yacc input syntax

This section has a description of the yacc input syntax as a yacc specification. Context dependencies and so forth are not considered. Ironically, although yacc accepts an LALR(1) grammar, 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 decides whether the next token (skipping blanks, new-lines, comments, and so on) 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 entries /
   %token     IDENTIFIER    / includes identifiers and literals /
   %token     C_IDENTIFIER  / identifier (but not literal) followed by a : /
   %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 /
   

   %token    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
          |   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 invalid with % type /
          |  IDENTIFIER NUMBER   / Note: invalid with % type /
          ;
   

      / rule 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 ';'
          ;