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A library to facilitate complex multi-host NixOS configurations.
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defaults = {inherit mkSystem pkgs specialArgs;}; settingNames = attrNames defaults; moduleLists = {inherit modules extraModules;}; tagFields = ["parents" "build" "select"] ++ settingNames ++ attrNames moduleLists; moduleFields = ["enable"];
fail = message: throw "prism: ${message}"; list = concatStringsSep ", ";
checkTag = name: tag: let unknownFields = filter (field: !(elem field tagFields)) (attrNames tag); build = if tag ? build then tag.build else !(tag ? select) && isChildTag name; in if !isAttrs tag then fail "tag '${name}' must be an attribute set" else if unknownFields != [] then fail "tag '${name}' has unknown fields: ${list unknownFields}" else if tag ? select && tag.build or false then fail "tag '${name}' is built, so it can't have a select" else if tag ? select && tag ? modules then fail "tag '${name}' has a select, so it can't set modules" else tag // {inherit name build;};
registry = let # `self` is a self-referrential fixed point to allow for parenting self = mapAttrs checkTag (tags self); in self;
childTags = childTagsOf registry; isChildTag = name: elem name childTags;
parentsOf = tag: map ( parent: if isAttrs parent && parent ? name && registry ? ${parent.name} then registry.${parent.name} else fail "tag '${tag.name}' has a parent that isn't a tag reference; use self.<tag>" ) (tag.parents or []);
# Names of `root` and every ancestor reachable through parents accepted by `follow`. # `genericClosure` does graph traversal and `follow` decides what counts as an edge. walk = follow: root: map (entry: entry.key) (genericClosure { startSet = [{key = root.name;}]; operator = {key}: map (parent: {key = parent.name;}) (filter follow (parentsOf registry.${key})); });
ancestorsOf = walk (_: true);
cyclicTags = root: filter (name: any (parent: elem name (ancestorsOf parent)) (parentsOf registry.${name})) (ancestorsOf root);
# A value defined by a tag overrides any defined by that tag's ancestors. # If two unrelated tags both define it, `nearest` has more than one entry, # which is ambiguous. resolve = root: what: defines: get: fallback: let definers = filter (name: defines registry.${name}) (ancestorsOf root); overridden = definer: any (other: other != definer && elem definer (ancestorsOf registry.${other})) definers; nearest = filter (definer: !overridden definer) definers; in if nearest == [] then fallback else if length nearest == 1 then get registry.${head nearest} else fail "'${root.name}' inherits ${what} from unrelated tags ${list nearest}; set it on '${root.name}'";
settingOf = root: name: resolve root name (tag: tag ? ${name}) (tag: tag.${name}) defaults.${name};
specialArgsOf = root: let argNames = attrNames (foldl' (args: name: args // registry.${name}.specialArgs or {}) specialArgs (ancestorsOf root)); resolveArg = arg: resolve root "specialArgs.${arg}" (tag: tag ? specialArgs.${arg}) (tag: tag.specialArgs.${arg}) specialArgs.${arg}; in listToAttrs (map (arg: { name = arg; value = resolveArg arg; }) argNames);
importModule = path: let module = import path; undefinedTags = filter (key: !(elem key moduleFields) && !(registry ? ${key})) (attrNames module); in if !isAttrs module then fail "${toString path} must be an attribute set of tag sections" else if undefinedTags != [] then fail "${toString path} uses undefined tags: ${list undefinedTags}" else {inherit path module;};
# Every ancestor's list merged with the mkSystems default. mergedOf = root: name: moduleLists.${name} ++ concatMap (tag: registry.${tag}.${name} or []) (ancestorsOf root);
uniquePaths = paths: map (entry: entry.path) (genericClosure { startSet = map (path: { key = toString path; inherit path; }) paths; operator = _: []; });
isActive = context: tag: if !(tag ? select) then true else let active = tag.select (context // {inherit tag;}); in if isBool active then active else fail "select of tag '${tag.name}' must return a bool";
sectionsOf = root: rootTags: { path, module, }: let active = walk (isActive { inherit root rootTags path module; tags = registry; }) root; in # `_file` makes module system errors point at the source file. map (name: { _file = path; imports = [module.${name}]; }) (filter (name: elem name active) (attrNames module));
buildSystem = root: let cycle = cyclicTags root; rootMkSystem = settingOf root "mkSystem"; rootPkgs = settingOf root "pkgs"; prismModules = filter (entry: entry.module.enable or true) (map importModule (uniquePaths (mergedOf root "modules"))); in if cycle != [] then fail "tags ${list cycle} form a cycle" else if rootMkSystem == null then fail "'${root.name}' has no mkSystem; set it in mkSystems or on a tag" else rootMkSystem { specialArgs = specialArgsOf root; modules = concatMap (sectionsOf root (ancestorsOf root)) prismModules ++ mergedOf root "extraModules" ++ optional (rootPkgs != null) {nixpkgs.pkgs = rootPkgs;}; };in listToAttrs (map (root: { inherit (root) name; value = buildSystem root; }) (filter (tag: tag.build or false) (attrValues registry)))