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* New manual material: example of extending Dia using Guile.

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doc/ref/ChangeLog
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@ -1,3 +1,11 @@
2002-04-20 Neil Jerram <neil@ossau.uklinux.net>
* program.texi (Scheme vs C): New node, with existing material
taken from chapter intro.
(Programming Overview): New intro para to introduce example of
Guile integration:
(Extending Dia): New node.
2002-04-17 Marius Vollmer <mvo@zagadka.ping.de>
* Makefile.am (CLEANFILES): Added guile.cps, guile.fns, guile.rns,

526
doc/ref/program.texi
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@ -11,10 +11,516 @@ 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.
Underlying this argument 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?
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
@section How One Might Extend Dia Using Guile
Dia is a free software program for drawing schematic diagrams like flow
charts and floor plans (REFFIXME). 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 Smobs:: 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
@subsection 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
@subsection 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{SMOB} 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{SMOBs} and @dfn{primitives} to the Scheme world.
The following subsections expand on these four points in turn.
@node Dia Smobs
@subsection 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 the idea of SMOBs comes in, and with it 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 SMOB.
The SMOB is what is passed into and out of Scheme code, and the
Scheme-specific C structure inside the SMOB 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 SMOB. 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 SMOB 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 SMOB type.
@item
Whenever you need to represent a C shape onto the Scheme level, create a
SMOB instance for it, and pass that.
@item
In primitive code that receives a shape SMOB 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 SMOB 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 SMOB value, it calls
the @dfn{SMOB free} function that was specified when defining the shape
SMOB 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 SMOB types, see
@ref{Defining New Types (Smobs)}.
@node Dia Primitives
@subsection 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
#define FUNC_NAME "square?"
static SCM square_p (SCM shape)
@{
struct dia_guile_shape * guile_shape;
/* Check that arg is really a shape SMOB. */
SCM_VALIDATE_SHAPE (SCM_ARG1, shape);
/* Access Scheme-specific shape structure. */
guile_shape = SCM_SMOB_DATA (shape);
/* Find out if underlying shape exists and is a
square; return answer as a Scheme boolean. */
return SCM_BOOL (guile_shape->c_shape &&
(guile_shape->c_shape->type == DIA_SQUARE));
@}
#undef FUNC_NAME
@end lisp
Notice how easy it is to chain through from the @code{SCM shape}
parameter that @code{square_p} receives --- which is a SMOB --- to the
Scheme-specific structure inside the SMOB, and thence to the underlying
C structure for the shape.
In this code, @code{SCM_SMOB_DATA} and @code{SCM_BOOL} are macros from
the standard Guile API. @code{SCM_VALIDATE_SHAPE} is a macro that you
should define as part of your SMOB definition: it checks that the passed
parameter is of the expected type. 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} (REFFIXME) 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
@subsection 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
@subsection 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 SMOB 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 SMOBs and have written primitive code as
described in the preceding subsections, you also need to insert calls to
declare your new SMOBs 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
@subsection 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 SMOBs 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 SMOB 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
SMOBs 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
SMOB 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
@section 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
@ -36,9 +542,9 @@ 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, further features that make programming easier ---
and more fun! --- are its powerful mechanisms for abstracting parts of
programs (closures --- @pxref{About Closure}) and for iteration
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
@ -51,12 +557,6 @@ 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.
@menu
* Testbed Example:: Example: using Guile in a testbed.
* Programming Options:: Options for Guile programming.
* User Programming:: How about application users?
@end menu
@node Testbed Example
@section Example: Using Guile for an Application Testbed