code completion without building a PSIFile?

Hi

I've just started writing a plugin, a plugin for a custom filetype. Is there a way to invoke the completionContributor without having a PSIFile build(i.e) without parsing the file to create the AST? I just want to insert some text to the file through the auto completion pop up. I can't seem to find any proper way to do this. I tried implementing a CompletionContributor and declared as an extension point but cant' seem to figure out what to pass to the extend(). can someone just guide me in the proper direction.

Thanks
Vishnu