diff --git a/erlang/apps/lib/src/getopt.erl b/erlang/apps/lib/src/getopt.erl
index d05a384..b90d9b0 100644
--- a/erlang/apps/lib/src/getopt.erl
+++ b/erlang/apps/lib/src/getopt.erl
@@ -165,28 +165,25 @@ find_longopt(_Prefix, _LongOpts, _Candidate) -> ?error.
 mkerror(Tag, Data) ->
   {error, {?MODULE, {Tag, Data}}}.
 
--spec format_error(term()) -> string().
+-spec format_error(term()) -> io_lib:chars().
 format_error(Reason) ->
   case Reason of
     {missing_argument, OptCh} ->
-      flatformat("missing argument to -~c", [OptCh]);
+      io_lib:format("missing argument to -~c", [OptCh]);
     {invalid_option, OptCh} ->
-      flatformat("invalid option -~c", [OptCh]);
+      io_lib:format("invalid option -~c", [OptCh]);
     {invalid_optstring, OptString} ->
-      flatformat("invalid optstring ~p", [OptString]);
+      io_lib:format("invalid optstring ~p", [OptString]);
     {invalid_argument_long, Prefix} ->
-      flatformat("invalid argument to --~s", [Prefix]);
+      io_lib:format("invalid argument to --~s", [Prefix]);
     {missing_argument_long, Prefix} ->
-      flatformat("missing argument to --~s", [Prefix]);
+      io_lib:format("missing argument to --~s", [Prefix]);
     {invalid_longopts, LongOpts} ->
-      flatformat("invalid longopts ~p", [LongOpts]);
+      io_lib:format("invalid longopts ~p", [LongOpts]);
     {invalid_option_long, Prefix} ->
-      flatformat("invalid option --~s", [Prefix]);
+      io_lib:format("invalid option --~s", [Prefix]);
     {ambiguous_option, Prefix} ->
-      flatformat("ambiguous option --~s", [Prefix]);
+      io_lib:format("ambiguous option --~s", [Prefix]);
     _ ->
-      flatformat("~p", [Reason])
+      io_lib:format("~p", [Reason])
   end.
-
-flatformat(Fmt, Args) ->
-  lists:flatten(io_lib:format(Fmt, Args)).
diff --git a/erlang/apps/lib/src/pdp10_stdio.erl b/erlang/apps/lib/src/pdp10_stdio.erl
index 88a7d8a..99619ee 100644
--- a/erlang/apps/lib/src/pdp10_stdio.erl
+++ b/erlang/apps/lib/src/pdp10_stdio.erl
@@ -150,25 +150,25 @@ stdin() ->
 stdout() ->
   do_open(stdout).
 
--spec format_error(term()) -> string().
+-spec format_error(term()) -> io_lib:chars().
 format_error(Reason) ->
   case Reason of
     {bad_request, Req} ->
-      flatformat("bad request ~p", [Req]);
+      io_lib:format("bad request ~p", [Req]);
     no_io_direction ->
       "no I/O direction";
     {bad_mode, Mode} ->
-      flatformat("bad mode ~p", [Mode]);
+      io_lib:format("bad mode ~p", [Mode]);
     {bad_fread, Size, NMemb} ->
-      flatformat("bad fread size ~p nmemb ~p", [Size, NMemb]);
+      io_lib:format("bad fread size ~p nmemb ~p", [Size, NMemb]);
     eof ->
       "end-of-file during fread";
     write_only ->
       "read from write-only file";
     {bad_whence, Whence} ->
-      flatformat("bad whence ~p", [Whence]);
+      io_lib:format("bad whence ~p", [Whence]);
     _ ->
-      flatformat("~p", [Reason])
+      io_lib:format("~p", [Reason])
   end.
 
 %% gen_server callbacks --------------------------------------------------------
@@ -700,6 +700,3 @@ mkerror(Reason) ->
 
 mkfileerror(Reason) ->
   {error, {file, Reason}}.
-
-flatformat(Fmt, Args) ->
-  lists:flatten(io_lib:format(Fmt, Args)).
diff --git a/erlang/apps/lib/src/strtol.erl b/erlang/apps/lib/src/strtol.erl
index 8f7521e..9b754ad 100644
--- a/erlang/apps/lib/src/strtol.erl
+++ b/erlang/apps/lib/src/strtol.erl
@@ -130,10 +130,10 @@ digit_value(C) ->
 mkerror(Tag) ->
   {error, {?MODULE, Tag}}.
 
--spec format_error(term()) -> string().
+-spec format_error(term()) -> io_lib:chars().
 format_error(Reason) ->
   case Reason of
     no_digits -> "no valid digits found";
     invalid_base -> "invalid base";
-    _ -> lists:flatten(io_lib:format("~p", [Reason]))
+    _ -> io_lib:format("~p", [Reason])
   end.