erlang io:format, and a hanging web application

Question

While I'm learning a new language, I'll typically put lots of silly println's to see what values are where at specific times. It usually suffices because the languages typically have available a tostring equivalent. In trying that same approach with erlang, my webapp just "hangs" when there's a value attempted to be printed that's not a list. This happens when variable being printed is a tuple instead of a list. There's no error, exception, nothing... just doesn't respond. Now, I'm muddling through by being careful about what I'm writing out and as I learn more, things are getting better. But I wonder, is there a way to more reliably to [blindly] print a value to stdout?
Thanks,

--tim

Answer 1

In Erlang, as in other languages, you can print your variables, no matter if they are a list, a tuple or anything else.

My feeling is that, for printing, you're doing something like (just a guess):

io:format("The value is: ~p.", A).

This is wrong, because you're supposed to pass a list of arguments:

io:format("The value is: ~p.", [A]).

Where A can be anything.

I usually find comfortable to use:

erlang:display/1

to print variables.

Also, tracing functions is usually a better way to debug an application, rather than using printouts. Please see:

http://aloiroberto.wordpress.com/2009/02/23/tracing-erlang-functions/

Answer 2

When developing webapps I use the error_logger module I usually define some macros like this

-ifdef(debug).
-define(idbg(FmtStr, Err), 
        error_logger:info_msg("~p (line ~p): " FmtStr "~n", 
                              [?MODULE, ?LINE | Err])).
-define(rdbg(Term), error_logger:info_report(Term)).
-else.
-define(idbg(_FmtStr, _Err), void).
-define(rdbg(_Term), void).
-endif.

You call the macros with something like:

code...
?rdbg(ErlangTerm),
other code...

During development you compile your modules with:

erlc -Ddebug *.erl

and so you get info messages in your erlang console.

Answer 3

Also make sure that there is no terminating process without link which could then cause other process to wait on something and not timeout-ing - hence strange hanging part.