Change jQuery effect with plugin options

Im trying to write a small jQuery plugin and be able to change the effects (fadeIn, slideDown etc etc) with options but am having no luck working out how to, for example if I was to add this

fx : fadeIn

then it would change this:

$('.foo').opts.fx('slow');

This issue I seem to have is that it doesen't like the placement of the option so am I suppost to wrap it somehow?

I'm no jQuery guru, but since there are no responses, I'll give it a try.

This seems to work for me:

if ($.isFunction($('#foo')[opts.fx])) { // check if it's a valid function
    $('#foo')[opts.fx]("slow");
} else { 
    // ... use default effect ...
}

See demo.