-*- outline -*-

* Overview

This directory includes an example program for extending Guile with a

new (and even useful) data type.

The `box' program created by this example is nearly identical to the

one produced in directory ../box, with one (important) difference: The

interpreter in this directory will place all defined primitive

procedures in a module called (box-module). That means that this

module must be used before the primitives can be accessed.

* Build Instructions

To build the example, simply type

make box

in this directory.

The resulting `box' program is a Guile interpreter which has one

additional data type called `box'.

* The Box Data Type

A box is simply an object for storing one other object in. It can be

used for passing parameters by reference, for example. You simply

store an object into a box, pass it to another procedure which can

store a new object into it and thus return a value via the box.

** Usage

Box objects are created with `make-box', set with `box-set!' and

examined with `box-ref'. Note that these procedures are placed in a

module called (box-module) and can thus only be accessed after using

this module. See the following example session for usage details:

** Example Session

$ ./box

guile> (use-modules (box-module))

guile> (define b (make-box))

guile> b

#<box #f>

guile> (box-set! b '(list of values))

guile> b

#<box (list of values)>

guile> (box-ref b)

(list of values)

guile> (quit)

$