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

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<H2><A NAME="sec:A.18"><SPAN class="sec-nr">A.18</SPAN> <SPAN class="sec-title">library(pio): 
Pure I/O</SPAN></A></H2>

<A NAME="pio"></A>
<A NAME="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.

<H3><A NAME="sec:A.18.1"><SPAN class="sec-nr">A.18.1</SPAN> <SPAN class="sec-title">library(pure_input): 
Pure Input from files</SPAN></A></H3>

<P><A NAME="sec:pureinput"></A>

<DL>
<DT><B>author</B><DD>- Ulrich Neumerkel <BR>
- Jan Wielemaker<DT><B>To be done</B><DD>- Provide support for 
alternative input readers, e.g. reading terms, tokens, etc. <BR>
- Support non-repositioning streams, such as sockets and pipes.
</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 NAME="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 NAME="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"><span class="pred-tag">[det]</span><A NAME="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>
<DT><B>To be done</B><DD> Enhance of lazy list throughout the system.
</DL>

</DD>
</DL>

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