116 lines
2.8 KiB
Plaintext
116 lines
2.8 KiB
Plaintext
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.\" This module is believed to contain source code proprietary to AT&T.
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.\" Use and redistribution is subject to the Berkeley Software License
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.\" Agreement and your Software Agreement with AT&T (Western Electric).
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.\"
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.\" @(#)ssb 8.1 (Berkeley) 6/8/93
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.\"
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.\" $FreeBSD$
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.SH
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Appendix B: Yacc Input Syntax
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.PP
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This Appendix has a description of the Yacc input syntax, as a Yacc specification.
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Context dependencies, etc., are not considered.
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Ironically, the Yacc input specification language
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is most naturally specified as an LR(2) grammar; the sticky
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part comes when an identifier is seen in a rule, immediately
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following an action.
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If this identifier is followed by a colon, it is the start of the
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next rule; otherwise
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it is a continuation of the current rule, which just happens to have
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an action embedded in it.
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As implemented, the lexical analyzer looks
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ahead after seeing an identifier, and
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decide whether the next token (skipping blanks, newlines, comments, etc.)
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is a colon.
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If so, it returns the token C_IDENTIFIER.
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Otherwise, it returns IDENTIFIER.
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Literals (quoted strings) are also returned as IDENTIFIERS,
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but never as part of C_IDENTIFIERs.
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.sp
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.nf
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.ta .6i 1.2i 1.8i 2.4i 3i 3.6i
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/* grammar for the input to Yacc */
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/* basic entities */
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%token IDENTIFIER /* includes identifiers and literals */
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%token C_IDENTIFIER /* identifier (but not literal) followed by colon */
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%token NUMBER /* [0-9]+ */
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/* reserved words: %type => TYPE, %left => LEFT, etc. */
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%token LEFT RIGHT NONASSOC TOKEN PREC TYPE START UNION
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%token MARK /* the %% mark */
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%token LCURL /* the %{ mark */
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%token RCURL /* the %} mark */
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/* ascii character literals stand for themselves */
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%start spec
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%%
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spec : defs MARK rules tail
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;
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tail : MARK { \fIIn this action, eat up the rest of the file\fR }
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| /* empty: the second MARK is optional */
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;
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defs : /* empty */
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| defs def
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;
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def : START IDENTIFIER
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| UNION { \fICopy union definition to output\fR }
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| LCURL { \fICopy C code to output file\fR } RCURL
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| ndefs rword tag nlist
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;
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rword : TOKEN
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| LEFT
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| RIGHT
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| NONASSOC
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| TYPE
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;
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tag : /* empty: union tag is optional */
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| \'<\' IDENTIFIER \'>\'
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;
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nlist : nmno
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| nlist nmno
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| nlist \',\' nmno
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;
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nmno : IDENTIFIER /* NOTE: literal illegal with %type */
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| IDENTIFIER NUMBER /* NOTE: illegal with %type */
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;
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/* rules section */
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rules : C_IDENTIFIER rbody prec
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| rules rule
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;
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rule : C_IDENTIFIER rbody prec
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| '|' rbody prec
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;
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rbody : /* empty */
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| rbody IDENTIFIER
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| rbody act
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;
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act : \'{\' { \fICopy action, translate $$, etc.\fR } \'}\'
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;
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prec : /* empty */
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| PREC IDENTIFIER
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| PREC IDENTIFIER act
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| prec \';\'
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;
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.fi
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.bp
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