diff --git a/erlang/apps/lib/src/error.erl b/erlang/apps/lib/src/error.erl
new file mode 100644
index 0000000..5aa967d
--- /dev/null
+++ b/erlang/apps/lib/src/error.erl
@@ -0,0 +1,47 @@
+%%% -*- erlang-indent-level: 2 -*-
+%%%
+%%% error message formatting
+%%% Copyright (C) 2019  Mikael Pettersson
+%%%
+%%% This file is part of pdp10-tools.
+%%%
+%%% pdp10-tools is free software: you can redistribute it and/or modify
+%%% it under the terms of the GNU General Public License as published by
+%%% the Free Software Foundation, either version 3 of the License, or
+%%% (at your option) any later version.
+%%%
+%%% pdp10-tools is distributed in the hope that it will be useful,
+%%% but WITHOUT ANY WARRANTY; without even the implied warranty of
+%%% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+%%% GNU General Public License for more details.
+%%%
+%%% You should have received a copy of the GNU General Public License
+%%% along with pdp10-tools.  If not, see <http://www.gnu.org/licenses/>.
+%%%
+%%%-----------------------------------------------------------------------------
+%%%
+%%% Format error terms for output in end-user visible diagnostics.
+%%%
+%%% Our standard representation of error terms is as {Module, Reason} 2-tuples,
+%%% where Module:format_error(Reason) returns a textual representation of Reason.
+%%% These error terms are typically returned as {error, {Module, Reason}} values
+%%% or thrown as error exceptions.
+%%%
+%%% This code will _not_ attempt to load Module.
+
+-module(error).
+-export([format/1]).
+
+-spec format(term()) -> io_lib:chars().
+format({Module, Reason} = Error) when is_atom(Module) ->
+  case erlang:function_exported(Module, format_error, 1) of
+    true ->
+      try Module:format_error(Reason)
+      catch _:_ -> default_format(Error)
+      end;
+    false -> default_format(Error)
+  end;
+format(Error) -> default_format(Error).
+
+default_format(Error) ->
+  io_lib:format("~p", [Error]).