Opencart 1.5+ calling custom function from module controller

Hi guys, I've been wondering if I can call custom function from module controller.
I've written some extensions before where I have a custom function called from the controller, for example:
if I edit admin/controller/sale/customer.php and after index() function add I can access it by using Now I have a module in catalog, could I access a function from it's controller, in the below example "foo"?
my_module.php: Thanks in advance for any insights.
B&P

It's not really clear what you want but if you want to access a function from within the controller just call it like this: You can also set up the call to the function to be echoed in the TPL (from the controller) either by href or by using AJAX.

Thanks for your reply. Sorry if I wasn't clear.
On pages that have their own url (route) like index.php?route=account/customer, I can access custom function by just appending function name to the url, like this: index.php?route=account/customer/my_function

But for modules, there are no urls or routes as they are loaded on the fly into other pages.
That was my question. I do need to access it from the .tpl, i.e. when I click a button it makes ajax call to this module's controller, if that does make any sense.

Example, I add a function my_fuction() to the account/customer.php
Then I can make an ajax call or access it directly through this url:
index.php?route=account/customer/my_function

It will return or output: foo

Now, if I add the same function to a module's controller, how would I access it. Modules are loaded into index.php and we can have them on any page and can have multiple instances of the same module on a single page.

Ah yes. I see. There is no difference at all to be honest. A controller is a controller. For example if you take the categories module (which displays categories on a floating box) . In the category.tpl you could put an ajax function that contacts a public function inside the module/category.php controller, just like you are planning on doing. No difference at all.

Could you demonstrate please. if categories box is on home page, how do I go about it? index.php?route=common/home/my_function ?
Because, obviously index.php?route=modules/category/my_function won't work (I did try it though..)

Thanks.

No, you are thinking about this the wrong way. Just because it is on the home page doesn't mean you mess with the home page controller. It's a module, and positioned on different pages depending upon what your settings are. It will look the same on all pages. Just mess with module/category.php and module/category.tpl

Yes, I am confused. thought I am getting hang of this MVC thing until this. Can you give an example of how would I call the controller function from the .tpl? I want to make an AJAX call to the controller from a button in my .tpl