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Annotation of 41BSD/cmd/yacc/yaccdiffs, revision 1.1

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        !            10:                       Yacc Differences
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        !            16: This document gives a short list of differences between  the
        !            17: new Yacc and earlier Yaccs.
        !            18: 
        !            19: _B_u_g_s _F_i_x_e_d
        !            20: 
        !            21: 1.   There was a bug which caused  Yacc  to  silently  steal
        !            22:      away in the night if an action had mismatched '' in it;
        !            23:      this is fixed.
        !            24: 
        !            25: 2.   A number of table size overflow conditions used  to  be
        !            26:      checked incorrectly or not at all; this is now better.
        !            27: 
        !            28: 3.   A bug which suppressed the printing of some rules  with
        !            29:      empty RHS's on the y.output file has been fixed.
        !            30: 
        !            31: _S_p_e_e_d_u_p_s, _S_h_r_i_n_k_s, _a_n_d _D_i_d_d_l_e_s
        !            32: 
        !            33: 1.   The old optimizer (-o) flag is now the default in Yacc.
        !            34:      At the same time, the Yacc process itself has been sped
        !            35:      up; the result is that Yacc takes  about  the  same  or
        !            36:      slightly  longer on short inputs, but is much faster on
        !            37:      long inputs.
        !            38: 
        !            39: 2.   The optimized parsers produced by Yacc are likely to be
        !            40:      2-3  times  faster  and 1-2k bytes smaller than the old
        !            41:      ones, for medium/large grammars.  The time to parse  is
        !            42:      now  essentially  independent  of  the grammar size; it
        !            43:      used to grow as the size of the grammar did.
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        !            45: 3.   The y.output file has been considerably reformatted, to
        !            46:      make  it easier to read.  The old "goto" table is gone;
        !            47:      the goto's for nonterminal symbols are now  printed  in
        !            48:      the  states  where  they  occur.   Rules  which  can be
        !            49:      reduced in a state have their rule number printed after
        !            50:      them,  in  ().   This makes it much easier to interpret
        !            51:      the "reduce n" actions.  The message "same  as  n"  has
        !            52:      been  removed; duplicate states are in fact duplicated,
        !            53:      saving shuffling and cross-referencing.
        !            54: 
        !            55: 4.   Various table sizes are somewhat bigger.
        !            56: 
        !            57: 5.   The form feed character, and the construction '\f', are
        !            58:      now  recognized;  form feed is ignored (=whitespace) on
        !            59:      input.
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        !            63: 
        !            64:                       January 14, 1977
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        !            70:                            - 2 -
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        !            74: 6.   The arrays "yysterm" and "yysnter" are no  longer  pro-
        !            75:      duced  on  output; they were little used, and took up a
        !            76:      surprising amount of space in the parser.
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        !            78: 7.   Rules in the input which are not reduced are  now  com-
        !            79:      plained  about; they may represent unreachable parts of
        !            80:      the grammar, botched precedence,  or  duplicate  rules.
        !            81:      As  with  conflicts,  a summary complaint, "n rules not
        !            82:      reduced", appears at the terminal; more information  is
        !            83:      on the y.output file.
        !            84: 
        !            85: _N_e_w _F_e_a_t_u_r_e_s
        !            86: 
        !            87: 1.   The actions are now  copied  into  the  middle  of  the
        !            88:      parser, rather than being gathered into a separate rou-
        !            89:      tine.  It's faster.  Also, you can return a value  from
        !            90:      yyparse (and stop parsing...) by saying `return(x);' in
        !            91:      an action.  There are  macros  which  simulate  various
        !            92:      interesting parsing actions:
        !            93: 
        !            94:      YYERROR  causes the parser to behave as if a syntax
        !            95:      error had been encountered (i.e., do error recovery)
        !            96:      YYACCEPT causes a return from yyparse with a value of 0
        !            97:      YYABORT  causes a return from yyparse with a value of 1
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        !           100: 2.   The repositioning of the actions may cause scope  prob-
        !           101:      lems  for  some people who include lexical analyzers in
        !           102:      funny places.  This can probably be  avoided  by  using
        !           103:      another  new  feature:  the `-d' option.  Invoking Yacc
        !           104:      with the -d option causes  the  #defines  generated  by
        !           105:      Yacc  to  be  written out onto a file called "y.tab.h",
        !           106:      (as well as on the "y.tab.c" file).  This can  then  be
        !           107:      included as desired in lexical analyzers, etc.
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        !           109: 3.   Actions  are  now  permitted  within  rules;  for  such
        !           110:      actions,  $$, $1, $2, etc. continue to have their usual
        !           111:      meanings.  An error  message  is  returned  if  any  $n
        !           112:      refers  to  a value lying to the right of the action in
        !           113:      the rule.  These internal actions are assumed to return
        !           114:      a  value,  which  is accessed through the $n mechanism.
        !           115:      In the y.output file, the actions are  referred  to  by
        !           116:      created  nonterminal  names  of  the  form  $$nnn.  All
        !           117:      actions within rules are assumed to  be  distinct.   If
        !           118:      some   actions   are   the   same,  Yacc  might  report
        !           119:      reduce/reduce conflicts  which  could  be  resolved  by
        !           120:      explicitly  identifying  identical actions; does anyone
        !           121:      have a good idea for a syntax to do this?  The  =  sign
        !           122:      may  now be omitted in action constructions of the form
        !           123:      ={  ...   }.
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        !           130:                       January 14, 1977
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        !           136:                            - 3 -
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        !           140: 4.   As a result of the rearrangement of rules,  people  who
        !           141:      thought they knew what $1 really turned into, and wrote
        !           142:      programs which referred to yypv[1], etc., are in  trou-
        !           143:      ble.  See Steve Johnson if you are really suffering.
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        !           196:                       January 14, 1977
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