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Annotation of 43BSDReno/share/doc/ps1/15.yacc/ss1, revision 1.1

1.1     ! root        1: .\"    @(#)ss1 6.1 (Berkeley) 5/8/86
        !             2: .\"
        !             3: .tr *\(**
        !             4: .tr |\(or
        !             5: .SH
        !             6: 1: Basic Specifications
        !             7: .PP
        !             8: Names refer to either tokens or nonterminal symbols.
        !             9: Yacc requires
        !            10: token names to be declared as such.
        !            11: In addition, for reasons discussed in Section 3, it is often desirable
        !            12: to include the lexical analyzer as part of the specification file;
        !            13: it may be useful to include other programs as well.
        !            14: Thus, every specification file consists of three sections:
        !            15: the
        !            16: .I declarations ,
        !            17: .I "(grammar) rules" ,
        !            18: and
        !            19: .I programs .
        !            20: The sections are separated by double percent ``%%'' marks.
        !            21: (The percent ``%'' is generally used in Yacc specifications as an escape character.)
        !            22: .PP
        !            23: In other words, a full specification file looks like
        !            24: .DS
        !            25: declarations
        !            26: %%
        !            27: rules
        !            28: %%
        !            29: programs
        !            30: .DE
        !            31: .PP
        !            32: The declaration section may be empty.
        !            33: Moreover, if the programs section is omitted, the second %% mark may be omitted also;
        !            34: thus, the smallest legal Yacc specification is
        !            35: .DS
        !            36: %%
        !            37: rules
        !            38: .DE
        !            39: .PP
        !            40: Blanks, tabs, and newlines are ignored except
        !            41: that they may not appear in names or multi-character reserved symbols.
        !            42: Comments may appear wherever a name is legal; they are enclosed
        !            43: in /* . . . */, as in C and PL/I.
        !            44: .PP
        !            45: The rules section is made up of one or more grammar rules.
        !            46: A grammar rule has the form:
        !            47: .DS
        !            48: A  :  BODY  ;
        !            49: .DE
        !            50: A represents a nonterminal name, and BODY represents a sequence of zero or more names and literals.
        !            51: The colon and the semicolon are Yacc punctuation.
        !            52: .PP
        !            53: Names may be of arbitrary length, and may be made up of letters, dot ``.'', underscore ``\_'', and
        !            54: non-initial digits.
        !            55: Upper and lower case letters are distinct.
        !            56: The names used in the body of a grammar rule may represent tokens or nonterminal symbols.
        !            57: .PP
        !            58: A literal consists of a character enclosed in single quotes ``\'''.
        !            59: As in C, the backslash ``\e'' is an escape character within literals, and all the C escapes
        !            60: are recognized.
        !            61: Thus
        !            62: .DS
        !            63: \'\en\'        newline
        !            64: \'\er\'        return
        !            65: \'\e\'\'       single quote ``\'''
        !            66: \'\e\e\'       backslash ``\e''
        !            67: \'\et\'        tab
        !            68: \'\eb\'        backspace
        !            69: \'\ef\'        form feed
        !            70: \'\exxx\'      ``xxx'' in octal
        !            71: .DE
        !            72: For a number of technical reasons, the
        !            73: \s-2NUL\s0
        !            74: character (\'\e0\' or 0) should never
        !            75: be used in grammar rules.
        !            76: .PP
        !            77: If there are several grammar rules with the same left hand side, the vertical bar ``|''
        !            78: can be used to avoid rewriting the left hand side.
        !            79: In addition,
        !            80: the semicolon at the end of a rule can be dropped before a vertical bar.
        !            81: Thus the grammar rules
        !            82: .DS
        !            83: A      :       B  C  D   ;
        !            84: A      :       E  F   ;
        !            85: A      :       G   ;
        !            86: .DE
        !            87: can be given to Yacc as
        !            88: .DS
        !            89: A      :       B  C  D
        !            90:        |       E  F
        !            91:        |       G
        !            92:        ;
        !            93: .DE
        !            94: It is not necessary that all grammar rules with the same left side appear together in the grammar rules section,
        !            95: although it makes the input much more readable, and easier to change.
        !            96: .PP
        !            97: If a nonterminal symbol matches the empty string, this can be indicated in the obvious way:
        !            98: .DS
        !            99: empty :   ;
        !           100: .DE
        !           101: .PP
        !           102: Names representing tokens must be declared; this is most simply done by writing
        !           103: .DS
        !           104: %token   name1  name2 . . .
        !           105: .DE
        !           106: in the declarations section.
        !           107: (See Sections 3 , 5, and 6 for much more discussion).
        !           108: Every name not defined in the declarations section is assumed to represent a nonterminal symbol.
        !           109: Every nonterminal symbol must appear on the left side of at least one rule.
        !           110: .PP
        !           111: Of all the nonterminal symbols, one, called the
        !           112: .I "start symbol" ,
        !           113: has particular importance.
        !           114: The parser is designed to recognize the start symbol; thus,
        !           115: this symbol represents the largest,
        !           116: most general structure described by the grammar rules.
        !           117: By default,
        !           118: the start symbol is taken to be the left hand side of the first
        !           119: grammar rule in the rules section.
        !           120: It is possible, and in fact desirable, to declare the start
        !           121: symbol explicitly in the declarations section using the %start keyword:
        !           122: .DS
        !           123: %start   symbol
        !           124: .DE
        !           125: .PP
        !           126: The end of the input to the parser is signaled by a special token, called the
        !           127: .I endmarker .
        !           128: If the tokens up to, but not including, the endmarker form a structure
        !           129: which matches the start symbol, the parser function returns to its caller
        !           130: after the endmarker is seen; it
        !           131: .I accepts
        !           132: the input.
        !           133: If the endmarker is seen in any other context, it is an error.
        !           134: .PP
        !           135: It is the job of the user-supplied lexical analyzer
        !           136: to return the endmarker when appropriate; see section 3, below.
        !           137: Usually the endmarker represents some reasonably obvious 
        !           138: I/O status, such as ``end-of-file'' or ``end-of-record''.

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