flatironinstitute/nixpack

---

flatironinstitute/nixpack.json

{
  "createdAt": "2021-07-27T22:01:37Z",
  "defaultBranch": "main",
  "description": "nix+spack = nixpack (spanix?)",
  "fullName": "flatironinstitute/nixpack",
  "homepage": null,
  "language": "Nix",
  "name": "nixpack",
  "pushedAt": "2025-11-20T00:09:14Z",
  "stargazersCount": 48,
  "topics": [],
  "updatedAt": "2025-10-13T10:19:01Z",
  "url": "https://github.com/flatironinstitute/nixpack"
}

nixpack = nix+spack

A hybrid of the nix package manager and spack where nix (without nixpkgs) is used to solve and manage packages, using the package repository, builds, and modules from spack.

If you love nix’s expressiveness and efficiency, but don’t need the purity of nixpkgs (in the sense of independence from the host system)… if you like the spack packages and package.py format, but are tired of managing roots and concretizations, this may be for you. Nix on the outside, spack on the inside.

Usage

1. Install and configure nix, sufficient to build derivations.
2. Edit (or copy) [default.nix]!(default.nix).
   • It’s recommended to set packs.spackSrc.rev to a fixed version of spack. Changing the spack version requires all packages to be rebuilt. If you want to update individual packages without a rebuild, you can put them in spack/repo/packages (or another repo in packs.repos).
   • Set packs.os and packs.global.target.
   • Set packs.spackConfig.config.source_cache and add any other custom spack config you want (nixpack ignores system and user spack config for purity, but will load default and site config from the spack repo itself).
   • Set bootstrapPacks.package.compiler to a pre-existing (system/external) compiler to be used to bootstrap.
   • Set packs.package.gcc to choose your default compiler, or packs.package.compiler to use something other than gcc.
   • Add any other package preferences to packs.package (versions, variants, virtual providers, etc.)
   • See packs.global.fixedDeps: by default multiple different instances of any given package may be built in order to satisfy requirements, but you may prefer to force only one version of each package, which will improve performance and build times.
3. Run nix-build -A pkgs.foo to build the spack package foo.
4. To build modules, configure packs.mods and run nix-build -A mods.

Flatiron Specific

We have our local Flatiron-specific configuration and repositories in [fi]!(fi), complete with views and modules, some of which may be more generally useful or at least helpful reference or template for creating a full working system. See the [README]!(fi/README.md) in that directory for more information.

Compatibility

nixpack uses an unmodified checkout of spack (as specified in spackSrc), and should work with other forks as well. However, it makes many assumptions about the internals of spack builds, so may not work on much older (or newer) versions.

Implementation and terminology

In nixpkgs, there’s mainly the concept of package, and arguments that can be overridden. In spack, there are packages and specs, and “spec” is used in many different ways. We define a few more specific concepts to merge the two.

package descriptor

The metadata for a spack package. These are generated by [spack/generate.py]!(spack/generate.py) from the spack repo package.pys and loaded into packs.repo. They look like this:

example = {
  namespace = "builtin";
  version = ["2.0" "1.2" "1.0"]; # in decreasing order of preference
  variants = {
    flag = true;
    option = ["a" "b" "c"]; # single-valued, first is default
    multi = {
      a = true;
      b = false;
    };
  };
  depends = {
    /* package preferences for dependencies (see below) */
    compiler = { # added implicitly if missing
      deptype = ["build" "link"];
    };
    deppackage = {
      version = "1.5:2.1";
      deptype = ["run" "test"];
    };
    notused = null;
  };
  provides = {
    virtual = "2:";
  };
  paths = {}; # paths to tools provided by this package (like `cc` for compilers)
  patches = []; # extra patches to apply (in addition to those in spack)
  conflicts = []; # any conflicts (non-empty means invalid)
};

Most things default to empty. This is not a complete build description, just the metadata necessary to resolve dependencies (concretize). In practice, these are constructed as functions that take a resolved package spec as an argument, so that dependencies and such be conditional on a specific version and variants.

You can build the repo using nix-build -A spackRepo (and see result).

package preferences

Constraints for a package that come from the user, or a depending package. These are used in package descriptor depends and in user global and per-package preferences. They look similar to package descriptors and can be used to override or constrain some of those values.

example = {
  version = "1.3:1.5";
  variants = {
    flag = true;
    option = "b";
    /* multi options can be specified as list of trues or explicitly */
    multi = ["a"];
    multi = {
      a = true;
      b = false;
    };
  };
  depends = {
    compiler = {
      name = "clang"; # use clang as the compiler virtual provider
    };
    deppackage = {
      version = ... # use a specific version for a dependency
    };
    virtualdep = {
      name = "provider";
      version = ...;
      ...
    };
    # dependencies can also be set to a specific package:
    builddep = packs.pkgs.builddep;
  };
  provides = {
    virtual = "version"; # this requires that this package provides virtual (not that it does)
  };
  patches = []; # extra patches to apply (in additon to those in the descriptor)
  extern = "/opt/local/mypackage"; # a prefix string or derivation (e.g., nixpkgs package) for an external installation (overrides depends)
  fixedDeps = false; # only use user preferences to resolve dependencies (see default.nix)
  target = "microarch"; # defaults to currentSystem (e.g., x86_64)
  verbose = true; # to enable nix-build -Q and nix-store -l (otherwise only spack keeps build logs)
  tests = false; # run tests (not implemented)
  resolver = ...; # where to find dependent packages (see default.nix)
};

package spec

A resolved (concrete) package specifier created by applying (optional) package preferences to a package descriptor. This looks just like a package descriptior but with concrete values. It also includes settings from prefereces like extern and target.

package

An actual derivation. These contain a spec metadata attribute. They also have a withPrefs function that can be used to make a new version of this package with updated prefs (unless they are extern).

compiler

Rather than spack’s dedicated %compiler concept, we introduce a new virtual “compiler” that all packages depend on and is provided by gcc and llvm (by default). By setting the package preference for compiler, you determine which compiler to use.

packs

The world, like nixpkgs. It contains pkgs with actual packages, as well as repo, view, modules, and other functions. See [packs/default.nix]!(packs/default.nix) for full details.

You can have one or more packs instances. Each instance is defined by a set of global user preferences, as passed to import ./packs. You can also create additional sets based on an existing one using packs.withPrefs. Thus, difference package sets can have different providers or package settings (like a different compiler, mpi version, blas provider, variants, etc.).

See [default.nix]!(default.nix) for preferences that can be set and their descriptions.

Bootstrapping

The default compiler is specified in [default.nix]!(default.nix) by compiler = bootstrapPacks.pkgs.gcc which means that the compiler used to build everything is packs comes from bootstrapPacks, and is built with the preferences and compiler defined there. bootstrapPacks in turn specifies a compiler of gcc with extern set, i.e., one from the host system. This compiler is used to build any other bootstrap packages, which are then used to build the main compiler. You could specify more extern packages in bootstrap to speed up bootstrapping.

You could also add additional bootstrap layers by setting the bootstrap compiler resolver to a different set. You could also replace specific dependencies or packages from a different packs set to bootstrap or modify other packages.