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                      Yacc Differences





This document gives a short list of differences between  the
new Yacc and earlier Yaccs.

_\bB_\bu_\bg_\bs _\bF_\bi_\bx_\be_\bd

1.   There was a bug which caused  Yacc  to  silently  steal
     away in the night if an action had mismatched '' in it;
     this is fixed.

2.   A number of table size overflow conditions used  to  be
     checked incorrectly or not at all; this is now better.

3.   A bug which suppressed the printing of some rules  with
     empty RHS's on the y.output file has been fixed.

_\bS_\bp_\be_\be_\bd_\bu_\bp_\bs, _\bS_\bh_\br_\bi_\bn_\bk_\bs, _\ba_\bn_\bd _\bD_\bi_\bd_\bd_\bl_\be_\bs

1.   The old optimizer (-o) flag is now the default in Yacc.
     At the same time, the Yacc process itself has been sped
     up; the result is that Yacc takes  about  the  same  or
     slightly  longer on short inputs, but is much faster on
     long inputs.

2.   The optimized parsers produced by Yacc are likely to be
     2-3  times  faster  and 1-2k bytes smaller than the old
     ones, for medium/large grammars.  The time to parse  is
     now  essentially  independent  of  the grammar size; it
     used to grow as the size of the grammar did.

3.   The y.output file has been considerably reformatted, to
     make  it easier to read.  The old "goto" table is gone;
     the goto's for nonterminal symbols are now  printed  in
     the  states  where  they  occur.   Rules  which  can be
     reduced in a state have their rule number printed after
     them,  in  ().   This makes it much easier to interpret
     the "reduce n" actions.  The message "same  as  n"  has
     been  removed; duplicate states are in fact duplicated,
     saving shuffling and cross-referencing.

4.   Various table sizes are somewhat bigger.

5.   The form feed character, and the construction '\f', are
     now  recognized;  form feed is ignored (=whitespace) on
     input.




                      January 14, 1977





                           - 2 -



6.   The arrays "yysterm" and "yysnter" are no  longer  pro-
     duced  on  output; they were little used, and took up a
     surprising amount of space in the parser.

7.   Rules in the input which are not reduced are  now  com-
     plained  about; they may represent unreachable parts of
     the grammar, botched precedence,  or  duplicate  rules.
     As  with  conflicts,  a summary complaint, "n rules not
     reduced", appears at the terminal; more information  is
     on the y.output file.

_\bN_\be_\bw _\bF_\be_\ba_\bt_\bu_\br_\be_\bs

1.   The actions are now  copied  into  the  middle  of  the
     parser, rather than being gathered into a separate rou-
     tine.  It's faster.  Also, you can return a value  from
     yyparse (and stop parsing...) by saying `return(x);' in
     an action.  There are  macros  which  simulate  various
     interesting parsing actions:

     YYERROR  causes the parser to behave as if a syntax
     error had been encountered (i.e., do error recovery)
     YYACCEPT causes a return from yyparse with a value of 0
     YYABORT  causes a return from yyparse with a value of 1


2.   The repositioning of the actions may cause scope  prob-
     lems  for  some people who include lexical analyzers in
     funny places.  This can probably be  avoided  by  using
     another  new  feature:  the `-d' option.  Invoking Yacc
     with the -d option causes  the  #defines  generated  by
     Yacc  to  be  written out onto a file called "y.tab.h",
     (as well as on the "y.tab.c" file).  This can  then  be
     included as desired in lexical analyzers, etc.

3.   Actions  are  now  permitted  within  rules;  for  such
     actions,  $$, $1, $2, etc. continue to have their usual
     meanings.  An error  message  is  returned  if  any  $n
     refers  to  a value lying to the right of the action in
     the rule.  These internal actions are assumed to return
     a  value,  which  is accessed through the $n mechanism.
     In the y.output file, the actions are  referred  to  by
     created  nonterminal  names  of  the  form  $$nnn.  All
     actions within rules are assumed to  be  distinct.   If
     some   actions   are   the   same,  Yacc  might  report
     reduce/reduce conflicts  which  could  be  resolved  by
     explicitly  identifying  identical actions; does anyone
     have a good idea for a syntax to do this?  The  =  sign
     may  now be omitted in action constructions of the form
     ={  ...   }.






                      January 14, 1977





                           - 3 -



4.   As a result of the rearrangement of rules,  people  who
     thought they knew what $1 really turned into, and wrote
     programs which referred to yypv[1], etc., are in  trou-
     ble.  See Steve Johnson if you are really suffering.




















































                      January 14, 1977