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1.1 root 1: .\" @(#)ssA 6.1 (Berkeley) 5/8/86
2: .\"
3: .SH
4: 10: Advanced Topics
5: .PP
6: This section discusses a number of advanced features
7: of Yacc.
8: .SH
9: Simulating Error and Accept in Actions
10: .PP
11: The parsing actions of error and accept can be simulated
12: in an action by use of macros YYACCEPT and YYERROR.
13: YYACCEPT causes
14: .I yyparse
15: to return the value 0;
16: YYERROR causes
17: the parser to behave as if the current input symbol
18: had been a syntax error;
19: .I yyerror
20: is called, and error recovery takes place.
21: These mechanisms can be used to simulate parsers
22: with multiple endmarkers or context-sensitive syntax checking.
23: .SH
24: Accessing Values in Enclosing Rules.
25: .PP
26: An action may refer to values
27: returned by actions to the left of the current rule.
28: The mechanism is simply the same as with ordinary actions,
29: a dollar sign followed by a digit, but in this case the
30: digit may be 0 or negative.
31: Consider
32: .DS
33: sent : adj noun verb adj noun
34: { \fIlook at the sentence\fR . . . }
35: ;
36:
37: adj : THE { $$ = THE; }
38: | YOUNG { $$ = YOUNG; }
39: . . .
40: ;
41:
42: noun : DOG
43: { $$ = DOG; }
44: | CRONE
45: { if( $0 == YOUNG ){
46: printf( "what?\en" );
47: }
48: $$ = CRONE;
49: }
50: ;
51: . . .
52: .DE
53: In the action following the word CRONE, a check is made that the
54: preceding token shifted was not YOUNG.
55: Obviously, this is only possible when a great deal is known about
56: what might precede the symbol
57: .I noun
58: in the input.
59: There is also a distinctly unstructured flavor about this.
60: Nevertheless, at times this mechanism will save a great
61: deal of trouble, especially when a few combinations are to
62: be excluded from an otherwise regular structure.
63: .SH
64: Support for Arbitrary Value Types
65: .PP
66: By default, the values returned by actions and the lexical analyzer are integers.
67: Yacc can also support
68: values of other types, including structures.
69: In addition, Yacc keeps track of the types, and inserts
70: appropriate union member names so that the resulting parser will
71: be strictly type checked.
72: The Yacc value stack (see Section 4)
73: is declared to be a
74: .I union
75: of the various types of values desired.
76: The user declares the union, and associates union member names
77: to each token and nonterminal symbol having a value.
78: When the value is referenced through a $$ or $n construction,
79: Yacc will automatically insert the appropriate union name, so that
80: no unwanted conversions will take place.
81: In addition, type checking commands such as
82: .I Lint\|
83: .[
84: Johnson Lint Checker 1273
85: .]
86: will be far more silent.
87: .PP
88: There are three mechanisms used to provide for this typing.
89: First, there is a way of defining the union; this must be
90: done by the user since other programs, notably the lexical analyzer,
91: must know about the union member names.
92: Second, there is a way of associating a union member name with tokens
93: and nonterminals.
94: Finally, there is a mechanism for describing the type of those
95: few values where Yacc can not easily determine the type.
96: .PP
97: To declare the union, the user includes in the declaration section:
98: .DS
99: %union {
100: body of union ...
101: }
102: .DE
103: This declares the Yacc value stack,
104: and the external variables
105: .I yylval
106: and
107: .I yyval ,
108: to have type equal to this union.
109: If Yacc was invoked with the
110: .B \-d
111: option, the union declaration
112: is copied onto the
113: .I y.tab.h
114: file.
115: Alternatively,
116: the union may be declared in a header file, and a typedef
117: used to define the variable YYSTYPE to represent
118: this union.
119: Thus, the header file might also have said:
120: .DS
121: typedef union {
122: body of union ...
123: } YYSTYPE;
124: .DE
125: The header file must be included in the declarations
126: section, by use of %{ and %}.
127: .PP
128: Once YYSTYPE is defined,
129: the union member names must be associated
130: with the various terminal and nonterminal names.
131: The construction
132: .DS
133: < name >
134: .DE
135: is used to indicate a union member name.
136: If this follows
137: one of the
138: keywords %token,
139: %left, %right, and %nonassoc,
140: the union member name is associated with the tokens listed.
141: Thus, saying
142: .DS
143: %left <optype> \'+\' \'\-\'
144: .DE
145: will cause any reference to values returned by these two tokens to be
146: tagged with
147: the union member name
148: .I optype .
149: Another keyword, %type, is
150: used similarly to associate
151: union member names with nonterminals.
152: Thus, one might say
153: .DS
154: %type <nodetype> expr stat
155: .DE
156: .PP
157: There remain a couple of cases where these mechanisms are insufficient.
158: If there is an action within a rule, the value returned
159: by this action has no
160: .I "a priori"
161: type.
162: Similarly, reference to left context values (such as $0 \- see the
163: previous subsection ) leaves Yacc with no easy way of knowing the type.
164: In this case, a type can be imposed on the reference by inserting
165: a union member name, between < and >, immediately after
166: the first $.
167: An example of this usage is
168: .DS
169: rule : aaa { $<intval>$ = 3; } bbb
170: { fun( $<intval>2, $<other>0 ); }
171: ;
172: .DE
173: This syntax has little to recommend it, but the situation arises rarely.
174: .PP
175: A sample specification is given in Appendix C.
176: The facilities in this subsection are not triggered until they are used:
177: in particular, the use of %type will turn on these mechanisms.
178: When they are used, there is a fairly strict level of checking.
179: For example, use of $n or $$ to refer to something with no defined type
180: is diagnosed.
181: If these facilities are not triggered, the Yacc value stack is used to
182: hold
183: .I int' s,
184: as was true historically.
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