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<title>avram - a virtual machine code interpreter: 2.5.3 Byte Transducers</title>

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<a name="Byte-Transducers"></a>
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="Line-Maps.html#Line-Maps" title="Previous section in reading order"> &lt; </a>]</td>
<td valign="middle" align="left">[<a href="Parameter-Mode-Interface.html#Parameter-Mode-Interface" title="Next section in reading order"> &gt; </a>]</td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left">[<a href="Virtual-Machine-Specification.html#Virtual-Machine-Specification" title="Beginning of this chapter or previous chapter"> &lt;&lt; </a>]</td>
<td valign="middle" align="left">[<a href="Filter-Mode-Interface.html#Filter-Mode-Interface" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="Library-Reference.html#Library-Reference" title="Next chapter"> &gt;&gt; </a>]</td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left">[<a href="avram.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="avram_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="Function-Index.html#Function-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="avram_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
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<hr size="1">
<a name="Byte-Transducers-1"></a>
<h3 class="subsection">2.5.3 Byte Transducers</h3>

<p>The interface used when the <code>--byte-transducer</code> option is selected
<a name="index-byte_002dtransducer-command-line-option-2"></a>
allows an application to serve as a persistent stream processor suitable
<a name="index-infinite-streams-2"></a>
for finite or infinite streams. The interface can be summarized by the
following points.
</p>
<ul>
<li> When it is first invoked, the function in the virtual code file is
applied to an argument of <code>nil</code>, and is expected to return a pair
<code>(<var>state</var>,<var>output</var>)</code>. The <var>state</var> format is
unconstrained. The <var>output</var> must be a character string that will
be written to standard output, but it may be the empty string.
</li><li> For each byte read from standard input, <code>avram</code> applies the function
to the pair <code>(<var>state</var>,<var>character</var>)</code>, using the state
obtained from previous evaluation, and the character whose code is the
byte. The purpose of the <var>state</var> field is therefore to provide a
way for the application to remember something from one invocation to
the next.
</li><li> The function is usually expected to return a pair
<code>(<var>state</var>,<var>output</var>)</code> for each input byte, so that the state
can be used on the next iteration, and the output can be written to standard
output as a character string.
</li><li> If the function ever returns a value of <code>nil</code>, the computation
terminates.
</li><li> If standard input comes to an end before the computation terminates,
the function will be applied to a pair of the form
<code>(<var>state</var>,nil)</code> thereafter, but may continue to return
<code>(<var>state</var>,<var>output</var>)</code> pairs for arbitrarily many more iterations.
The <code>EOF</code> character is not explicitly passed to the function, but
the end is detectable insofar as <code>nil</code> is not a representation for
any character.
</li></ul>

<p>Unlike the situation with line maps, the output character strings
do not have line breaks automatically appended, and the application must
include them explicitly if required. The convention for
<a name="index-Unix-4"></a>
line breaks is system dependent. On Unix and GNU/Linux systems, character
code 10 indicates a line break, but other systems may use character code
13 followed by character code 10. See <a href="Character-Table.html#Character-Table">Character Table</a> for the
<a name="index-character-codes-2"></a>
representations of characters having these codes.
</p>
<hr size="1">
<table cellpadding="1" cellspacing="1" border="0">
<tr><td valign="middle" align="left">[<a href="Line-Maps.html#Line-Maps" title="Previous section in reading order"> &lt; </a>]</td>
<td valign="middle" align="left">[<a href="Parameter-Mode-Interface.html#Parameter-Mode-Interface" title="Next section in reading order"> &gt; </a>]</td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left">[<a href="Virtual-Machine-Specification.html#Virtual-Machine-Specification" title="Beginning of this chapter or previous chapter"> &lt;&lt; </a>]</td>
<td valign="middle" align="left">[<a href="Filter-Mode-Interface.html#Filter-Mode-Interface" title="Up section"> Up </a>]</td>
<td valign="middle" align="left">[<a href="Library-Reference.html#Library-Reference" title="Next chapter"> &gt;&gt; </a>]</td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left"> &nbsp; </td>
<td valign="middle" align="left">[<a href="avram.html#Top" title="Cover (top) of document">Top</a>]</td>
<td valign="middle" align="left">[<a href="avram_toc.html#SEC_Contents" title="Table of contents">Contents</a>]</td>
<td valign="middle" align="left">[<a href="Function-Index.html#Function-Index" title="Index">Index</a>]</td>
<td valign="middle" align="left">[<a href="avram_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
</tr></table>
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