io_lib_pretty - a nice secret module

There are some modules in the erlang stdlib that aren't exactly advertised, but are quite useful. My newest discovery is io_lib_pretty. It hasn't got a manpage, but there are some docs if you less `locate io_lib_pretty.erl`.

io_lib_pretty is the module used by the shell to print records in a nicely formatted way. This isn't possible using plain io:format but can make program output a lot nicer.

Take for example a logging program that deals with records that look like this:

5> L = #logMessage{actor=23507, server_ip = <<123,234,123,234>>}.
#logMessage{actor = 23507,
            server_ip = <<"{\352{\352">>,
            timestamp = undefined,
            level = undefined,
            log_filename = undefined,
            message = undefined}

If you just try to print it out, you get:

7> io:format("Logged: ~p", [L]).
Logged: {logMessage,23507,<<"{\352{\352">>,undefined,undefined,undefined,undefined}ok

Pretty useless output.

Using io_lib_pretty you can get:

9> io:format(io_lib_pretty:print(L, fun(logMessage, 6) -> [actor, server_ip, timestamp, level, log_filename, message] end)).
#logMessage{actor = 23507,
            server_ip = <<"{\352{\352">>,
            timestamp = undefined,
            level = undefined,
            log_filename = undefined,
            message = undefined}ok

Just like the shell. I listed the record information manually in the function above, but you can easily use the record_info macro to accomplish the same without code duplication. Or even easier, use the exprecs parse transform (pretty printing example available there).

Next time: how to load record definitions dynamically at runtime.