Define {builder,fs}.nix for utils.nix

Re-export everything from utils.nix.

utils/fs.nix

@@ -0,0 +1,41 @@
+{ pkgs, ... }:
+
+# `fs` contains utility functions for working with the filesystem.
+
+let
+  inherit (builtins) attrNames hasAttr map readDir;
+  inherit (pkgs.lib) filterAttrs;
+in {
+  # Returns a list of all of the regular files in `dir`.
+  files = dir:
+    map (name: dir + "/${name}")
+      (attrNames
+        (filterAttrs (_: type: type == "regular") (readDir dir)));
+
+  # Returns a list of all of the directories in `dir`.
+  dirs = dir:
+    map (name: dir + "/${name}")
+      (attrNames
+        (filterAttrs (_: type: type == "directory") (readDir dir)));
+
+  # Returns a list of paths to all of the `name` files starting at `dir`.
+  find = name: dir:
+    if hasAttr name (readDir dir) then
+      [ (dir + name) ] ++ concatMap findAllDefaultNix (dirs dir)
+    else
+      concatMap findAllDefaultNix (dirs dir);
+
+  # Looks for `name` in `dir`; if it cannot find it, it checks the parent
+  # directory.
+  resolve = name: dir:
+    if hasAttr name (readDir dir) then
+      dir + "/${name}"
+    else
+      # This prevents the function from infinitely recursing and eventually
+      # stack overflowing.
+      if (dirOf dir) == dir then
+        null
+      else
+        resolve name (dirOf dir);
+  };
+}