lib.fileset.fileFilter: Minor cleanups and more tests

      fileFilter (file: hasPrefix "." file.name) ./.

      # Include all regular files (not symlinks or others) in the current directory

      fileFilter (file: file.type == "regular")

      fileFilter (file: file.type == "regular") ./.

  */

  fileFilter =

    /*

    fileset:

    if ! isFunction predicate then

      throw ''

        lib.fileset.fileFilter: First argument is of type ${typeOf predicate}, but it should be a function.''

        lib.fileset.fileFilter: First argument is of type ${typeOf predicate}, but it should be a function instead.''

    else

      _fileFilter predicate

        (_coerce "lib.fileset.fileFilter: Second argument" fileset);

        _differenceTree (path + "/${name}") lhsValue (rhs.${name} or null)

      ) (_directoryEntries path lhs);

  # Filters all files in a file set based on a predicate

  # Type: ({ name, type, ... } -> Bool) -> FileSet -> FileSet

  _fileFilter = predicate: fileset:

    let

      # Check the predicate for a path and a filesetTree, returning a new filesetTree

      # Type: Path -> filesetTree -> filesetTree

      recurse = path: tree:

        mapAttrs (name: subtree:

          if isAttrs subtree || subtree == "directory" then

## File filter

# The first argument needs to be a function

expectFailure 'fileFilter null (abort "this is not needed")' 'lib.fileset.fileFilter: First argument is of type null, but it should be a function instead.'

# The second argument can be a file set or an existing path

expectFailure 'fileFilter (file: abort "this is not needed") null' 'lib.fileset.fileFilter: Second argument is of type null, but it should be a file set or a path instead.'

expectFailure 'fileFilter (file: abort "this is not needed") ./a' 'lib.fileset.fileFilter: Second argument \('"$work"'/a\) is a path that does not exist.'

# The predicate is not called when there's no files

tree=()

checkFileset 'fileFilter (file: abort "this is not needed") ./.'