So far we have considered what Guile programming means for an application developer. But what if you are instead using an existing Guile-based application, and want to know what your options are for programming and extending this application?
The answer to this question varies from one application to another, because the options available depend inevitably on whether the application developer has provided any hooks for you to hang your own code on and, if there are such hooks, what they allow you to do.1 For example...
In the last two cases, what you can do is, by definition, restricted by the application, and you should refer to the application's own manual to find out your options.
The most well known example of the first case is Emacs, with its extension language Emacs Lisp: as well as being a text editor, Emacs supports the loading and execution of arbitrary Emacs Lisp code. The result of such openness has been dramatic: Emacs now benefits from user-contributed Emacs Lisp libraries that extend the basic editing function to do everything from reading news to psychoanalysis and playing adventure games. The only limitation is that extensions are restricted to the functionality provided by Emacs's built-in set of primitive operations. For example, you can interact and display data by manipulating the contents of an Emacs buffer, but you can't pop-up and draw a window with a layout that is totally different to the Emacs standard.
This situation with a Guile application that supports the loading of arbitrary user code is similar, except perhaps even more so, because Guile also supports the loading of extension libraries written in C. This last point enables user code to add new primitive operations to Guile, and so to bypass the limitation present in Emacs Lisp.
At this point, the distinction between an application developer and an application user becomes rather blurred. Instead of seeing yourself as a user extending an application, you could equally well say that you are developing a new application of your own using some of the primitive functionality provided by the original application. As such, all the discussions of the preceding sections of this chapter are relevant to how you can proceed with developing your extension.
 Of course, in the world of free software, you always have the freedom to modify the application's source code to your own requirements. Here we are concerned with the extension options that the application has provided for without your needing to modify its source code.