Extendable/customizable applications

Question

This idea is about a specific type of feature:
Namely,
the ability to define actions on objects (mail in a mail client, file in a file manager, etc).  Such an action would usually be an invocation of an external program or script, passing along some context-dependent variables which the "main program" provides you  (eg in a mail program: the sender address, subject, contents, etc.  in a filemanager: the filename, location,last change date etc),
Such scripts ("actions") would be triggered by opening a menu on the item (eg right click a mail in mail program or file in file manager) or a user-defined keyboard shortcut.
Such a feature is actually very common in file managers, and in a slightly less degree in window managers.
But here is the thing: what is holding back developers of web browsers, email clients, music programs, todo managers, calendars, ... of also implementing some like this?
I haven't seen such a feature (yet) in programs such as mail clients or web browsers, even though I think it's fairly easy to implement.
Actually I'm looking for a new (gui) mail client and web browser and I was thinking I could really use something like that. (eg I could write scripts to convert emails into calendar items, todo's etc.)
But, imho such a feature is beneficial for any kind of application, whether it be a cli-based todo program or a GUI email client.  Especially for people who really like to customize and fine-tune their workflow.
Or, to make this discussion a bit less theoretical: who has good practical examples of software implementing such features? (other then file managers).

Answer 1

Dieter@be wrote:But here is the thing: what is holding back developers of web browsers, email clients, music programs, todo managers, calendars, ... of also implementing some like this?
I haven't seen such a feature (yet) in programs such as mail clients or web browsers, even though I think it's fairly easy to implement.
Firefox and thunderbird extensions both offer the ability to do things like this. It would probably take a matter of minutes to write an extension that does some of the things you've suggested.
Or, to make this discussion a bit less theoretical: who has good practical examples of software implementing such features? (other then file managers).
JEdit has a superb event-oriented plugin architecture that seems similar to what you describe. Any decent text editor has a plugin/scripting feature that can be customized to do it.
While the specific feature you ask for may not be that common, the general plugin/hooks/extension architecture is certainly very common. It may be more complex than what you ask, but its also more powerful, and certainly encompasses the use cases you provided.
Dusty