{
  pkgs ? import <nixpkgs> {},
  system ? builtins.currentSystem,
}: let
  inherit (pkgs) lib;
  sources = builtins.fromJSON (lib.strings.fileContents ./sources.json);

  # mkBinaryInstall makes a derivation that installs Zig from a binary.
  mkBinaryInstall = {
    url,
    version,
    sha256,
  }:
    pkgs.stdenv.mkDerivation {
      inherit version;

      pname = "zig";
      src = pkgs.fetchurl {inherit url sha256;};
      dontConfigure = true;
      dontBuild = true;
      dontFixup = true;
      installPhase = ''
        mkdir -p $out/{doc,bin,lib}
        [ -d docs ] && cp -r docs/* $out/doc
        [ -d doc ] && cp -r doc/* $out/doc
        cp -r lib/* $out/lib
        substituteInPlace $out/lib/std/zig/system.zig \
          --replace "/usr/bin/env" "${pkgs.lib.getExe' pkgs.coreutils "env"}"
        cp zig $out/bin/zig
      '';
    };

  # The packages that are tagged releases
  taggedPackages =
    lib.attrsets.mapAttrs
    (k: v: mkBinaryInstall {inherit (v.${system}) version url sha256;})
    (lib.attrsets.filterAttrs
      (k: v: (builtins.hasAttr system v) && (v.${system}.url != null) && (v.${system}.sha256 != null))
      (builtins.removeAttrs sources ["master"]));

  # The master packages
  masterPackages =
    lib.attrsets.mapAttrs' (
      k: v:
        lib.attrsets.nameValuePair
        (
          if k == "latest"
          then "master"
          else ("master-" + k)
        )
        (mkBinaryInstall {inherit (v.${system}) version url sha256;})
    )
    (lib.attrsets.filterAttrs
      (k: v: (builtins.hasAttr system v) && (v.${system}.url != null))
      sources.master);

  # This determines the latest /released/ version.
  latest = lib.lists.last (
    builtins.sort
    (x: y: (builtins.compareVersions x y) < 0)
    (builtins.attrNames taggedPackages)
  );
in
  # We want the packages but also add a "default" that just points to the
  # latest released version.
  taggedPackages // masterPackages // {"default" = taggedPackages.${latest};}