1 files changed, 65 insertions, 0 deletions
diff --git a/lib/by-name-overlay.nix b/lib/by-name-overlay.nix
new file mode 100644
index 00000000..cc8bcb67
--- /dev/null
+++ b/lib/by-name-overlay.nix
@@ -0,0 +1,65 @@
+{warn}:
+# Adapted from this: https://github.com/NixOS/nixpkgs/blob/1814b56453c91192f6d5a6276079948f9fe96c18/pkgs/top-level/by-name-overlay.nix
+# This file should not depend on `pkgs` and thus not use `lib`.
+{
+  baseDirectory,
+  fileName,
+  finalizeFunction,
+}: let
+  # Takes a list of attrs as input and returns one merged attr set.
+  flattenAttrs = list:
+    if builtins.isList list
+    then
+      builtins.foldl' (acc: elem:
+        if builtins.isList elem
+        # Merging them with `//` is okay here, as we can be sure that the attr names are
+        # unique (they were separate dictionary after all)
+        then acc // (flattenAttrs elem)
+        else acc // elem) {}
+      list
+    else list;
+
+  # From nixpkgs/lib {{{
+  # These functions are taken straight out of the `nixpkgs/lib`.
+  # We can't depended on `pkgs` (and thus on `lib`), because the `pkgs` module argument
+  # is only defined in the `nixpkgs` module (which is imported through this function).
+  mapAttrsToList = f: attrs:
+    builtins.map (name: f name attrs.${name}) (builtins.attrNames attrs);
+
+  nameValuePair = name: value: {inherit name value;};
+  filterAttrs = pred: set:
+    builtins.listToAttrs (builtins.concatMap (name: let
+      v = set.${name};
+    in
+      if pred name v
+      then [(nameValuePair name v)]
+      else []) (builtins.attrNames set));
+  # }}}
+
+  # Module files for a single shard
+  # Type: String -> String -> ListOf Path
+  namesForShard = shard: type:
+    if type != "directory"
+    then warn "Ignored non-directory, whilst importing by-name directory (${fileName}): '${shard}'" {}
+    else let
+      mkPath = name: _type: let
+        path = baseDirectory + "/${shard}/${name}" + "/${fileName}";
+      in
+        if builtins.pathExists path
+        then path
+        else warn "'${builtins.toString path}' does not exist. Skipped" null;
+    in
+      filterAttrs (name: value: value != null)
+      (builtins.mapAttrs
+        mkPath
+        (builtins.readDir (baseDirectory + "/${shard}")));
+
+  # A list of all module paths.
+  # These can the be simply injected into `import`
+  files = flattenAttrs (mapAttrsToList namesForShard (builtins.readDir baseDirectory));
+  output =
+    builtins.mapAttrs
+    finalizeFunction
+    files;
+in
+  output