is there a lib function to upper-case the first character in a string? I have

mutFirstChar =
    f: s:
    let
      firstChar = f (lib.substring 0 1 s);
      rest = lib.substring 1 (-1) s;
    in
    firstChar + rest;
  toTitle = s: mutFirstChar lib.toUpper s;
  darwinAppName = toTitle binaryName + ".app";

but I feel like there probably should be a builtin for this?