127 lines
6 KiB
HTML
127 lines
6 KiB
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Dia Primitives (Guile Reference Manual)</title>
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<meta name="description" content="Dia Primitives (Guile Reference Manual)">
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<meta name="keywords" content="Dia Primitives (Guile Reference Manual)">
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<meta name="distribution" content="global">
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<meta name="Generator" content=".texi2any-real">
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<meta name="viewport" content="width=device-width,initial-scale=1">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Extending-Dia.html" rel="up" title="Extending Dia">
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<link href="Dia-Hook.html" rel="next" title="Dia Hook">
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<link href="Dia-Objects.html" rel="prev" title="Dia Objects">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Dia-Primitives">
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<div class="nav-panel">
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<p>
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Next: <a href="Dia-Hook.html" accesskey="n" rel="next">Providing a Hook for the Evaluation of Scheme Code</a>, Previous: <a href="Dia-Objects.html" accesskey="p" rel="prev">How to Represent Dia Data in Scheme</a>, Up: <a href="Extending-Dia.html" accesskey="u" rel="up">How One Might Extend Dia Using Guile</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<h4 class="subsubsection" id="Writing-Guile-Primitives-for-Dia"><span>5.7.1.4 Writing Guile Primitives for Dia<a class="copiable-link" href="#Writing-Guile-Primitives-for-Dia"> ¶</a></span></h4>
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<p>Once the details of object representation are decided, writing the
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primitive function code that you need is usually straightforward.
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</p>
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<p>A primitive is simply a C function whose arguments and return value are
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all of type <code class="code">SCM</code>, and whose body does whatever you want it to do.
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As an example, here is a possible implementation of the <code class="code">square?</code>
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primitive:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">static SCM square_p (SCM shape)
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{
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struct dia_guile_shape * guile_shape;
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/* Check that arg is really a shape object. */
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scm_assert_foreign_object_type (shape_type, shape);
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/* Access Scheme-specific shape structure. */
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guile_shape = scm_foreign_object_ref (shape, 0);
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/* Find out if underlying shape exists and is a
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square; return answer as a Scheme boolean. */
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return scm_from_bool (guile_shape->c_shape &&
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(guile_shape->c_shape->type == DIA_SQUARE));
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}
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</pre></div>
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<p>Notice how easy it is to chain through from the <code class="code">SCM shape</code>
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parameter that <code class="code">square_p</code> receives — which is a foreign object
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— to the Scheme-specific structure inside the foreign object, and
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thence to the underlying C structure for the shape.
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</p>
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<p>In this code, <code class="code">scm_assert_foreign_object_type</code>,
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<code class="code">scm_foreign_object_ref</code>, and <code class="code">scm_from_bool</code> are from the
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standard Guile API. We assume that <code class="code">shape_type</code> was given to us
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when we made the shape foreign object type, using
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<code class="code">scm_make_foreign_object_type</code>. The call to
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<code class="code">scm_assert_foreign_object_type</code> ensures that <var class="var">shape</var> is indeed
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a shape. This is needed to guard against Scheme code using the
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<code class="code">square?</code> procedure incorrectly, as in <code class="code">(square? "hello")</code>;
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Scheme’s latent typing means that usage errors like this must be caught
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at run time.
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</p>
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<p>Having written the C code for your primitives, you need to make them
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available as Scheme procedures by calling the <code class="code">scm_c_define_gsubr</code>
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function. <code class="code">scm_c_define_gsubr</code> (see <a class="pxref" href="Primitive-Procedures.html">Primitive Procedures</a>)
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takes arguments that specify the Scheme-level name for the primitive and
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how many required, optional and rest arguments it can accept. The
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<code class="code">square?</code> primitive always requires exactly one argument, so the
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call to make it available in Scheme reads like this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">scm_c_define_gsubr ("square?", 1, 0, 0, square_p);
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</pre></div>
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<p>For where to put this call, see the subsection after next on the
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structure of Guile-enabled code (see <a class="pxref" href="Dia-Structure.html">Top-level Structure of Guile-enabled Dia</a>).
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Dia-Hook.html">Providing a Hook for the Evaluation of Scheme Code</a>, Previous: <a href="Dia-Objects.html">How to Represent Dia Data in Scheme</a>, Up: <a href="Extending-Dia.html">How One Might Extend Dia Using Guile</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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