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- <a name="Library-combinator"></a>
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- <h4 class="subsubsection">2.7.16.1 Library combinator</h4>
- <p>The simplest and fastest method of interfacing to an external library
- is by way of a virtual machine combinator called <code>library</code>. It
- takes two non-empty character strings as arguments to a virtual code
- program of the form implied by the following property.
- </p><dl compact="compact">
- <dt> <em>T33</em></dt>
- <dd><p>[[<code>library</code>]] (<code><var>x</var></code>,<code><var>y</var></code>) = <code>((nil,nil),((<var>x</var>,<var>y</var>),(nil,nil)))</code>
- </p></dd>
- </dl>
- <p>Intuitively, <var>x</var> is the name of a library and <var>y</var> is the name
- of a function within the library. For example, if <var>x</var> is
- <code>'math'</code> and <var>y</var> is <code>'sqrt'</code>, then
- <code>library</code>(<var>x</var>,<var>y</var>) represents the function that computes
- the square root of a floating point number as defined by the host
- machine’s native C implementation, normally in IEEE double precision
- format. Different functions and libraries may involve other argument
- and result types, such as complex numbers, arrays, sparse matrices, or
- arbitrary precision numbers. A list of currently supported external
- library names with their functions and calling conventions is given in
- <a href="External-Libraries.html#External-Libraries">External Libraries</a>.
- </p>
- <p>On the virtual code side, all function arguments and results
- regardless of their types are encoded as nested pairs of <code>nil</code>,
- as always, and may be manipulated or stored as any other data,
- including storage and retrieval from files in ‘<tt>.avm</tt>’ virtual
- code format (<a href="File-Format.html#File-Format">File Format</a>). However, on the C side,
- various memory management and caching techniques are employed to
- maintain this facade while allowing the libraries to operate on data
- in their native format. The details are given more fully in the API
- documentation, particularly in <a href="Type-Conversions.html#Type-Conversions">Type Conversions</a> and
- <a href="External-Library-Maintenance.html#External-Library-Maintenance">External Library Maintenance</a>.
- </p>
- <p>While this style is fast and convenient, it is limited either to
- libraries that have already been built into the virtual machine, or to
- those for which the user is prepared to implement a new interface
- module in C as described in <a href="Implementing-new-library-functions.html#Implementing-new-library-functions">Implementing new library functions</a>.
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