/usr/lib/swi-prolog/doc/Manual/pio.html

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<h2 id="sec:pio"><a id="sec:A.19"><span class="sec-nr">A.19</span> <span class="sec-title">library(pio): 
Pure I/O</span></a></h2>

<a id="sec:pio"></a>
<a id="sec:lib:pio"></a>

<p>This library provides pure list-based I/O processing for Prolog, 
where the communication to the actual I/O device is performed 
transparently through coroutining. This module itself is just an 
interface to the actual implementation modules.

<p><h3 id="sec:pureinput"><a id="sec:A.19.1"><span class="sec-nr">A.19.1</span> <span class="sec-title">library(pure_input): 
Pure Input from files</span></a></h3>

<p><a id="sec:pureinput"></a>

<dl class="tags">
<dt class="mtag">author</dt>
<dd>
- Ulrich Neumerkel <br>
- Jan Wielemaker</dd>
<dt class="mtag">To be done</dt>
<dd>
- Provide support for alternative input readers, e.g. reading terms, 
tokens, etc. <br>
- Support non-repositioning streams, such as sockets and pipes.
</dd>
</dl>

<p>This module is part of <code>pio.pl</code>, dealing with <i>pure</i> <i>input</i>: 
processing input streams from the outside world using pure predicates, 
notably grammar rules (DCG). Using pure predicates makes 
non-deterministic processing of input much simpler.

<p>Pure input uses coroutining (<a class="pred" href="coroutining.html#freeze/2">freeze/2</a>) 
to read input from the external source into a list <i>on demand</i>. The 
overhead of lazy reading is more than compensated for by using block 
reads based on <a class="pred" href="chario.html#read_pending_input/3">read_pending_input/3</a>.

<dl class="latex">
<dt class="pubdef"><span class="pred-tag">[nondet]</span><a id="phrase_from_file/2"><strong>phrase_from_file</strong>(<var>:Grammar, 
+File</var>)</a></dt>
<dd class="defbody">
Process the content of <var>File</var> using the DCG rule <var>Grammar</var>. 
The space usage of this mechanism depends on the length of the not 
committed part of <var>Grammar</var>. Committed parts of the temporary 
list are reclaimed by the garbage collector, while the list is extended 
on demand. Here is a very simple definition for searching a string in a 
file:

<pre class="code">
... --&gt; []|[_],... .

file_contains(File, Pattern) :-
        phrase_from_file((..., Pattern, ...), File).

match_count(File, Pattern, Count) :-
        findall(x, file_contains(File, Pattern), Xs),
        length(Xs, Count).
</pre>

<p>This can be called as (note that the pattern must be a string (code 
list)):

<pre class="code">
?- match_count('pure_input.pl', "file", Count).
</pre>

</dd>
<dt class="pubdef"><span class="pred-tag">[nondet]</span><a id="phrase_from_file/3"><strong>phrase_from_file</strong>(<var>:Grammar, 
+File, +Options</var>)</a></dt>
<dd class="defbody">
As <a class="pred" href="pio.html#phrase_from_file/2">phrase_from_file/2</a>, 
providing additional <var>Options</var>. <var>Options</var> are passed 
to <a class="pred" href="IO.html#open/4">open/4</a>, except for <code>buffer_size</code>, 
which is passed to
<a class="pred" href="IO.html#set_stream/2">set_stream/2</a>. If not 
specified, the default buffer size is 512 bytes. Of particular 
importance are the <a class="pred" href="IO.html#open/4">open/4</a> 
options <code>type</code> and <code>encoding</code>.</dd>
<dt class="pubdef"><a id="phrase_from_stream/2"><strong>phrase_from_stream</strong>(<var>:Grammer, 
+Stream</var>)</a></dt>
<dd class="defbody">
Helper for <a class="pred" href="pio.html#phrase_from_file/3">phrase_from_file/3</a>. 
This predicate cooperates with
<a class="pred" href="pio.html#syntax_error/3">syntax_error/3</a> to 
generate syntax error locations for grammars.</dd>
<dt class="pubdef"><a id="syntax_error/3"><strong>syntax_error</strong>(<var>+Error</var>)</a><code>//</code></dt>
<dd class="defbody">
Throw the syntax error <var>Error</var> at the current location of the 
input. This predicate is designed to be called from the handler of <a class="pred" href="pio.html#phrase_from_file/3">phrase_from_file/3</a>.

<dl class="tags">
<dt class="tag">throws</dt>
<dd>
<code>error(syntax_error(Error), Location)</code>
</dd>
</dl>

</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="lazy_list_location/3"><strong>lazy_list_location</strong>(<var>-Location</var>)</a><code>//</code></dt>
<dd class="defbody">
Determine current (error) location in a lazy list. True when
<var>Location</var> is an (error) location term that represents the 
current location in the DCG list.
<table class="arglist">
<tr><td><var>Location</var> </td><td>is a term <code>file(Name, Line, LinePos, CharNo)</code> 
or
<code>stream(Stream, Line, LinePos, CharNo)</code> if no file is 
associated to the stream RestLazyList. Finally, if the Lazy list is 
fully materialized (ends in <code>[]</code>), <var>Location</var> is 
unified with <code>end_of_file-CharCount</code>. </td></tr>
</table>

<dl class="tags">
<dt class="tag">See also</dt>
<dd>
<a class="pred" href="pio.html#lazy_list_character_count/3">lazy_list_character_count/3</a> 
only provides the character count.
</dd>
</dl>

</dd>
<dt class="pubdef"><a id="lazy_list_character_count/3"><strong>lazy_list_character_count</strong>(<var>-CharCount</var>)</a><code>//</code></dt>
<dd class="defbody">
True when <var>CharCount</var> is the current character count in the 
Lazy list. The character count is computed by finding the distance to 
the next frozen tail of the lazy list. <var>CharCount</var> is one of:

<p>
<ul class="compact">
<li>An integer
<li>A term end_of_file-Count
</ul>

<dl class="tags">
<dt class="tag">See also</dt>
<dd>
<a class="pred" href="pio.html#lazy_list_location/3">lazy_list_location/3</a> 
provides full details of the location for error reporting.
</dd>
</dl>

</dd>
<dt class="pubdef"><span class="pred-tag">[det]</span><a id="stream_to_lazy_list/2"><strong>stream_to_lazy_list</strong>(<var>+Stream, 
-List</var>)</a></dt>
<dd class="defbody">
Create a lazy list representing the character codes in <var>Stream</var>. 
It must be possible to reposition <var>Stream</var>. <var>List</var> is 
a list that ends in a delayed goal. <var>List</var> can be unified 
completely transparent to a (partial) list and processed transparently 
using DCGs, but please be aware that a lazy list is not the same as a 
materialized list in all respects.

<p>Typically, this predicate is used as a building block for more high 
level safe predicates such as <a class="pred" href="pio.html#phrase_from_file/2">phrase_from_file/2</a>.

<dl class="tags">
<dt class="tag">To be done</dt>
<dd>
Enhance of lazy list throughout the system.
</dd>
</dl>

</dd>
</dl>

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swi-prolog-nox 6.6.6-5 / usr / lib / swi-prolog / doc / Manual / pio.html
