@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2014
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Programming Overview
@section 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, @emph{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.

@menu
* Extending Dia::               How one might extend Dia using Guile.
* Scheme vs C::                 Why Scheme is more hackable than C.
* Testbed Example::             Example: using Guile in a testbed.
* Programming Options::         Options for Guile programming.
* User Programming::            How about application users?
@end menu


@node Extending Dia
@subsection How One Might Extend Dia Using Guile

Dia is a free software program for drawing schematic diagrams like flow
charts and floor plans (@uref{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.

@menu
* Dia Objective::               Deciding why you want to add Guile.
* Dia Steps::                   Four steps required to add Guile.
* Dia Objects::                 How to represent Dia data in Scheme.
* Dia Primitives::              Writing Guile primitives for Dia.
* Dia Hook::                    Providing a hook for Scheme evaluation.
* Dia Structure::               Overall structure for adding Guile.
* Dia Advanced::                Going further with Dia and Guile.
@end menu


@node Dia Objective
@subsubsection 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?

@itemize @bullet
@item
Most importantly, the @dfn{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.

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

@item
The code that handles input events, which indicate that the application
user is wanting to do something.
@end itemize

@noindent
(In other words, a textbook example of the @dfn{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:

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


@node Dia Steps
@subsubsection 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 @code{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 @dfn{foreign object}
interface to create a new Scheme data type for your objects.

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

The following subsections expand on these four points in turn.


@node Dia Objects
@subsubsection 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.

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

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

@itemize @bullet
@item
Whatever the representation, it has to be decodable again by the C code
for the @code{square?} and @code{change-fill-pattern} primitives.  In
other words, a primitive like @code{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.

@item
The representation must also cope with Scheme code holding on to the
value for later use.  What happens if the Scheme code stores
@code{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,
@code{square?}?

@item
The lifetime and memory allocation of objects that exist @emph{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 @code{shape} argument passes
transiently in and out of the Scheme world, it would be quite wrong the
@strong{delete} the underlying C shape just because the Scheme code has
finished evaluation.  How do we avoid this happening?
@end itemize

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 @code{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
@code{struct dia_shape}):

@itemize @bullet
@item
Define a new Scheme-specific structure that @emph{points} to the
underlying C structure:

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

@item
Add a field to @code{struct dia_shape} that points to its @code{struct
dia_guile_shape} if it has one ---

@lisp
struct dia_shape
@{
  @dots{}
  struct dia_guile_shape * guile_shape;
@}
@end lisp

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

@item
Wrap @code{struct dia_guile_shape} as a foreign object type.

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

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

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 @dfn{finalizer} function that was specified when defining
the shape foreign object type.  To maintain the correctness of the
@code{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 @code{guile_shape} field to NULL.

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


@node Dia Primitives
@subsubsection 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 @code{SCM}, and whose body does whatever you want it to do.
As an example, here is a possible implementation of the @code{square?}
primitive:

@lisp
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));
@}
@end lisp

Notice how easy it is to chain through from the @code{SCM shape}
parameter that @code{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, @code{scm_assert_foreign_object_type},
@code{scm_foreign_object_ref}, and @code{scm_from_bool} are from the
standard Guile API.  We assume that @code{shape_type} was given to us
when we made the shape foreign object type, using
@code{scm_make_foreign_object_type}.  The call to
@code{scm_assert_foreign_object_type} ensures that @var{shape} is indeed
a shape.  This is needed to guard against Scheme code using the
@code{square?}  procedure incorrectly, as in @code{(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 @code{scm_c_define_gsubr}
function.  @code{scm_c_define_gsubr} (@pxref{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
@code{square?} primitive always requires exactly one argument, so the
call to make it available in Scheme reads like this:

@lisp
scm_c_define_gsubr ("square?", 1, 0, 0, square_p);
@end lisp

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


@node Dia Hook
@subsubsection 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 @kbd{@key{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 @code{scm_c_eval_string} function.


@node Dia Structure
@subsubsection Top-level Structure of Guile-enabled Dia

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

@itemize @bullet
@item
@code{main ()}

@itemize @bullet
@item
do lots of initialization and setup stuff
@item
enter Gtk main loop
@end itemize
@end itemize

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
@code{scm_boot_guile} like this:

@itemize @bullet
@item
@code{main ()}

@itemize @bullet
@item
do lots of initialization and setup stuff
@item
@code{scm_boot_guile (argc, argv, inner_main, NULL)}
@end itemize

@item
@code{inner_main ()}

@itemize @bullet
@item
define all foreign object types
@item
export primitives to Scheme using @code{scm_c_define_gsubr}
@item
enter Gtk main loop
@end itemize
@end itemize

In other words, you move the guts of what was previously in your
@code{main} function into a new function called @code{inner_main}, and
then add a @code{scm_boot_guile} call, with @code{inner_main} as a
parameter, to the end of @code{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 @emph{inside} the dynamic scope
of the @code{scm_boot_guile} call, but also @emph{before} any code is
run that could possibly use them --- the beginning of @code{inner_main}
is an ideal place for this.


@node Dia Advanced
@subsubsection 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
@code{dia_change_fill_pattern} function:

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

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

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

  @dots{}

  dia_change_fill_pattern (d_shape, d_pattern);

  return SCM_UNSPECIFIED;
@}
@end lisp

At this point, it makes sense to keep @code{dia_change_fill_pattern} and
@code{change_fill_pattern} separate, because
@code{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
@code{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 @code{change_fill_pattern}, like this:

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

  @dots{}

  /* real pattern change work */

  return SCM_UNSPECIFIED;
@}
@end lisp

So further Guile integration progressively @emph{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 @code{dia_change_fill_pattern} and
@code{change_fill_pattern}, the practical upshot of such a change is
that you would no longer have to keep the @code{dia_shape} and
@code{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 @code{dia_shape} structure
directly, and send @code{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 @dots{}


@node Scheme vs C
@subsection 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:

@itemize @bullet
@item
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.

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

@item
They provide high level features such as container objects and exception
handling that make common programming tasks easier.
@end itemize

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


@node Testbed Example
@subsection 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''@footnote{A @dfn{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.)


@node Programming Options
@subsection 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.

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:

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

@item
How do you design the interface between the C and Scheme parts of your
application?
@end itemize

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.

@menu
* Available Functionality::     What functionality is already available?
* Basic Constraints::           Functional and performance constraints.
* Style Choices::               Your preferred programming style.
* Program Control::             What controls program execution?
@end menu


@node Available Functionality
@subsubsection 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:

@itemize @bullet
@item
standard Scheme

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

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

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
@code{(gtk gtk)}, which makes Gtk+ drawing functions available in
Scheme, and @code{(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 @dfn{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 @ref{Defining New
Foreign Object Types}, @ref{Primitive Procedures}, and @ref{Foreign
Function Interface}.


@node Basic Constraints
@subsubsection Functional and Performance Constraints

@c REFFIXME

@node Style Choices
@subsubsection Your Preferred Programming Style

@c REFFIXME

@node Program Control
@subsubsection What Controls Program Execution?

@c REFFIXME

@node User Programming
@subsection How About Application Users?

So far we have considered what Guile programming means for an
application developer.  But what if you are instead @emph{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.@footnote{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@dots{}

@itemize @bullet
@item
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.

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

@item
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.
@end itemize

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.


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