What I would really like to see is a statically typed language on top of the Erlang VM (dialyzer was not amazing last time I checked it, which was admittedly years ago).
-record(outside_error, {
reason :: term()
}).
-record(inside_error, {
reason :: binary()
}).
-spec to_inside_error(#outside_error{}) -> #inside_error{}.
to_inside_error(#outside_error{reason = Reason}) ->
#inside_error{reason = Reason}.