## 4.13 DCG Grammar rules

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 of characters or tokens and for generating a flat list from a term.

Grammar rules look like ordinary clauses using ``-->`/2` for separating the head and body rather than ``:-`/2`. Expanding grammar rules is done by expand_term/2, which adds two additional arguments to each term for representing the difference list.

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 `{`...`}` is interpreted as plain 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.

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

    integer(I) -->
            digit(D0),
            digits(D),
            { number_codes(I, [D0|D])
            }.

    digits([D|T]) -->
            digit(D), !,
            digits(T).
    digits([]) -->
            [].

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

Grammar rule sets are called using the built-in predicates phrase/2 and phrase/3:

phrase(:DCGBody, ?List)
  Equivalent to `phrase(DCGBody, InputList, [])`.
phrase(:DCGBody, ?List, ?Rest)
  True when DCGBody applies to the difference List/Rest. Although DCGBody is typically a callable term that denotes a grammar rule, it can be any term that is valid as the body of a DCG rule.

  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.

call_dcg(:DCGBody, ?State0, ?State)
  As phrase/3, but without type checking State0 and State. This allows for using DCG rules for threading an arbitrary state variable. This predicate was introduced after type checking was added to phrase/3.71After discussion with Samer Abdallah.

  A portable solution for threading state through a DCG can be implemented by wrapping the state in a list and use the DCG semicontext facility. Subsequently, the following predicates may be used to access and modify the state:72This solution was proposed by Markus Triska.

      state(S), [S] --> [S].
      state(S0, S), [S] --> [S0].

As stated above, grammar rules are a general interface to difference lists. To illustrate, we show a DCG-based implementation of reverse/2:

    reverse(List, Reversed) :-
            phrase(reverse(List), Reversed).

    reverse([])    --> [].
    reverse([H|T]) --> reverse(T), [H].