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<title>
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The Header2Scheme Home Page
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</title>
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<center>
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<img src="./h2s2.jpg" width = 400 height = 300> <p>
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<h1>
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Header2Scheme:
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</h1>
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<h3>
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An Automatic C++ to Scheme Interface Generator
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</h3>
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</center>
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<hr size = 5>
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<p>
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<b> Header2Scheme version 1.4 </b> is now available, and contains a few <a
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href="#CHANGES">bug fixes and enhancements</a>. The source code is available
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for <a href="#DOWNLOAD">download.</a>
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<p>
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<h2>
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The Problem
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</h2>
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There are many situations in which more text-based interactivity is
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desired in a C or C++ program than simple printf's and
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scanf's. Unfortunately, to create a full-fledged interpreter is a
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challenging and time-consuming task. Tcl is one solution that many
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people choose, but has several disadvantages, among them that it is
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slow and stores all data types as character strings. Scheme is a very
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elegant interpreted language for which there are many implementations
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of interpreters. Unfortunately, up until now, there have been
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relatively few useful libraries for which Scheme bindings have been
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created, so acceptance of Scheme as a general interpreted language
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solution has been minimal.
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<p>
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<h2>
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One solution
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</h2>
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Header2Scheme is a program which reads in a directory tree full of C++
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header files and <i> compiles </i> them into C++ code. This new code,
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when compiled and linked with SCM (<a href =
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"http://swissnet.ai.mit.edu/~jaffer">Aubrey Jaffer</a>'s Scheme
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interpreter) implements the back end for a Scheme interface to the
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classes defined by these header files.
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<p>
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<h2>
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Why SCM?
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</h2>
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I looked at a few Scheme interpreters before deciding on SCM; two of
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these included Scheme48 and libScheme. Each had its advantages, but SCM had
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the distinction of being the fastest of the pack. Since my work focuses
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mainly on real-time graphics applications, SCM had the obvious
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advantage. (However, libScheme had a very elegant, object-oriented
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programming interface which was very appealing.)
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<h2>
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Basic syntax defined by Header2Scheme
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</h2>
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Given a C++ class:
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<pre>
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class Foo {
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public:
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int memberFunction();
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Yabba *class_variable;
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};
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</pre>
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The Scheme interface becomes
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<pre>
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(define my-foo (new-foo))
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(define my-int (-> my-foo 'memberfunction))
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(define my-yabba (-> my-foo 'class_variable))
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</pre>
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An alternative calling interface is
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<pre>
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(foo::memberfunction my-foo)
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(foo::class_variable my-foo)
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</pre>
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<hr size = 3>
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<h2>
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More on the backend
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</h2>
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When interfacing C++ and Scheme, there are several issues to take into
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consideration:
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<ul>
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<li> <a href = "#CONVERSION"> conversion of data structures </a>
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<li> <a href = "#POINTERS"> pointers and references </a>
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<li> <a href = "#TYPECHECKING"> type checking and overloaded functions </a>
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<li> <a href = "#INHERITANCE"> inheritance </a>
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</ul>
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I have tried to address all of these problems in Header2Scheme.
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<p>
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<h2> <a name = "CONVERSION"> Conversion of data structures </a> </h2>
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Basic C types often have analagous Scheme data structures:
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<ul>
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<li> arrays/vectors
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<li> strings
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<li> ints, floats
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</ul>
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Header2Scheme generates code to convert fixed-length C arrays into Scheme
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vectors and back. (See the section below on pointers and references.)
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Strings have essentially the same representation in Scheme and C, modulo a
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type tag in Scheme. Floats and ints are likewise similarly represented and
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have all the necessary conversion code generated by Header2Scheme.
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<p>
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More sophisticated C data structures (i.e. the ones which Header2Scheme is
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designed to provide access to) are represented in the Scheme backend as
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pointers with a type tag.
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<p>
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<h2> <a name = "POINTERS"> Pointers and references </a> </h2>
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All "complicated" C structures are represented in the Scheme backend
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essentially as C pointers with a type tag. On the Scheme side of things,
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there are no notions of pointers, references or dereferencing; everything
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is an "object".
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<p>
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When making a function call (see the section below on type checking)
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pointers are automatically dereferenced.
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<p>
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<h2> <a name = "TYPECHECKING"> Type checking and overloaded functions </a>
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</h2>
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One of the most elegant aspects of Scheme is that there is no type
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checking; everything is considered to be "data". Because Header2Scheme
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fundamentally is dealing with C, however, it necessarily enforces some type
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checking.
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<p>
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When you call a member function of a class via Scheme, the code that
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Header2Scheme produced checks the Scheme arguments' types and attempts to
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find the appropriate (possibly overloaded) C++ function. If one can't be
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found, the interpreter produces a wrong type error (as opposed to a
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segmentation fault). Otherwise, the arguments are converted to their C
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equivalents, the function is called, and the return value, if any, is
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converted to Scheme format and returned to the interpreter.
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<p>
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In Scheme, all arguments to functions are passed by
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value. Unfortunately, C++ allows passing pointers and references to
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variables during function calls, and many C functions would become
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unusable if they were unable to side-effect their parameters. Because
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of this, the backend that Header2Scheme generates allows side-effects
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to propagate, where possible, to all arguments that are passed by
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reference (either via a pointer or a reference). (Scheme integers and
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characters can not be mutated because they are always passed by
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value.) It should be noted that this can cause unpleasant and
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unexpected effects in code:
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<pre>
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class SbVec3f {
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(...)
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void getValue(float &x, float &y, float &z);
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(...)
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};
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> (define x 0.0)
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> (define y 0.0)
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> (define z 0.0)
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> (define my-list (list x y z))
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> my-list
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(0.0 0.0 0.0)
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> (define my-vec (new-SbVec3f 3 4 5))
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> (-> my-vec 'getValue x y z)
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> x
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3.0
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> y
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4.0
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> z
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5.0
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> my-list
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#(3.0 4.0 5.0)
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</pre>
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If, on the other hand, the command which had mutated x had been the
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standard Scheme
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<pre>
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> (set! x 2.0)
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> my-list
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#(0.0 0.0 0.0)
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</pre>
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my-list is (correctly) not mutated in this example.
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<h2> <a name = "INHERITANCE"> Inheritance </a> </h2>
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Consider the following two classes:
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<pre>
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class a {
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void foo();
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};
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class b : public a {
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void bar();
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};
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</pre>
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The member function "foo" can be called from an object of type "a" or
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"b". However, the member function "bar" can only be called from an object
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of type "b". Standard Scheme has no built-in notion of inheritance, so the
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question remains of how to decide at runtime whether a given member
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function can correctly be called for a given Scheme object.
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<p>
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Header2Scheme solves this problem by reconstructing its own idea of the
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desired class hierarchy in a pre-processing step while the backend to the
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Scheme interface is being generated. This hierarchy is then reconstructed
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when the Scheme interpreter is run, and is referenced to resolve run-time
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questions of type checking.
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<p>
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<hr size=3>
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<h2>
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What has been done with Header2Scheme?
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</h2>
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I have used Header2Scheme to create a Scheme binding for Open Inventor, a
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3D graphics toolkit developed by Silicon Graphics. This package is called
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<b>Ivy</b>, and is available from <a href = "../Ivy/">the Ivy home page.</a>
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<p>
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<h2>
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<a name = "CHANGES">
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Changes in version 1.4
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</h2>
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<ul>
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<li> Fixed bugs in generated code for pointers returned from functions.
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<li> Added long/short qualifiers in typedefs file for ints and doubles, and
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output them in the argument extraction code so references to these types
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work.
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<li> Rewrote the C++ wrappers for the Scheme interpreter so they work
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better; more consistent user interface (prompt is always there when
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interpreter is listening) and more robust (Ctrl-C no longer crashes the
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interpreter).
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</ul>
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<p>
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<h2>
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<a name = "DOWNLOAD">
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Downloading Header2Scheme
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</a>
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</h2>
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<a href = "h2s-1.4.tar.gz">h2s-1.4.tar.gz</a> (290K) contains the source
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code and documentation for Header2Scheme. Header2Scheme is Copyright 1995
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Kenneth B. Russell and is covered under the GNU General Public License.
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<p>
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<h2>
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Other links
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</h2>
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<a href = "http://www.red-bean.com/guile">GUILE</a> is a collaborative
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effort to make embedded Scheme more ubiquitous.
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<p>
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<a href = "http://www.swig.org">SWIG</a> is a more up-to-date glue code
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generator than Header2Scheme which supports multiple language bindings.
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<h2>
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Feedback
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</h2>
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Please <a href = "mailto:kbrussel@media.mit.edu">send me email</a> if you
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have any comments, questions or suggestions, or have problems with the
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distribution.
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<p>
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<hr size = 5>
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<p>
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<i> <a href="../index.html"> Kenneth B. Russell - kbrussel@media.mit.edu
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</a> </i>
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<p>
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$Id: index.html,v 1.15 2001/06/02 05:20:33 kbrussel Exp $
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