Grammar rules form a comfortable interface to difference-lists. 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.
Grammar rules look like ordinary clauses using 2 for separating the head and body rather then 2. Expanding grammar rules is done by expand_term2, 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.
![\begin{code}
integer(I) -->
digit(D0),
digits(D),
{ number_chars(I, [D0\vert ...
....
digits([]) -->
.
\par digit(D) -->
[D],
{ code_type(D, digit)
}.
\end{code}](img37.png)
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
{...} 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, 2,
/2, 2 and 0) can be used in grammar
rules.
Grammar rule-sets are called using the builtin predicates phrase2 and phrase3:
![\begin{code}
?- phrase(integer(X), ''42 times'', Rest).
\par X = 42
Rest = [32, 116, 105, 109, 101, 115]
\end{code}](img38.png)