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avram


			
				
					
						
						
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<h4 class="subsection">2.5.3 Byte Transducers</h4>

<p>The interface used when the <code>--byte-transducer</code> option is selected
<a name="index-g_t_0040code_007bbyte_002dtransducer_007d-command-line-option-197"></a>allows an application to serve as a persistent stream processor suitable
<a name="index-infinite-streams-198"></a>for finite or infinite streams. The interface can be summarized by the
following points.

     <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>(</code><var>state</var><code>,</code><var>output</var><code>)</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>For each byte read from standard input, <code>avram</code> applies the function
to the pair <code>(</code><var>state</var><code>,</code><var>character</var><code>)</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>The function is usually expected to return a pair
<code>(</code><var>state</var><code>,</code><var>output</var><code>)</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>If the function ever returns a value of <code>nil</code>, the computation
terminates. 
<li>If standard input comes to an end before the computation terminates,
the function will be applied to a pair of the form
<code>(</code><var>state</var><code>,nil)</code> thereafter, but may continue to return
<code>(</code><var>state</var><code>,</code><var>output</var><code>)</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. 
</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-199"></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-200"></a>representations of characters having these codes.

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