An Overview of Guile Programming
================================

Guile is designed as an extension language interpreter that is straightforward to integrate with applications written in C (and C++). The big win here for the application developer is that Guile integration, as the Guile web page says, ``lowers your project's hacktivation energy.''  Lowering the hacktivation energy means that you, as the application developer, and your users, reap the benefits that flow from being able to extend the application in a high level extension language rather than in plain old C.

In abstract terms, it's difficult to explain what this really means and what the integration process involves, so instead let's begin by jumping straight into an example of how you might integrate Guile into an existing program, and what you could expect to gain by so doing.  With that example under our belts, we'll then return to a more general analysis of the arguments involved and the range of programming options available.

How One Might Extend Dia Using Guile
------------------------------------

Dia is a free software program for drawing schematic diagrams like flow charts and floor plans (http://www.gnome.org/projects/dia/). This section conducts the thought experiment of adding Guile to Dia.  In so doing, it aims to illustrate several of the steps and considerations involved in adding Guile to applications in general.

Deciding Why You Want to Add Guile
..................................

First off, you should understand why you want to add Guile to Dia at all, and that means forming a picture of what Dia does and how it does it.  So, what are the constituents of the Dia application?

  * Most importantly, the application domain objects --- in other words, the concepts that differentiate Dia from another application such as a word processor or spreadsheet: shapes, templates, connectors, pages, plus the properties of all these things.

  * The code that manages the graphical face of the application, including the layout and display of the objects above.

  * The code that handles input events, which indicate that the application user is wanting to do something.

(In other words, a textbook example of the model - view - controller paradigm.)

Next question: how will Dia benefit once the Guile integration is complete?  Several (positive!) answers are possible here, and the choice is obviously up to the application developers.  Still, one answer is that the main benefit will be the ability to manipulate Dia's application domain objects from Scheme.

Suppose that Dia made a set of procedures available in Scheme, representing the most basic operations on objects such as shapes, connectors, and so on.  Using Scheme, the application user could then write code that builds upon these basic operations to create more complex procedures.  For example, given basic procedures to enumerate the objects on a page, to determine whether an object is a square, and to change the fill pattern of a single shape, the user can write a Scheme procedure to change the fill pattern of all squares on the current page:

     (define (change-squares'-fill-pattern new-pattern)
       (for-each-shape current-page
         (lambda (shape)
           (if (square? shape)
               (change-fill-pattern shape new-pattern)))))

Four Steps Required to Add Guile
................................

Assuming this objective, four steps are needed to achieve it.

First, you need a way of representing your application-specific objects --- such as shape in the previous example --- when they are passed into the Scheme world.  Unless your objects are so simple that they map naturally into builtin Scheme data types like numbers and strings, you will probably want to use Guile's foreign object interface to create a new Scheme data type for your objects.

Second, you need to write code for the basic operations like for-each-shape and square? such that they access and manipulate your existing data structures correctly, and then make these operations available as primitives on the Scheme level.

Third, you need to provide some mechanism within the Dia application that a user can hook into to cause arbitrary Scheme code to be evaluated.

Finally, you need to restructure your top-level application C code a little so that it initializes the Guile interpreter correctly and declares your foreign objects and primitives to the Scheme world.

The following subsections expand on these four points in turn.

How to Represent Dia Data in Scheme
...................................

For all but the most trivial applications, you will probably want to allow some representation of your domain objects to exist on the Scheme level.  This is where foreign objects come in, and with them issues of lifetime management and garbage collection.

To get more concrete about this, let's look again at the example we gave earlier of how application users can use Guile to build higher-level functions from the primitives that Dia itself provides.

     (define (change-squares'-fill-pattern new-pattern)
       (for-each-shape current-page
         (lambda (shape)
           (if (square? shape)
               (change-fill-pattern shape new-pattern)))))

Consider what is stored here in the variable shape.  For each shape on the current page, the for-each-shape primitive calls (lambda (shape) ...) with an argument representing that shape.  Question is: how is that argument represented on the Scheme level?  The issues are as follows.

  * Whatever the representation, it has to be decodable again by the C code for the square? and change-fill-pattern primitives.  In other words, a primitive like square? has somehow to be able to turn the value that it receives back into something that points to the underlying C structure describing a shape.

  * The representation must also cope with Scheme code holding on to the value for later use.  What happens if the Scheme code stores shape in a global variable, but then that shape is deleted (in a way that the Scheme code is not aware of), and later on some other Scheme code uses that global variable again in a call to, say, square??

  * The lifetime and memory allocation of objects that exist only in the Scheme world is managed automatically by Guile's garbage collector using one simple rule: when there are no remaining references to an object, the object is considered dead and so its memory is freed.  But for objects that exist in both C and Scheme, the picture is more complicated; in the case of Dia, where the shape argument passes transiently in and out of the Scheme world, it would be quite wrong the delete the underlying C shape just because the Scheme code has finished evaluation.  How do we avoid this happening?

One resolution of these issues is for the Scheme-level representation of a shape to be a new, Scheme-specific C structure wrapped up as a foreign object.  The foreign object is what is passed into and out of Scheme code, and the Scheme-specific C structure inside the foreign object points to Dia's underlying C structure so that the code for primitives like square? can get at it.

To cope with an underlying shape being deleted while Scheme code is still holding onto a Scheme shape value, the underlying C structure should have a new field that points to the Scheme-specific foreign object.  When a shape is deleted, the relevant code chains through to the Scheme-specific structure and sets its pointer back to the underlying structure to NULL.  Thus the foreign object value for the shape continues to exist, but any primitive code that tries to use it will detect that the underlying shape has been deleted because the underlying structure pointer is NULL.

So, to summarize the steps involved in this resolution of the problem (and assuming that the underlying C structure for a shape is struct dia_shape):

  * Define a new Scheme-specific structure that points to the underlying C structure:

          struct dia_guile_shape
          {
            struct dia_shape * c_shape;   /* NULL => deleted */
          }

  * Add a field to struct dia_shape that points to its struct dia_guile_shape if it has one ---

          struct dia_shape
          {
            ...
            struct dia_guile_shape * guile_shape;
          }

     --- so that C code can set guile_shape->c_shape to NULL when the underlying shape is deleted.

  * Wrap struct dia_guile_shape as a foreign object type.

  * Whenever you need to represent a C shape onto the Scheme level, create a foreign object instance for it, and pass that.

  * In primitive code that receives a shape foreign object instance, check the c_shape field when decoding it, to find out whether the underlying C shape is still there.

As far as memory management is concerned, the foreign object values and their Scheme-specific structures are under the control of the garbage collector, whereas the underlying C structures are explicitly managed in exactly the same way that Dia managed them before we thought of adding Guile.

When the garbage collector decides to free a shape foreign object value, it calls the finalizer function that was specified when defining the shape foreign object type.  To maintain the correctness of the guile_shape field in the underlying C structure, this function should chain through to the underlying C structure (if it still exists) and set its guile_shape field to NULL.

For full documentation on defining and using foreign object types, see Defining New Foreign Object Types.

Writing Guile Primitives for Dia
................................

Once the details of object representation are decided, writing the primitive function code that you need is usually straightforward.

A primitive is simply a C function whose arguments and return value are all of type SCM, and whose body does whatever you want it to do. As an example, here is a possible implementation of the square? primitive:

     static SCM square_p (SCM shape)
     {
       struct dia_guile_shape * guile_shape;

       /* Check that arg is really a shape object. */
       scm_assert_foreign_object_type (shape_type, shape);

       /* Access Scheme-specific shape structure. */
       guile_shape = scm_foreign_object_ref (shape, 0);

       /* Find out if underlying shape exists and is a
          square; return answer as a Scheme boolean. */
       return scm_from_bool (guile_shape->c_shape &&
                             (guile_shape->c_shape->type == DIA_SQUARE));
     }

Notice how easy it is to chain through from the SCM shape parameter that square_p receives --- which is a foreign object --- to the Scheme-specific structure inside the foreign object, and thence to the underlying C structure for the shape.

In this code, scm_assert_foreign_object_type, scm_foreign_object_ref, and scm_from_bool are from the standard Guile API.  We assume that shape_type was given to us when we made the shape foreign object type, using scm_make_foreign_object_type.  The call to scm_assert_foreign_object_type ensures that SHAPE is indeed a shape.  This is needed to guard against Scheme code using the square?  procedure incorrectly, as in (square? "hello"); Scheme's latent typing means that usage errors like this must be caught at run time.

Having written the C code for your primitives, you need to make them available as Scheme procedures by calling the scm_c_define_gsubr function.  scm_c_define_gsubr (see Primitive Procedures) takes arguments that specify the Scheme-level name for the primitive and how many required, optional and rest arguments it can accept.  The square? primitive always requires exactly one argument, so the call to make it available in Scheme reads like this:

     scm_c_define_gsubr ("square?", 1, 0, 0, square_p);

For where to put this call, see the subsection after next on the structure of Guile-enabled code (see Dia Structure).

Providing a Hook for the Evaluation of Scheme Code
..................................................

To make the Guile integration useful, you have to design some kind of hook into your application that application users can use to cause their Scheme code to be evaluated.

Technically, this is straightforward; you just have to decide on a mechanism that is appropriate for your application.  Think of Emacs, for example: when you type ESC :, you get a prompt where you can type in any Elisp code, which Emacs will then evaluate.  Or, again like Emacs, you could provide a mechanism (such as an init file) to allow Scheme code to be associated with a particular key sequence, and evaluate the code when that key sequence is entered.

In either case, once you have the Scheme code that you want to evaluate, as a null terminated string, you can tell Guile to evaluate it by calling the scm_c_eval_string function.

Top-level Structure of Guile-enabled Dia
........................................

Let's assume that the pre-Guile Dia code looks structurally like this:

  * main ()

       * do lots of initialization and setup stuff

       * enter Gtk main loop

When you add Guile to a program, one (rather technical) requirement is that Guile's garbage collector needs to know where the bottom of the C stack is.  The easiest way to ensure this is to use scm_boot_guile like this:

  * main ()

       * do lots of initialization and setup stuff

       * scm_boot_guile (argc, argv, inner_main, NULL)

  * inner_main ()

       * define all foreign object types

       * export primitives to Scheme using scm_c_define_gsubr

       * enter Gtk main loop

In other words, you move the guts of what was previously in your main function into a new function called inner_main, and then add a scm_boot_guile call, with inner_main as a parameter, to the end of main.

Assuming that you are using foreign objects and have written primitive code as described in the preceding subsections, you also need to insert calls to declare your new foreign objects and export the primitives to Scheme.  These declarations must happen inside the dynamic scope of the scm_boot_guile call, but also before any code is run that could possibly use them --- the beginning of inner_main is an ideal place for this.

Going Further with Dia and Guile
................................

The steps described so far implement an initial Guile integration that already gives a lot of additional power to Dia application users.  But there are further steps that you could take, and it's interesting to consider a few of these.

In general, you could progressively move more of Dia's source code from C into Scheme.  This might make the code more maintainable and extensible, and it could open the door to new programming paradigms that are tricky to effect in C but straightforward in Scheme.

A specific example of this is that you could use the guile-gtk package, which provides Scheme-level procedures for most of the Gtk+ library, to move the code that lays out and displays Dia objects from C to Scheme.

As you follow this path, it naturally becomes less useful to maintain a distinction between Dia's original non-Guile-related source code, and its later code implementing foreign objects and primitives for the Scheme world.

For example, suppose that the original source code had a dia_change_fill_pattern function:

     void dia_change_fill_pattern (struct dia_shape * shape,
                                   struct dia_pattern * pattern)
     {
       /* real pattern change work */
     }

During initial Guile integration, you add a change_fill_pattern primitive for Scheme purposes, which accesses the underlying structures from its foreign object values and uses dia_change_fill_pattern to do the real work:

     SCM change_fill_pattern (SCM shape, SCM pattern)
     {
       struct dia_shape * d_shape;
       struct dia_pattern * d_pattern;

       ...

       dia_change_fill_pattern (d_shape, d_pattern);

       return SCM_UNSPECIFIED;
     }

At this point, it makes sense to keep dia_change_fill_pattern and change_fill_pattern separate, because dia_change_fill_pattern can also be called without going through Scheme at all, say because the user clicks a button which causes a C-registered Gtk+ callback to be called.

But, if the code for creating buttons and registering their callbacks is moved into Scheme (using guile-gtk), it may become true that dia_change_fill_pattern can no longer be called other than through Scheme.  In which case, it makes sense to abolish it and move its contents directly into change_fill_pattern, like this:

     SCM change_fill_pattern (SCM shape, SCM pattern)
     {
       struct dia_shape * d_shape;
       struct dia_pattern * d_pattern;

       ...

       /* real pattern change work */

       return SCM_UNSPECIFIED;
     }

So further Guile integration progressively reduces the amount of functional C code that you have to maintain over the long term.

A similar argument applies to data representation.  In the discussion of foreign objects earlier, issues arose because of the different memory management and lifetime models that normally apply to data structures in C and in Scheme.  However, with further Guile integration, you can resolve this issue in a more radical way by allowing all your data structures to be under the control of the garbage collector, and kept alive by references from the Scheme world.  Instead of maintaining an array or linked list of shapes in C, you would instead maintain a list in Scheme.

Rather like the coalescing of dia_change_fill_pattern and change_fill_pattern, the practical upshot of such a change is that you would no longer have to keep the dia_shape and dia_guile_shape structures separate, and so wouldn't need to worry about the pointers between them.  Instead, you could change the foreign object definition to wrap the dia_shape structure directly, and send dia_guile_shape off to the scrap yard.  Cut out the middle man!

Finally, we come to the holy grail of Guile's free software / extension language approach.  Once you have a Scheme representation for interesting Dia data types like shapes, and a handy bunch of primitives for manipulating them, it suddenly becomes clear that you have a bundle of functionality that could have far-ranging use beyond Dia itself.  In other words, the data types and primitives could now become a library, and Dia becomes just one of the many possible applications using that library --- albeit, at this early stage, a rather important one!

In this model, Guile becomes just the glue that binds everything together.  Imagine an application that usefully combined functionality from Dia, Gnumeric and GnuCash --- it's tricky right now, because no such application yet exists; but it'll happen some day ...

Why Scheme is More Hackable Than C
----------------------------------

Underlying Guile's value proposition is the assumption that programming in a high level language, specifically Guile's implementation of Scheme, is necessarily better in some way than programming in C.  What do we mean by this claim, and how can we be so sure?

One class of advantages applies not only to Scheme, but more generally to any interpretable, high level, scripting language, such as Emacs Lisp, Python, Ruby, or TeX's macro language.  Common features of all such languages, when compared to C, are that:

  * They lend themselves to rapid and experimental development cycles, owing usually to a combination of their interpretability and the integrated development environment in which they are used.

  * They free developers from some of the low level bookkeeping tasks associated with C programming, notably memory management.

  * They provide high level features such as container objects and exception handling that make common programming tasks easier.

In the case of Scheme, particular features that make programming easier --- and more fun! --- are its powerful mechanisms for abstracting parts of programs (closures --- see About Closure) and for iteration (see while do).

The evidence in support of this argument is empirical: the huge amount of code that has been written in extension languages for applications that support this mechanism.  Most notable are extensions written in Emacs Lisp for GNU Emacs, in TeX's macro language for TeX, and in Script-Fu for the Gimp, but there is increasingly now a significant code eco-system for Guile-based applications as well, such as Lilypond and GnuCash.  It is close to inconceivable that similar amounts of functionality could have been added to these applications just by writing new code in their base implementation languages.

Example: Using Guile for an Application Testbed
-----------------------------------------------

As an example of what this means in practice, imagine writing a testbed for an application that is tested by submitting various requests (via a C interface) and validating the output received.  Suppose further that the application keeps an idea of its current state, and that the ``correct'' output for a given request may depend on the current application state.  A complete ``white box'' (A white box test plan is one that incorporates knowledge of the internal design of the application under test.) test plan for this application would aim to submit all possible requests in each distinguishable state, and validate the output for all request/state combinations.

To write all this test code in C would be very tedious.  Suppose instead that the testbed code adds a single new C function, to submit an arbitrary request and return the response, and then uses Guile to export this function as a Scheme procedure.  The rest of the testbed can then be written in Scheme, and so benefits from all the advantages of programming in Scheme that were described in the previous section.

(In this particular example, there is an additional benefit of writing most of the testbed in Scheme.  A common problem for white box testing is that mistakes and mistaken assumptions in the application under test can easily be reproduced in the testbed code.  It is more difficult to copy mistakes like this when the testbed is written in a different language from the application.)

A Choice of Programming Options
-------------------------------

The preceding arguments and example point to a model of Guile programming that is applicable in many cases.  According to this model, Guile programming involves a balance between C and Scheme programming, with the aim being to extract the greatest possible Scheme level benefit from the least amount of C level work.

The C level work required in this model usually consists of packaging and exporting functions and application objects such that they can be seen and manipulated on the Scheme level.  To help with this, Guile's C language interface includes utility features that aim to make this kind of integration very easy for the application developer.  These features are documented later in this part of the manual: see REFFIXME.

This model, though, is really just one of a range of possible programming options.  If all of the functionality that you need is available from Scheme, you could choose instead to write your whole application in Scheme (or one of the other high level languages that Guile supports through translation), and simply use Guile as an interpreter for Scheme.  (In the future, we hope that Guile will also be able to compile Scheme code, so lessening the performance gap between C and Scheme code.)  Or, at the other end of the C--Scheme scale, you could write the majority of your application in C, and only call out to Guile occasionally for specific actions such as reading a configuration file or executing a user-specified extension.  The choices boil down to two basic questions:

  * Which parts of the application do you write in C, and which in Scheme (or another high level translated language)?

  * How do you design the interface between the C and Scheme parts of your application?

These are of course design questions, and the right design for any given application will always depend upon the particular requirements that you are trying to meet.  In the context of Guile, however, there are some generally applicable considerations that can help you when designing your answers.

What Functionality is Already Available?
........................................

Suppose, for the sake of argument, that you would prefer to write your whole application in Scheme.  Then the API available to you consists of:

  * standard Scheme

  * plus the extensions to standard Scheme provided by Guile in its core distribution

  * plus any additional functionality that you or others have packaged so that it can be loaded as a Guile Scheme module.

A module in the last category can either be a pure Scheme module --- in other words a collection of utility procedures coded in Scheme --- or a module that provides a Scheme interface to an extension library coded in C --- in other words a nice package where someone else has done the work of wrapping up some useful C code for you.  The set of available modules is growing quickly and already includes such useful examples as (gtk gtk), which makes Gtk+ drawing functions available in Scheme, and (database postgres), which provides SQL access to a Postgres database.

Given the growing collection of pre-existing modules, it is quite feasible that your application could be implemented by combining a selection of these modules together with new application code written in Scheme.

If this approach is not enough, because the functionality that your application needs is not already available in this form, and it is impossible to write the new functionality in Scheme, you will need to write some C code.  If the required function is already available in C (e.g. in a library), all you need is a little glue to connect it to the world of Guile.  If not, you need both to write the basic code and to plumb it into Guile.

In either case, two general considerations are important.  Firstly, what is the interface by which the functionality is presented to the Scheme world?  Does the interface consist only of function calls (for example, a simple drawing interface), or does it need to include objects of some kind that can be passed between C and Scheme and manipulated by both worlds.  Secondly, how does the lifetime and memory management of objects in the C code relate to the garbage collection governed approach of Scheme objects?  In the case where the basic C code is not already written, most of the difficulties of memory management can be avoided by using Guile's C interface features from the start.

For the full documentation on writing C code for Guile and connecting existing C code to the Guile world, see REFFIXME.

Functional and Performance Constraints
......................................

Your Preferred Programming Style
................................

What Controls Program Execution?
................................

How About Application Users?
----------------------------

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. (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.)  For example...

  * If the application permits you to load and execute any Guile code, the world is your oyster.  You can extend the application in any way that you choose.

  * A more cautious application might allow you to load and execute Guile code, but only in a safe environment, where the interface available is restricted by the application from the standard Guile API.

  * Or a really fearful application might not provide a hook to really execute user code at all, but just use Scheme syntax as a convenient way for users to specify application data or configuration options.

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.