4.14.4 Semantics of Grammar Rules

Grammar rules are best explained in terms of an interpreter. The semantics of phrase/3 is shown as if defined by the interpreter shown below. The interpreter's case analysis is as follows:

control constructs

(Including cuts and module prefixes). The interpreter implements the semantics of the construct, descending into its argument. Note that other built-in predicates are not treated this way.

lists

Treated as terminal symbols.

curly brackets

Treated as procedure calls.

callable terms with functor N/A

A grammar rule with head functor N/A is looked up using the imaginary predicate -->/2, unified against, and its body is interpreted. If none exists, this is treated as a procedure call to a predicate N/A+2.

non-callable terms

Raise error exception.

The following points are worth noting:

• The code below defines what constructs of and to what depth grammar rule bodies are interpreted, as opposed to being treated as non-terminals.
• Throughout the interpretation, we must keep track of the module context.
• The head non-terminal of a grammar rule is optionally followed by a sequence of terminals. This feature is not supported by the interpreter, but is supported in the actual implementation.
• As a general rule, the last argument is unified after any side-effects, including cuts. This is in line with the rule that output arguments should not be unified before a cut (see Eff Overview). In other words, grammar rules are steadfast.
• The last clause gives a clue to how grammar rules are actually implemented, i.e. by compile-time transformation to ordinary Prolog clauses. A grammar rule with head functor N/A is transformed to a Prolog clause with head functor N/A+2, the extra arguments being S0 and S. -->/2 is not a predicate.

The interpreter is as follows, slightly simplified:

     phrase(M:Body, S0, S) :-
             phrase(Body, M, S0, S).
     
     phrase(Var, M, S0, S) :- \+callable(Var), !,
             must_be(Var, callable, phrase(M:Var,S0,S), 1).
     phrase(M:Body, _, S0, S) :- !,
             phrase(Body, M, S0, S).
     phrase(!, _, S0, S) :- !,
             cut relevant choicepoints,
             S0 = S.                 % unification AFTER action
     phrase((A, B), M, S0, S) :- !,
             phrase(A, M, S0, S1),
             phrase(B, M, S1, S).
     phrase((A -> B), M, S0, S) :- !,
         (   phrase(A, M, S0, S1) ->
             phrase(B, M, S1, S)
         ).
     phrase((A -> B ; C), M, S0, S) :- !,
         (   phrase(A, M, S0, S1) ->
             phrase(B, M, S1, S)
         ;   phrase(C, M, S0, S)
         ).
     phrase((A ; B), M, S0, S) :- !,
         (   phrase(A, M, S0, S)
         ;   phrase(B, M, S0, S)
         ).
     phrase(\+(A), M, S0, S) :- !,
         (   phrase(A, M, S0, _) ->
             fail
         ;   S0 = S
         ).
     phrase(_^A, M, S0, S) :- !,
             phrase(A, M, S0, S).
     phrase(do(Iter,Body), M, S0, S) :- !,
         (   Iter,
             fromto(S0,S1,S2,S)
         do  phrase(Body, M, S1, S2)
         ).
     phrase(if(A,B,C), M, S0, S) :- !,
             if(phrase(A, M, S0, S1),
                phrase(B, M, S1, S),
                phrase(C, M, S0, S)).
     phrase(once(A), M, S0, S) :- !,
         (   phrase(A, M, S0, S1) ->
             S1 = S                  % unification AFTER call
         ).
     phrase([], _, S0, S) :- !,
             S0 = S.
     phrase([H|T], M, S0, S) :- !,
             S0 = [H|S1],
             phrase(T, M, S1, S).
     phrase({G}, M, S0, S) :- !,
             call(M:G),              % Please note: transparent to cuts
             S0 = S.                 % unification AFTER call
     phrase(NT, M, S0, S) :-
             \+ \+(M:NT --> Rhs), !, % grammar rule exists?
             (M:NT --> Rhs),
             phrase(Rhs, M, S0, S).
     phrase(NT, M, S0, S) :-
             call(M:NT, S0, S).      % otherwise, treat as procedure call

As mentioned above, grammar rules are merely a convenient abbreviation for ordinary Prolog clauses. Each grammar rule is translated into a Prolog clause as it is compiled. This translation is exemplified below.

The procedural interpretation of a grammar rule is that it takes an input list of symbols or character codes, analyzes some initial portion of that list, and produces the remaining portion (possibly enlarged) as output for further analysis. The arguments required for the input and output lists are not written explicitly in a grammar rule, but are added when the rule is translated into an ordinary Prolog clause. The translations shown differ from the output of listing/[0,1] in that internal translations such as variable renaming are not represented. This is done in the interests of clarity. For example, a rule such as (A) will be depicted as translating into (B) rather than (C).

     p(X) --> q(X). (A)

     p(X, S0, S) :-
             q(X, S0, S). (B)

     p(A, B, C) :-
             q(A, B, C). (C)

If there is more than one non-terminal on the right-hand side, as in (D) the corresponding input and output arguments are identified, translating into (E):

     p(X, Y) --> q(X), r(X, Y), s(Y). (D)

     p(X, Y, S0, S) :- (E)
         q(X, S0, S1),
         r(X, Y, S1, S2),
         s(Y, S2, S).

Terminals are translated using the built-in predicate =/2. For instance, (F) is translated into (G):

     p(X) --> [go, to], q(X), [stop]. (F)

     p(X, S0, S) :- (G)
         S0 = [go,to|S1],
         q(X, S1, S2),
         S2 = [stop|S].

Extra conditions expressed as explicit procedure calls, enclosed in curly braces, naturally translate into themselves. For example (H) translates to (I):

     p(X) --> [X], {integer(X), X > 0}, q(X). (H)

     p(X, S0, S) :- (I)
         S0 = [X|S1],
         integer(X),
         X > 0,
         q(X, S1, S).

Terminals on the left-hand side of a rule, enclosed in square brackets, also translate into a unification. For example, (J) becomes (K):

     is(N), [not] --> [aint]. (J)

     is(N, S0, S) :- (K)
         S0 = [aint|S1],
         S = [not|S1].

Disjunction and other control constructs have a fairly obvious translation. For example, (L), a rule that equates phrases like “(sent) a letter to him” and “(sent) him a letter”, translates to (M):

     args(X, Y) --> (L)
             dir(X), [to], indir(Y) |
             indir(Y), dir(X).

     args(X, Y, S0, S) :- (M)
         (   dir(X, S0, S1),
             S1 = [to|S2],
             indir(Y, S2, S)
         |   indir(Y, S0, S1),
             dir(X, S1, S)
         ).

In order to look at these translations, declare the grammar rules dynamic and use listing/[0,1]. However, bear in mind that a grammar rule with head functor N/A is transformed to a Prolog clause with head functor N/A+2. For example, the following declaration for grammar rule (L) would enable you to list its translation, (M):

      :- dynamic args/4.