Yaws process died: {{badmatch,<<>>} - debugging

Question

I'm going over a very basic erlang book while using yaws. I'm editing a single yaws file and refreshing the browser. Often (3rd time now) the process will just start to show this error. and i look and look for a syntax error or anything, and eventually i just restart the process and everything works. without any change to the source file.
right now this is the source file that triggered the error this last time
<erl>
out(Arg) ->
{ehtml,
{table, [{width, "100%"}],
{tr,[],
[{td, [{width, "50%"}], "hello world!"},
{td, [{width, "50%"}], "hi again."}]
}
}
}.
</erl>
I tried searching the error, but where all the search results have a meaningful context like "no access" all i get is "<<>>"
=ERROR REPORT==== 26-Nov-2013::20:17:32 ===
Yaws process died: {{badmatch,<<>>},
[{yaws_server,skip_data,2,
[{file,"yaws_server.erl"},{line,2951}]},
{yaws_server,deliver_dyn_file,6,
[{file,"yaws_server.erl"},{line,2717}]},
{yaws_server,aloop,4,
[{file,"yaws_server.erl"},{line,1152}]},
{yaws_server,acceptor0,2,
[{file,"yaws_server.erl"},{line,1013}]},
{proc_lib,init_p_do_apply,3,
[{file,"proc_lib.erl"},{line,227}]}]}
Some version info:
Yaws 1.94
Debian GNU/Linux 7.2 (wheezy)
Linux 3.2.0-4-amd64 #1 SMP Debian 3.2.51-1 x86_64 GNU/Linux
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:4:4] [async-threads:0] [hipe] [kernel-poll:false]
Any ideas what this is telling me?
Also, any suggestions for debuggers that are good for beginners is very welcome.

Answer 1

For debugging, I think using Erlang tracing will be helpful. We want to figure out why the yaws_server:skip_data/2 function would be getting a badmatch exception, and specifically why it's getting an empty binary passed to it as that's the only way it could encounter that error. So we need to trace that condition. Try these steps (and don't forget the trailing period on each Erlang shell command):
Run yaws in interactive mode: yaws -i
Once yaws comes up, hit enter to get an Erlang shell prompt.
Create a tracing function for dbg so we get a reasonably formatted backtrace from the trace data: F = fun({trace,_,_,_,Dump},[]) -> io:format("~s~n", [binary_to_list(Dump)]), [] end.
Turn on tracing with this command: dbg:tracer(process, {F, []}).
Trace calls in all processes: dbg:p(all, call).
Now trace the condition of yaws_server:skip_data/2 getting an empty binary as a first argument, and when it does, get a backtrace:
dbg:tpl(yaws_server,skip_data,dbg:fun2ms(fun([<<>>, _]) -> message(process_dump()) end)).
With this in place, start hitting your .yaws page until you provoke the condition, at which point a backtrace will be displayed in your Erlang shell. If you get that backtrace, please copy it into a gist or pastebin and post a link to it as a follow-up here.

Answer 2

If I am correct the output of the out function is supposed to be a list. I didn't check your whole code but the following should work:
<erl>
out(Arg) ->
[{ehtml,
{table, [{width, "100%"}],
{tr,[],
[{td, [{width, "50%"}], "hello world!"},
{td, [{width, "50%"}], "hi again."}]
}
}
}].
</erl>