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

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<H2><A NAME="sec:4.12"><SPAN class="sec-nr">4.12</SPAN> <SPAN class="sec-title">DCG 
Grammar rules</SPAN></A></H2>

<A NAME="sec:DCG"></A>

<P><A NAME="idx:DCG:582"></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 as for generating a flat 
list from a term. Unfortunately, Definite Clause Grammar (DCG) handling 
is not part of the Prolog standard. Most Prolog engines implement DCG, 
but the details differ slightly.

<P>Grammar rules look like ordinary clauses using --&gt;/2 for 
separating the head and body rather than :-/2 . Expanding grammar rules 
is done by <A NAME="idx:expandterm2:583"></A><A class="pred" href="consulting.html#expand_term/2">expand_term/2</A>, 
which adds two additional argument to each term for representing the 
difference list. We will illustrate the behaviour by defining a rule-set 
for parsing an integer.

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

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

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

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

<P>Grammar rule-sets are called using the built-in predicates <A NAME="idx:phrase2:584"></A><A class="pred" href="DCG.html#phrase/2">phrase/2</A> 
and <A NAME="idx:phrase3:585"></A><A class="pred" href="DCG.html#phrase/3">phrase/3</A>:

<DL>
<DT class="pubdef"><A NAME="phrase/2"><STRONG>phrase</STRONG>(<VAR>+RuleSet, 
+InputList</VAR>)</A></DT>
<DD class="defbody">
Equivalent to <CODE>phrase(<VAR>RuleSet</VAR>, <VAR>InputList</VAR>, [])</CODE>.
</DD>
<DT class="pubdef"><A NAME="phrase/3"><STRONG>phrase</STRONG>(<VAR>+RuleSet, 
+InputList, -Rest</VAR>)</A></DT>
<DD class="defbody">
Activate the rule-set with given name. `InputList' is the list of tokens 
to parse, `Rest' is unified with the remaining tokens if the sentence is 
parsed correctly. The example below calls the rule-set `integer' defined 
above.

<PRE class="code">
?- phrase(integer(X), "42 times", Rest).

X = 42
Rest = [32, 116, 105, 109, 101, 115]
</PRE>

<P></DD>
</DL>

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