Deconstruction-Functions.html 7.1 KB

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  58. <h3 class="subsection">3.1.6 Deconstruction Functions</h3>
  59. <p>A fast native implementation of the deconstruction operation is provided
  60. <a name="index-deconstruction-3"></a>
  61. by the functions declared in &lsquo;<tt>decons.h</tt>&rsquo;.
  62. </p>
  63. <dl>
  64. <dt><a name="index-avm_005finitialize_005fdecons"></a><u>Function:</u> void <b>avm_initialize_decons</b><i> ()</i></dt>
  65. <dd><p>This should be called prior to the first call to
  66. <code>avm_deconstruction</code>, so as to initialize some necessary internal
  67. data structures. Results will be undefined if it is not.
  68. </p></dd></dl>
  69. <dl>
  70. <dt><a name="index-avm_005fcount_005fdecons"></a><u>Function:</u> void <b>avm_count_decons</b><i> ()</i></dt>
  71. <dd><p>For ecologically sound memory management, this function should be called
  72. at the end of a run to verify that there have been no leaks due to the
  73. deconstruction functions, which there won&rsquo;t be unless the code in
  74. &lsquo;<tt>decons.c</tt>&rsquo; has been ineptly modified. An error message to the
  75. effect of unreclaimed &ldquo;points&rdquo; could be the result otherwise.
  76. </p></dd></dl>
  77. <dl>
  78. <dt><a name="index-avm_005fdeconstruction"></a><u>Function:</u> list <b>avm_deconstruction</b><i> (list <var>pointer</var>, list <var>operand</var>, int *<var>fault</var>)</i></dt>
  79. <dd><p>Deconstructions are performed by this function, as described in
  80. <a href="Field.html#Field">Field</a>. In the <code>silly</code> program notation (<a href="A-Simple-Lisp-Like-Language.html#A-Simple-Lisp-Like-Language">A Simple Lisp Like Language</a>), this function computes the value
  81. of ([[<code>field</code>]] <code><var>pointer</var></code>) <code><var>operand</var></code>.
  82. </p>
  83. <p>For example, using the fixed list <code>avm_join(NULL,NULL)</code> as the
  84. <code><var>pointer</var></code> parameter will cause a copy of the operand itself
  85. to be returned as the result. A <code><var>pointer</var></code> equal to
  86. <code>avm_join(NULL,avm_join(NULL,NULL))</code> will cause a copy of
  87. <code>operand-&gt;tail</code> to be returned, and so on. A <code>NULL</code>
  88. <code><var>pointer</var></code> causes an internal error.
  89. </p>
  90. <p>If the deconstruction is invalid, as in the case of the tail of an empty
  91. list, the invalid deconstruction error message is returned as the
  92. result, and the <code>*<var>fault</var></code> parameter is set to a non-zero value.
  93. The <code>*<var>fault</var></code> parameter is also set to a non-zero value in the event
  94. of a memory overflow, and the memory overflow message is returned.
  95. </p></dd></dl>
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