( Writing Extensions for Guile

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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.


* A Sample Guile Extension
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