guile/doc/ref/scheme-modules.texi

@page
@node Modules
@chapter Modules
@cindex modules

When programs become large, naming conflicts can occur when a function
or global variable defined in one file has the same name as a function
or global variable in another file.  Even just a @emph{similarity}
between function names can cause hard-to-find bugs, since a programmer
might type the wrong function name.

The approach used to tackle this problem is called @emph{information
encapsulation}, which consists of packaging functional units into a
given name space that is clearly separated from other name spaces.
@cindex encapsulation
@cindex information encapsulation
@cindex name space

The language features that allow this are usually called @emph{the
module system} because programs are broken up into modules that are
compiled separately (or loaded separately in an interpreter).

Older languages, like C, have limited support for name space
manipulation and protection.  In C a variable or function is public by
default, and can be made local to a module with the @code{static}
keyword.  But you cannot reference public variables and functions from
another module with different names.

More advanced module systems have become a common feature in recently
designed languages: ML, Python, Perl, and Modula 3 all allow the
@emph{renaming} of objects from a foreign module, so they will not
clutter the global name space.
@cindex name space - private

@menu
* Scheme and modules::          How modules are handled in standard Scheme.
* The Guile module system::     How Guile does it.
* Dynamic Libraries::           Loading libraries of compiled code at run time.
@end menu


@node Scheme and modules
@section Scheme and modules

Scheme, as defined in R5RS, does @emph{not} have a module system at all.

Aubrey Jaffer, mostly to support his portable Scheme library SLIB,
implemented a provide/require mechanism for many Scheme implementations.
Library files in SLIB @emph{provide} a feature, and when user programs
@emph{require} that feature, the library file is loaded in.

For example, the file @file{random.scm} in the SLIB package contains the
line

@smalllisp
(provide 'random)
@end smalllisp

so to use its procedures, a user would type

@smalllisp
(require 'random)
@end smalllisp

and they would magically become available, @emph{but still have the same
names!}  So this method is nice, but not as good as a full-featured
module system.


@node The Guile module system
@section The Guile module system

In 1996 Tom Lord implemented a full-featured module system for Guile which
allows loading Scheme source files into a private name space.  This system has
been in available since Guile version 1.4.
@c fixme: Actually, was it available before?  1.4 seems a bit late...

For Guile version 1.5.0 and later, the system has been improved to have better
integration from C code, more fine-grained user control over interfaces, and
documentation.

Although it is anticipated that the module system implementation will
change in the future, the Scheme programming interface described in this
manual should be considered stable.  The C programming interface is
considered relatively stable, although at the time of this writing,
there is still some flux.
@c fixme: Review: Need better C code interface commentary.

@menu
* General Information about Modules::  Guile module basics.
* Using Guile Modules::         How to use existing modules.
* Creating Guile Modules::      How to package your code into modules.
* More Module Procedures::      Low-level module code.
* Module System Quirks::        Strange things to be aware of.
* Included Guile Modules::      Which modules come with Guile?
@end menu

@node General Information about Modules
@subsection General Information about Modules

A Guile module is a collection of named procedures, variables and
macros, altogether called the @dfn{bindings}, since they bind, or
associate, a symbol (the name) to a Scheme object (procedure, variable,
or macro).  Within a module, all bindings are visible.  Certain bindings
can be declared @dfn{public}, in which case they are added to the
module's so-called @dfn{export list}; this set of public bindings is
called the module's @dfn{public interface} (@pxref{Creating Guile
Modules}).

A client module @dfn{uses} a providing module's bindings by either
accessing the providing module's public interface, or by building a
custom interface (and then accessing that).  In a custom interface, the
client module can @dfn{select} which bindings to access and can also
algorithmically @dfn{rename} bindings.  In contrast, when using the
providing module's public interface, the entire export list is available
without renaming (@pxref{Using Guile Modules}).

To use a module, it must be found and loaded.  All Guile modules have a
unique @dfn{module name}, which is a list of one or more symbols.
Examples are @code{(ice-9 popen)} or @code{(srfi srfi-11)}.  When Guile
searches for the code of a module, it constructs the name of the file to
load by concatenating the name elements with slashes between the
elements and appending a number of file name extensions from the list
@code{%load-extensions} (REFFIXME).  The resulting file name is then
searched in all directories in the variable @code{%load-path}.  For
example, the @code{(ice-9 popen)} module would result in the filename
@code{ice-9/popen.scm} and searched in the installation directory of
Guile and in all other directories in the load path.

@c FIXME::martin:  Not sure about this, maybe someone knows better?
Every module has a so-called syntax transformer associated with it.
This is a procedure which performs all syntax transformation for the
time the module is read in and evaluated.  When working with modules,
you can manipulate the current syntax transformer using the
@code{use-syntax} syntactic form or the @code{#:use-syntax} module
definition option (@pxref{Creating Guile Modules}).

Please note that there are some problems with the current module system
you should keep in mind (@pxref{Module System Quirks}).  We hope to
address these eventually.


@node Using Guile Modules
@subsection Using Guile Modules

To use a Guile module is to access either its public interface or a
custom interface (@pxref{General Information about Modules}).  Both
types of access are handled by the syntactic form @code{use-modules},
which accepts one or more interface specifications and, upon evaluation,
arranges for those interfaces to be available to the current module.
This process may include locating and loading code for a given module if
that code has not yet been loaded (REFFIXME %load-path).

An @dfn{interface specification} has one of two forms.  The first
variation is simply to name the module, in which case its public
interface is the one accessed.  For example:

@smalllisp
(use-modules (ice-9 popen))
@end smalllisp

Here, the interface specification is @code{(ice-9 popen)}, and the
result is that the current module now has access to @code{open-pipe},
@code{close-pipe}, @code{open-input-pipe}, and so on (@pxref{Included
Guile Modules}).

Note in the previous example that if the current module had already
defined @code{open-pipe}, that definition would be overwritten by the
definition in @code{(ice-9 popen)}.  For this reason (and others), there
is a second variation of interface specification that not only names a
module to be accessed, but also selects bindings from it and renames
them to suit the current module's needs.  For example:

@smalllisp
(use-modules ((ice-9 popen)
              :select ((open-pipe . pipe-open) close-pipe)
              :rename (symbol-prefix-proc 'unixy:)))
@end smalllisp

Here, the interface specification is more complex than before, and the
result is that a custom interface with only two bindings is created and
subsequently accessed by the current module.  The mapping of old to new
names is as follows:

@c Use `smallexample' since `table' is ugly.  --ttn
@smallexample
(ice-9 popen) sees:             current module sees:
open-pipe                       unixy:pipe-open
close-pipe                      unixy:close-pipe
@end smallexample

This example also shows how to use the convenience procedure
@code{symbol-prefix-proc}.

@c begin (scm-doc-string "boot-9.scm" "symbol-prefix-proc")
@deffn procedure symbol-prefix-proc prefix-sym
Return a procedure that prefixes its arg (a symbol) with
@var{prefix-sym}.
@c Insert gratuitous C++ slam here.  --ttn
@end deffn

@c begin (scm-doc-string "boot-9.scm" "use-modules")
@deffn syntax use-modules spec @dots{}
Resolve each interface specification @var{spec} into an interface and
arrange for these to be accessible by the current module.  The return
value is unspecified.

@var{spec} can be a list of symbols, in which case it names a module
whose public interface is found and used.

@var{spec} can also be of the form:

@smalllisp
 (MODULE-NAME [:select SELECTION] [:rename RENAMER])
@end smalllisp

in which case a custom interface is newly created and used.
@var{module-name} is a list of symbols, as above; @var{selection} is a
list of selection-specs; and @var{renamer} is a procedure that takes a
symbol and returns its new name.  A selection-spec is either a symbol or
a pair of symbols @code{(ORIG . SEEN)}, where @var{orig} is the name in
the used module and @var{seen} is the name in the using module.  Note
that @var{seen} is also passed through @var{renamer}.

The @code{:select} and @code{:rename} clauses are optional.  If both are
omitted, the returned interface has no bindings.  If the @code{:select}
clause is omitted, @var{renamer} operates on the used module's public
interface.

Signal error if module name is not resolvable.
@end deffn


@c FIXME::martin: Is this correct, and is there more to say?
@c FIXME::martin: Define term and concept `system transformer' somewhere.

@deffn syntax use-syntax module-name
Load the module @code{module-name} and use its system
transformer as the system transformer for the currently defined module,
as well as installing it as the current system transformer.
@end deffn


@node Creating Guile Modules
@subsection Creating Guile Modules

When you want to create your own modules, you have to take the following
steps:

@itemize @bullet
@item
Create a Scheme source file and add all variables and procedures you wish
to export, or which are required by the exported procedures.

@item
Add a @code{define-module} form at the beginning.

@item
Export all bindings which should be in the public interface, either
by using @code{define-public} or @code{export} (both documented below).
@end itemize

@c begin (scm-doc-string "boot-9.scm" "define-module")
@deffn syntax define-module module-name [options @dots{}]
@var{module-name} is of the form @code{(hierarchy file)}.  One
example of this is

@smalllisp
(define-module (ice-9 popen))
@end smalllisp

@code{define-module} makes this module available to Guile programs under
the given @var{module-name}.

The @var{options} are keyword/value pairs which specify more about the
defined module.  The recognized options and their meaning is shown in
the following table.

@c fixme: Should we use "#:" or ":"?

@table @code
@item #:use-module @var{interface-specification}
Equivalent to a @code{(use-modules @var{interface-specification})}
(@pxref{Using Guile Modules}).

@item #:use-syntax @var{module}
Use @var{module} when loading the currently defined module, and install
it as the syntax transformer.

@item #:autoload @var{module} @var{symbol}
Load @var{module} whenever @var{symbol} is accessed.

@item #:export @var{list}
Export all identifiers in @var{list}, which must be a list of symbols.
This is equivalent to @code{(export @var{list})} in the module body.

@item #:no-backtrace
Tell Guile not to record information for procedure backtraces when
executing the procedures in this module.

@item #:pure
Create a @dfn{pure} module, that is a module which does not contain any
of the standard procedure bindings except for the syntax forms.  This is
useful if you want to create @dfn{safe} modules, that is modules which
do not know anything about dangerous procedures.
@end table

@end deffn
@c end

@deffn syntax export variable @dots{}
Add all @var{variable}s (which must be symbols) to the list of exported
bindings of the current module.
@end deffn

@c begin (scm-doc-string "boot-9.scm" "define-public")
@deffn syntax define-public @dots{}
Equivalent to @code{(begin (define foo ...) (export foo))}.
@end deffn
@c end


@node More Module Procedures
@subsection More Module Procedures

@c FIXME::martin: Review me!

@c FIXME::martin: Should this procedure be documented and supported
@c   at all?

The procedures in this section are useful if you want to dig into the
innards of Guile's module system.  If you don't know precisely what you
do, you should probably avoid using any of them.

@deffn primitive standard-eval-closure module
Return an eval closure for the module @var{module}.
@end deffn


@node Module System Quirks
@subsection Module System Quirks

Although the programming interfaces are relatively stable, the Guile
module system itself is still evolving.  Here are some situations where
usage surpasses design.

@itemize @bullet

@item
When using a module which exports a macro definition, the other module
must export all bindings the macro expansion uses, too, because the
expanded code would otherwise not be able to see these definitions and
issue a ``variable unbound'' error, or worse, would use another binding
which might be present in the scope of the expansion.

@item
When two or more used modules export bindings with the same names, the
last accessed module wins, and the exported binding of that last module
will silently be used.  This might lead to hard-to-find errors because
wrong procedures or variables are used.  To avoid this kind of
@dfn{name-clash} situation, use a custom interface specification
(@pxref{Using Guile Modules}).  (We include this entry for the possible
benefit of users of Guile versions previous to 1.5.0, when custom
interfaces were added to the module system.)

@item
[Add other quirks here.]

@end itemize


@node Included Guile Modules
@subsection Included Guile Modules

@c FIXME::martin: Review me!

Some modules are included in the Guile distribution; here are references
to the entries in this manual which describe them in more detail:

@table @strong
@item boot-9
boot-9 is Guile's initialization module, and it is always loaded when
Guile starts up.

@item (ice-9 debug)
Mikael Djurfeldt's source-level debugging support for Guile
(@pxref{Debugger User Interface}).

@item (ice-9 threads)
Guile's support for multi threaded execution (@pxref{Scheduling}).

@item (ice-9 rdelim)
Line- and character-delimited input (@pxref{Line/Delimited}).

@item (ice-9 rw)
Block string input/output (@pxref{Block Reading and Writing}).

@item (ice-9 documentation)
Online documentation (REFFIXME).

@item (srfi srfi-1)
A library providing a lot of useful list and pair processing
procedures (@pxref{SRFI-1}).

@item (srfi srfi-2)
Support for @code{and-let*} (@pxref{SRFI-2}).

@item (srfi srfi-4)
Support for homogeneous numeric vectors (@pxref{SRFI-4}).

@item (srfi srfi-6)
Support for some additional string port procedures (@pxref{SRFI-6}).

@item (srfi srfi-8)
Multiple-value handling with @code{receive} (@pxref{SRFI-8}).

@item (srfi srfi-9)
Record definition with @code{define-record-type} (@pxref{SRFI-9}).

@item (srfi srfi-10)
Read hash extension @code{#,()} (@pxref{SRFI-10}).

@item (srfi srfi-11)
Multiple-value handling with @code{let-values} and @code{let-values*}
(@pxref{SRFI-11}).

@item (srfi srfi-13)
String library (@pxref{SRFI-13}).

@item (srfi srfi-14)
Character-set library (@pxref{SRFI-14}).

@item (srfi srfi-17)
Getter-with-setter support (@pxref{SRFI-17}).

@item (ice-9 slib)
This module contains hooks for using Aubrey Jaffer's portable Scheme
library SLIB from Guile (@pxref{SLIB}).

@c FIXME::martin: This module is not in the distribution.  Remove it
@c from here?
@item (ice-9 jacal)
This module contains hooks for using Aubrey Jaffer's symbolic math
packge Jacal from Guile (@pxref{JACAL}).
@end table


@node Dynamic Libraries
@section Dynamic Libraries

Most modern Unices have something called @dfn{shared libraries}.  This
ordinarily means that they have the capability to share the executable
image of a library between several running programs to save memory and
disk space.  But generally, shared libraries give a lot of additional
flexibility compared to the traditional static libraries.  In fact,
calling them `dynamic' libraries is as correct as calling them `shared'.

Shared libraries really give you a lot of flexibility in addition to the
memory and disk space savings.  When you link a program against a shared
library, that library is not closely incorporated into the final
executable.  Instead, the executable of your program only contains
enough information to find the needed shared libraries when the program
is actually run.  Only then, when the program is starting, is the final
step of the linking process performed.  This means that you need not
recompile all programs when you install a new, only slightly modified
version of a shared library.  The programs will pick up the changes
automatically the next time they are run.

Now, when all the necessary machinery is there to perform part of the
linking at run-time, why not take the next step and allow the programmer
to explicitly take advantage of it from within his program?  Of course,
many operating systems that support shared libraries do just that, and
chances are that Guile will allow you to access this feature from within
your Scheme programs.  As you might have guessed already, this feature
is called @dfn{dynamic linking}@footnote{Some people also refer to the
final linking stage at program startup as `dynamic linking', so if you
want to make yourself perfectly clear, it is probably best to use the
more technical term @dfn{dlopening}, as suggested by Gordon Matzigkeit
in his libtool documentation.}

As with many aspects of Guile, there is a low-level way to access the
dynamic linking apparatus, and a more high-level interface that
integrates dynamically linked libraries into the module system.

@menu
* Low level dynamic linking::
* Extensions::
@end menu

@node Low level dynamic linking
@subsection Low level dynamic linking

When using the low level procedures to do your dynamic linking, you have
complete control over which library is loaded when and what get's done
with it.

@deffn primitive dynamic-link library
Find the shared library denoted by @var{library} (a string) and link it
into the running Guile application.  When everything works out, return a
Scheme object suitable for representing the linked object file.
Otherwise an error is thrown.  How object files are searched is system
dependent.

Normally, @var{library} is just the name of some shared library file
that will be searched for in the places where shared libraries usually
reside, such as in @file{/usr/lib} and @file{/usr/local/lib}.
@end deffn

@deffn primitive dynamic-object? val
Determine whether @var{val} represents a dynamically linked object file.
@end deffn

@deffn primitive dynamic-unlink dynobj
Unlink the indicated object file from the application.  The argument
@var{dynobj} should be one of the values returned by
@code{dynamic-link}.  When @code{dynamic-unlink} has been called on
@var{dynobj}, it is no longer usable as an argument to the functions
below and you will get type mismatch errors when you try to.
@end deffn

@deffn primitive dynamic-func function dynobj
Search the C function indicated by @var{function} (a string or symbol)
in @var{dynobj} and return some Scheme object that can later be used
with @code{dynamic-call} to actually call this function.  Right now,
these Scheme objects are formed by casting the address of the function
to @code{long} and converting this number to its Scheme representation.

Regardless whether your C compiler prepends an underscore @samp{_} to
the global names in a program, you should @strong{not} include this
underscore in @var{function}.  Guile knows whether the underscore is
needed or not and will add it when necessary.
@end deffn

@deffn primitive dynamic-call function dynobj
Call the C function indicated by @var{function} and @var{dynobj}.  The
function is passed no arguments and its return value is ignored.  When
@var{function} is something returned by @code{dynamic-func}, call that
function and ignore @var{dynobj}.  When @var{function} is a string (or
symbol, etc.), look it up in @var{dynobj}; this is equivalent to

@smallexample
(dynamic-call (dynamic-func @var{function} @var{dynobj} #f))
@end smallexample

Interrupts are deferred while the C function is executing (with
@code{SCM_DEFER_INTS}/@code{SCM_ALLOW_INTS}).
@end deffn

@deffn primitive dynamic-args-call function dynobj args
Call the C function indicated by @var{function} and @var{dynobj}, just
like @code{dynamic-call}, but pass it some arguments and return its
return value.  The C function is expected to take two arguments and
return an @code{int}, just like @code{main}:

@smallexample
int c_func (int argc, char **argv);
@end smallexample

The parameter @var{args} must be a list of strings and is converted into
an array of @code{char *}.  The array is passed in @var{argv} and its
size in @var{argc}.  The return value is converted to a Scheme number
and returned from the call to @code{dynamic-args-call}.
@end deffn

When dynamic linking is disabled or not supported on your system,
the above functions throw errors, but they are still available.

Here is a small example that works on GNU/Linux:

@smallexample
(define libc-obj (dynamic-link "libc.so"))
libc-obj
@result{} #<dynamic-object "libc.so">
(dynamic-args-call 'rand libc-obj '())
@result{} 269167349
(dynamic-unlink libc-obj)
libc-obj
@result{} #<dynamic-object "libc.so" (unlinked)>
@end smallexample

As you can see, after calling @code{dynamic-unlink} on a dynamically
linked library, it is marked as @samp{(unlinked)} and you are no longer
able to use it with @code{dynamic-call}, etc.  Whether the library is
really removed from you program is system-dependent and will generally
not happen when some other parts of your program still use it.  In the
example above, @code{libc} is almost certainly not removed from your
program because it is badly needed by almost everything.

The functions to call a function from a dynamically linked library,
@code{dynamic-call} and @code{dynamic-args-call}, are not very powerful.
They are mostly intended to be used for calling specially written
initialization functions that will then add new primitives to Guile.
For example, we do not expect that you will dynamically link
@file{libX11} with @code{dynamic-link} and then construct a beautiful
graphical user interface just by using @code{dynamic-call} and
@code{dynamic-args-call}.  Instead, the usual way would be to write a
special Guile<->X11 glue library that has intimate knowledge about both
Guile and X11 and does whatever is necessary to make them inter-operate
smoothly.  This glue library could then be dynamically linked into a
vanilla Guile interpreter and activated by calling its initialization
function.  That function would add all the new types and primitives to
the Guile interpreter that it has to offer.

From this setup the next logical step is to integrate these glue
libraries into the module system of Guile so that you can load new
primitives into a running system just as you can load new Scheme code.

There is, however, another possibility to get a more thorough access to
the functions contained in a dynamically linked library.  Anthony Green
has written @file{libffi}, a library that implements a @dfn{foreign
function interface} for a number of different platforms.  With it, you
can extend the Spartan functionality of @code{dynamic-call} and
@code{dynamic-args-call} considerably.  There is glue code available in
the Guile contrib archive to make @file{libffi} accessible from Guile.

@node Extensions
@subsection Writing Dynamically Loadable Extensions

XXX - document @code{load-extension}, @code{scm_register_extension}

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