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- <a name="Byte-Transducers"></a>
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- <td valign="middle" align="left">[<a href="Parameter-Mode-Interface.html#Parameter-Mode-Interface" title="Next section in reading order"> > </a>]</td>
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- <td valign="middle" align="left">[<a href="Virtual-Machine-Specification.html#Virtual-Machine-Specification" title="Beginning of this chapter or previous chapter"> << </a>]</td>
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- <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>
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- <tr><td valign="middle" align="left">[<a href="Line-Maps.html#Line-Maps" title="Previous section in reading order"> < </a>]</td>
- <td valign="middle" align="left">[<a href="Parameter-Mode-Interface.html#Parameter-Mode-Interface" title="Next section in reading order"> > </a>]</td>
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- <td valign="middle" align="left">[<a href="Library-Reference.html#Library-Reference" title="Next chapter"> >> </a>]</td>
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- <td valign="middle" align="left">[<a href="avram_abt.html#SEC_About" title="About (help)"> ? </a>]</td>
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