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

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<h2 id="sec:DCG"><a id="sec:4.12"><span class="sec-nr">4.12</span> <span class="sec-title">DCG 
Grammar rules</span></a></h2>

<a id="sec:DCG"></a>

<p><a id="idx:DCG:680"></a><a id="idx:serialize:681"></a><a id="idx:deserialize:682"></a>Grammar 
rules form a comfortable interface to <em>difference lists</em>. They 
are designed both to support writing parsers that build a parse tree 
from a list of characters or tokens and for generating a flat list from 
a term.

<p>Grammar rules look like ordinary clauses using <code><code>--&gt;</code>/2</code> 
for separating the head and body rather than <code><code>:-</code>/2</code>. 
Expanding grammar rules is done by <a id="idx:expandterm2:683"></a><a class="pred" href="consulting.html#expand_term/2">expand_term/2</a>, 
which adds two additional arguments to each term for representing the 
difference list.

<p>The body of a grammar rule can contain three types of terms. A 
callable term is interpreted as a reference to a grammar rule. Code 
between
<code>{</code>...<code>}</code> is interpreted as plain Prolog code, and 
finally, a list is interpreted as a sequence of <em>literals</em>. The 
Prolog control-constructs (<code><code>\+</code>/1</code>, <code><code>-&gt;</code>/2</code>, <code><code>;</code>/<code>;</code></code>2,
<code><code>,</code>/2</code> and <code><code>!</code>/0</code>) can be 
used in grammar rules.

<p>We illustrate the behaviour by defining a rule set for parsing an 
integer.

<pre class="code">
integer(I) --&gt;
        digit(D0),
        digits(D),
        { number_codes(I, [D0|D])
        }.

digits([D|T]) --&gt;
        digit(D), !,
        digits(T).
digits([]) --&gt;
        [].

digit(D) --&gt;
        [D],
        { code_type(D, digit)
        }.
</pre>

<p>Grammar rule sets are called using the built-in predicates <a id="idx:phrase2:684"></a><a class="pred" href="DCG.html#phrase/2">phrase/2</a> 
and <a id="idx:phrase3:685"></a><a class="pred" href="DCG.html#phrase/3">phrase/3</a>:

<dl class="latex">
<dt class="pubdef"><a id="phrase/2"><strong>phrase</strong>(<var>:DCGBody, 
?List</var>)</a></dt>
<dd class="defbody">
Equivalent to <code>phrase(<var>DCGBody</var>, <var>InputList</var>, [])</code>.
</dd>
<dt class="pubdef"><a id="phrase/3"><strong>phrase</strong>(<var>:DCGBody, 
?List, ?Rest</var>)</a></dt>
<dd class="defbody">
True when <var>DCGBody</var> applies to the difference
<var>List</var>/<var>Rest</var>. Although <var>DCGBody</var> is 
typically a
<em>callable</em> term that denotes a grammar rule, it can be any term 
that is valid as the body of a DCG rule.

<p>The example below calls the rule set `integer' defined in <a class="sec" href="DCG.html">section 
4.12</a>, binding <var>Rest</var> to the remainder of the input after 
matching the integer.

<pre class="code">
?- phrase(integer(X), "42 times", Rest).
X = 42
Rest = [32, 116, 105, 109, 101, 115]
</pre>

<p>The next example exploits a complete body.

<pre class="code">
digit_weight(W) --&gt;
        [D],
        { code_type(D, digit(W)) }.

?- phrase(("Version ",
           digit_weight(Major),".",digit_weight(Minor)),
          "Version 3.4").
Major = 3,
Minor = 4.
</pre>

<p>See also <a id="idx:portraytext1:686"></a><span class="pred-ext">portray_text/1</span>, 
which can be used to print lists of character codes as a string to the 
top level and debugger to facilitate debugging DCGs that process 
character codes. The library <code>library(apply_macros)</code> compiles <a id="idx:phrase3:687"></a><a class="pred" href="DCG.html#phrase/3">phrase/3</a> 
if the argument is sufficiently instantiated, eliminating the runtime 
overhead of translating <var>DCGBody</var> and meta-calling.
</dd>
</dl>

<p>As stated above, grammar rules are a general interface to difference 
lists. To illustrate, we show a DCG-based implementation of
<a id="idx:reverse2:688"></a><a class="pred" href="lists.html#reverse/2">reverse/2</a>:

<pre class="code">
reverse(List, Reversed) :-
        phrase(reverse(List), Reversed).

reverse([])    --&gt; [].
reverse([H|T]) --&gt; reverse(T), [H].
</pre>

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / DCG.html
