.

For example, this will compile and run the main executable:

nix develop -c cabal -- run bar

ghcid

Using Multiple Home Units you can use ghcid to auto-recompile and auto-rerun the bar program whenever any Haskell source changes, including from libraries (foo). To do this, you must pass --enable-multi-repl along with the list of libraries to reload, which list should be in the correct order. viz.:

ghcid -T Main.main -c 'cabal repl --enable-multi-repl bar foo'

Now, try modifying ./foo/src/Foo.hs and ghcid should instantly re-compile and re-run bar with the new changes. A demo can be seen here.

How it works

The nixpkgs tree

The nixpkgs release tag uses raw functions from nixpkgs.

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.

The master tree

The master branch uses haskell-flake which abstracts much of what we explained above, such that your flake.nix is as small as possible.

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. ↩