This Appendix has a description of the Yacc input syntax, as a Yacc specification. Context dependencies, etc., are not considered. Ironically, the Yacc input specification language is most naturally specified as an LR(2) grammar; the sticky part comes when an identifier is seen in a rule, immediately following an action. If this identifier is followed by a colon, it is the start of the next rule; otherwise it is a continuation of the current rule, which just happens to have an action embedded in it. As implemented, the lexical analyzer looks ahead after seeing an identifier, and decide whether the next token (skipping blanks, newlines, comments, etc.) is a colon. If so, it returns the token C_IDENTIFIER. Otherwise, it returns IDENTIFIER. Literals (quoted strings) are also returned as IDENTIFIERS, but never as part of C_IDENTIFIERs.

 /* grammar for the input to Yacc */

 /* basic entities */
%token IDENTIFIER /* includes identifiers and literals */
%token C_IDENTIFIER /* identifier (but not literal) followed by colon */
%token NUMBER /* [0-9]+ */

 /* reserved words: %type => TYPE, %left => LEFT, etc. */

%token LEFT RIGHT NONASSOC TOKEN PREC TYPE START UNION

%token MARK /* the %% mark */
%token LCURL /* the %{ mark */
%token RCURL /* the %} mark */

 /* ascii character literals stand for themselves */

%start spec

%%

spec : defs MARK rules tail
 ;

tail : MARK { In this action, eat up the rest of the file }
 | /* empty: the second MARK is optional */
 ;

defs : /* empty */
 | defs def
 ;

def : START IDENTIFIER
 | UNION { Copy union definition to output }
 | LCURL { Copy C code to output file } RCURL
 | ndefs rword tag nlist
 ;
rword : TOKEN
 | LEFT
 | RIGHT
 | NONASSOC
 | TYPE
 ;

tag : /* empty: union tag is optional */
 | \'<\' IDENTIFIER \'>\'
 ;

nlist : nmno
 | nlist nmno
 | nlist \',\' nmno
 ;

nmno : IDENTIFIER /* NOTE: literal illegal with %type */
 | IDENTIFIER NUMBER /* NOTE: illegal with %type */
 ;

 /* rules section */

rules : C_IDENTIFIER rbody prec
 | rules rule
 ;

rule : C_IDENTIFIER rbody prec
 | '|' rbody prec
 ;

rbody : /* empty */
 | rbody IDENTIFIER
 | rbody act
 ;

act : \'{\' { Copy action, translate $$, etc. } \'}\'
 ;

prec : /* empty */
 | PREC IDENTIFIER
 | PREC IDENTIFIER act
 | prec \';\'
 ;