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5.4 Writing Extensions for Guile

The previous sections have briefly explained how to write programs that make use of an embedded Guile interpreter. But sometimes, all you want to do is make new primitive procedures and data types available to the Scheme programmer. Writing a new version of guile is inconvenient in this case and it would in fact make the life of the users of your new features needlessly hard.

For example, suppose that there is a program guile-db that is a version of Guile with additional features for accessing a database. People who want to write Scheme programs that use these features would have to use guile-db instead of the usual guile program. Now suppose that there is also a program guile-gtk that extends Guile with access to the popular Gtk+ toolkit for graphical user interfaces. People who want to write GUIs in Scheme would have to use guile-gtk. Now, what happens when you want to write a Scheme application that uses a GUI to let the user access a database? You would have to write a third program that incorporates both the database stuff and the GUI stuff. This might not be easy (because guile-gtk might be a quite obscure program, say) and taking this example further makes it easy to see that this approach can not work in practice.

It would have been much better if both the database features and the GUI feature had been provided as libraries that can just be linked with guile. Guile makes it easy to do just this, and we encourage you to make your extensions to Guile available as libraries whenever possible.

You write the new primitive procedures and data types in the normal fashion, and link them into a shared library instead of into a stand-alone program. The shared library can then be loaded dynamically by Guile.