The example below calls the rule set integer//1 defined in section 4.13 and available from library(library(dcg/basics)), binding Rest to the remainder of the input after matching the integer.

?- [library(dcg/basics)].
?- atom_codes('42 times', Codes),
   phrase(integer(X), Codes, Rest).
X = 42
Rest = [32, 116, 105, 109, 101, 115]

The next example exploits a complete body. Given the following definition of digit_weight//1 , we can pose the query below.

digit_weight(W) -->
        [D],
        { code_type(D, digit(W)) }.

?- atom_codes('Version 3.4', Codes),
   phrase(("Version ",
           digit_weight(Major),".",digit_weight(Minor)),
          Codes).
Major = 3,
Minor = 4.

The SWI-Prolog implementation of phrase/3 verifies that the List and Rest arguments are unbound, bound to the empty list or a list cons cell. Other values raise a type error.^{70The ISO standard allows for both raising a type error and accepting any term as input and output. Note the tail of the list is not checked for performance reasons.} The predicate call_dcg/3 is provided to use grammar rules with terms that are not lists.

Note that the syntax for lists of codes changed in SWI-Prolog version 7 (see section 5.2). If a DCG body is translated, both "text" and `text` is a valid code-list literal in version 7. A version 7 string ("text") is not acceptable for the second and third arguments of phrase/3. This is typically not a problem for applications as the input of a DCG rarely appears in the source code. For testing in the toplevel, one must use double quoted text in versions prior to 7 and back quoted text in version 7 or later.

See also portray_text/1, 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 library(apply_macros) compiles phrase/3 if the argument is sufficiently instantiated, eliminating the runtime overhead of translating DCGBody and meta-calling.