Erlang and method_missing

Question

I come from a Java and Ruby background and I was wondering if there is any equivalent of "method_missing" in Erlang. I have looked at the documentation and the closest I can see is the use of erl_eval and erl_parse, but I wanted to know if there is any other way?

Answer 1

I'm using a similar pattern on the calling side where a module may supply a log level classification function (which may return invalid results). I do it like this:

-spec categorize(atom(), integer()) -> log_classification().
categorize(Module, Code) ->
    case catch(Module:ns_log_cat(Code)) of
        info -> info;
        warn -> warn;
        crit -> crit;
        _ -> info % Anything unknown is info (this includes {'EXIT', Reason})
        end.

Since there isn't a method (in general, you could certainly create an OO behavior and implement it with messages and then have method_missing, the callee doesn't know I attempted anything, so it can't interpret it.

Answer 2

There is the 'error_handler' module which is called when you attempt to call an undefined function.

process_flag(error_handler, Module).

The module needs to export undefined_function/3 and undefined_lambda/3. You can see how this should be implemented in the standard handler module, error_handler.

You need to be careful in your own error_handler not to break the standard code loading system. Erlang usually tries to load the module from an undefined function error and re-run the function before giving up and signalling an error. You usually want to try the standard error_handler first and fall back to your new behaviour if that fails.

Answer 3

If you are into "defensive coding", you can call Module:module_info(exports) to see if the function you're about to call is exported or not.

Answer 4

Erlang has functions not methods and generally trying to call an undefined function is considered to be an error.

The compiler generates an error if you try to call a non-existent function within the same module. Trying to call a function in another module will generate an error at runtime, the undef error. How you wish to handle such an error depends on your application.