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diff --git a/filesystem.nix b/filesystem.nix
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+# Functions for querying information about the filesystem
+# without copying any files to the Nix store.
+{ lib }:
+
+# Tested in lib/tests/filesystem.sh
+let
+  inherit (builtins)
+    readDir
+    pathExists
+    storeDir
+    ;
+
+  inherit (lib.trivial)
+    inPureEvalMode
+    ;
+
+  inherit (lib.path)
+    deconstruct
+    ;
+
+  inherit (lib.path.components)
+    fromSubpath
+    ;
+
+  inherit (lib.lists)
+    take
+    length
+    ;
+
+  inherit (lib.filesystem)
+    pathType
+    ;
+in
+
+{
+
+  /*
+    The type of a path. The path needs to exist and be accessible.
+    The result is either "directory" for a directory, "regular" for a regular file, "symlink" for a symlink, or "unknown" for anything else.
+
+    Type:
+      pathType :: Path -> String
+
+    Example:
+      pathType /.
+      => "directory"
+
+      pathType /some/file.nix
+      => "regular"
+  */
+  pathType =
+    let
+      storeDirComponents = fromSubpath "./${storeDir}";
+
+      legacy = path:
+        if ! pathExists path
+        # Fail irrecoverably to mimic the historic behavior of this function and
+        # the new builtins.readFileType
+        then abort "lib.filesystem.pathType: Path ${toString path} does not exist."
+        # The filesystem root is the only path where `dirOf / == /` and
+        # `baseNameOf /` is not valid. We can detect this and directly return
+        # "directory", since we know the filesystem root can't be anything else.
+        else if dirOf path == path
+        then "directory"
+        else (readDir (dirOf path)).${baseNameOf path};
+
+      legacyPureEval = path:
+        let
+          # This is a workaround for the fact that `lib.filesystem.pathType` doesn't work correctly on /nix/store/...-name paths in pure evaluation mode. For file set combinators it's safe to assume that
+          pathParts = deconstruct path;
+          assumeDirectory =
+            storeDirComponents == take (length pathParts.components - 1) pathParts.components;
+        in
+        if assumeDirectory then
+          "directory"
+        else
+          legacy path;
+
+    in
+    if builtins ? readFileType then
+      builtins.readFileType
+    # Nix <2.14 compatibility shim
+    else if inPureEvalMode then
+      legacyPureEval
+    else
+      legacy;
+
+  /*
+    Whether a path exists and is a directory.
+
+    Type:
+      pathIsDirectory :: Path -> Bool
+
+    Example:
+      pathIsDirectory /.
+      => true
+
+      pathIsDirectory /this/does/not/exist
+      => false
+
+      pathIsDirectory /some/file.nix
+      => false
+  */
+  pathIsDirectory = path:
+    pathExists path && pathType path == "directory";
+
+  /*
+    Whether a path exists and is a regular file, meaning not a symlink or any other special file type.
+
+    Type:
+      pathIsRegularFile :: Path -> Bool
+
+    Example:
+      pathIsRegularFile /.
+      => false
+
+      pathIsRegularFile /this/does/not/exist
+      => false
+
+      pathIsRegularFile /some/file.nix
+      => true
+  */
+  pathIsRegularFile = path:
+    pathExists path && pathType path == "regular";
+
+  /*
+    A map of all haskell packages defined in the given path,
+    identified by having a cabal file with the same name as the
+    directory itself.
+
+    Type: Path -> Map String Path
+  */
+  haskellPathsInDir =
+    # The directory within to search
+    root:
+    let # Files in the root
+        root-files = builtins.attrNames (builtins.readDir root);
+        # Files with their full paths
+        root-files-with-paths =
+          map (file:
+            { name = file; value = root + "/${file}"; }
+          ) root-files;
+        # Subdirectories of the root with a cabal file.
+        cabal-subdirs =
+          builtins.filter ({ name, value }:
+            builtins.pathExists (value + "/${name}.cabal")
+          ) root-files-with-paths;
+    in builtins.listToAttrs cabal-subdirs;
+  /*
+    Find the first directory containing a file matching 'pattern'
+    upward from a given 'file'.
+    Returns 'null' if no directories contain a file matching 'pattern'.
+
+    Type: RegExp -> Path -> Nullable { path : Path; matches : [ MatchResults ]; }
+  */
+  locateDominatingFile =
+    # The pattern to search for
+    pattern:
+    # The file to start searching upward from
+    file:
+    let go = path:
+          let files = builtins.attrNames (builtins.readDir path);
+              matches = builtins.filter (match: match != null)
+                          (map (builtins.match pattern) files);
+          in
+            if builtins.length matches != 0
+              then { inherit path matches; }
+              else if path == /.
+                then null
+                else go (dirOf path);
+        parent = dirOf file;
+        isDir =
+          let base = baseNameOf file;
+              type = (builtins.readDir parent).${base} or null;
+          in file == /. || type == "directory";
+    in go (if isDir then file else parent);
+
+
+  /*
+    Given a directory, return a flattened list of all files within it recursively.
+
+    Type: Path -> [ Path ]
+  */
+  listFilesRecursive =
+    # The path to recursively list
+    dir:
+    lib.flatten (lib.mapAttrsToList (name: type:
+    if type == "directory" then
+      lib.filesystem.listFilesRecursive (dir + "/${name}")
+    else
+      dir + "/${name}"
+  ) (builtins.readDir dir));
+
+}