A grammar file must be of the form:
<declarations>
%%
<rules>
%%
<classes>
The <declarations> and
<classes> sections may be empty. If the
<classes> section is empty, the second %%
may be omitted. Anywhere in a grammar file, Java comments (i.e.,
C-style or C++-style) may be used and are accordingly
ignored. Javadoc-style comments are also supported and may be used for
documenting a grammar (invoking
Jacc with the -doc option).
What goes in each section is detailed next (see also the sorted index giving the complete listing of all the Jacc commands).
The foregoing commands essentially behave as they do in yacc. The only yacc command not supported is %union which does not make sense for Java. This also means that the use of union member tags in %token, %left, %right, and %nonassoc is not supported. Note that the yacc %type command is not implemented; it is rendered useless by the %nodeclass command, which achieves similar, and more, effects.
In addition to the commands above, the following new commands are available:
<head> : <body> ;
where <head> is a non-terminal symbol of the grammar, and
<body> is a (possibly empty) sequence of (terminal or
non-terminal) grammar symbols, or semantic actions. A terminal symbol is an
identifier that has been defined in the <declarations> part
using the %token command, or any character sequence between single or
double quotes. A non-terminal symbol is any identifier appearing in a rule's
<head>, or in a rule's <body>, but is
not a terminal symbol. A semantic action is anything of the form
{ <Java statements> }.
As in yacc, the short hand:
<head> : <body>1 | ... | <body>n ;
may be used instead of:
<head> : <body>1 ;
<head> : ...
<head> : <body>n ;
As in yacc, so-called pseudo-variables of the form $$, $1, ..., $n, ..., where n is a number, may be used in a semantic action's statements. As in yacc again, $$ stands for the value returned by the rule's semantic action, and $n stands for the value returned by the parse of the n-th element in the rule's <body> to the left of the semantic action where it appears. Also as in yacc, if n is 0 or negative, the pseudo-variable refers to the value returned by a rule recognized before this rule ($0 for the value of the rule immediately preceding, $-1 for the one before that, etc...). The value returned by a rule is selected as follows:
As in yacc, the precedence and associativity associated to a rule are, by default, those of the rightmost terminal symbol occurring in it. If the rule contains no terminal symbol, its precedence is set to to the minimum value and its associativity to none. In order to override this default behavior, the %prec command may be used (at most once) anywhere in the body of a rule. One simply uses %prec token (or %prec looseness specifier) to confer to the rule the precedence and associativity of a specific token token (or using a looseness number and a specifier as for defining dynamic operators).
The type of the value returned by a semantic action is ParseNode. This class is the type of objects pushed on the semantic evaluation stack. It comes with two predefined attributes that may be used in a rule's semantic action: a numeric one (nvalue) of type double, and a symbolic one (svalue) of type String. If attributes of other types are needed, one may use the %nodeclass command for the appropriate symbol, which has for effect to make the type of the value it returns a subclass of ParseNode containing the desired class members.