X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/cd48d21d944380129d7a2ce68617ea31bf939aab..95021767a1e93fa7b83f8a6c5ec5fd5f8064dd2c:/NEWS diff --git a/NEWS b/NEWS index 0b6b2341..119521cc 100644 --- a/NEWS +++ b/NEWS @@ -1,11 +1,178 @@ Bison News ---------- -Changes in version 2.3+: +Changes in version 2.3a+ (????-??-??): + +* The -g and --graph options now output graphs in Graphviz DOT format, + not VCG format. + +* An experimental directive %language specifies the language of the + generated parser, which can be C (the default) or C++. This + directive affects the skeleton used, and the names of the generated + files if the grammar file's name ends in ".y". + +* The grammar file may now specify the name of the parser header file using + %defines. For example: + + %defines "parser.h" + +* The `=' that used to be required in the following declarations is now + deprecated: + + %file-prefix "parser" + %name-prefix "c_" + %output "parser.c" + +* Revised warning: unset or unused mid-rule values + + Since Bison 2.2, Bison has warned about mid-rule values that are set but not + used within any of the actions of the parent rule. For example, Bison warns + about unused $2 in: + + exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; }; + + Now, Bison also warns about mid-rule values that are used but not set. For + example, Bison warns about unset $$ in the mid-rule action in: + + exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; }; + + However, Bison now disables both of these warnings by default since they + sometimes prove to be false alarms in existing grammars employing the Yacc + constructs $0 or $-N (where N is some positive integer). + + To enable these warnings, specify the flag `--warnings=midrule-values' or + `-W', which is a synonym for `--warnings=all'. + +* Bison now recognizes two separate kinds of default %destructor's and + %printer's: + + 1. Place `<*>' in a %destructor/%printer symbol list to define a default + %destructor/%printer for all grammar symbols for which you have formally + declared semantic type tags. + + 2. Place `<>' in a %destructor/%printer symbol list to define a default + %destructor/%printer for all grammar symbols without declared semantic + type tags. + + Bison no longer supports the `%symbol-default' notation from Bison 2.3a. + `<*>' and `<>' combined achieve the same effect with one exception: Bison no + longer applies any %destructor to a mid-rule value if that mid-rule value is + not actually ever referenced using either $$ or $n in a semantic action. + + The default %destructor's and %printer's are experimental. More user + feedback will help to determine whether they should become permanent + features. + + See the section `Freeing Discarded Symbols' in the Bison manual for further + details. + +* The Yacc prologue alternatives from Bison 2.3a have been rewritten as the + following directives: + + 1. %code {CODE} + + Other than semantic actions, this is probably the most common place you + should write verbatim code for the parser implementation. It replaces + the traditional Yacc prologue, `%{CODE%}', for most purposes. Compare + with: + + - `%{CODE%}' appearing after the first `%union {CODE}' in a grammar + file. While Bison will continue to support `%{CODE%}' for backward + compatibility, `%code {CODE}' is cleaner as its functionality does + not depend on its position in the grammar file relative to any + `%union {CODE}'. Specifically, `%code {CODE}' always inserts your + CODE into the parser code file after the usual contents of the + parser header file. + - `%after-header {CODE}', which only Bison 2.3a supported. + + 2. %requires {CODE} + + This is the right place to write dependency code for externally exposed + definitions required by Bison. Such exposed definitions are those + usually appearing in the parser header file. Thus, this is the right + place to define types referenced in `%union {CODE}' directives, and it + is the right place to override Bison's default YYSTYPE and YYLTYPE + definitions. Compare with: + + - `%{CODE%}' appearing before the first `%union {CODE}' in a grammar + file. Unlike `%{CODE%}', `%requires {CODE}' inserts your CODE both + into the parser code file and into the parser header file since + Bison's required definitions should depend on it in both places. + - `%start-header {CODE}', which only Bison 2.3a supported. + + 3. %provides {CODE} + + This is the right place to write additional definitions you would like + Bison to expose externally. That is, this directive inserts your CODE + both into the parser header file and into the parser code file after + Bison's required definitions. Compare with: + + - `%end-header {CODE}', which only Bison 2.3a supported. + + 4. %code-top {CODE} + + Occasionally it is desirable to insert code near the top of the parser + code file. For example: + + %code-top { + #define _GNU_SOURCE + #include + } + + Compare with: + + - `%{CODE%}' appearing before the first `%union {CODE}' in a grammar + file. `%code-top {CODE}' is cleaner as its functionality does not + depend on its position in the grammar file relative to any + `%union {CODE}'. + - `%before-header {CODE}', which only Bison 2.3a supported. + + If you have multiple occurrences of any one of the above four directives, + Bison will concatenate the contents in the order they appear in the grammar + file. + + The prologue alternatives are experimental. More user feedback will help to + determine whether they should become permanent features. + + Also see the new section `Prologue Alternatives' in the Bison manual. + +Changes in version 2.3a, 2006-09-13: + +* Instead of %union, you can define and use your own union type + YYSTYPE if your grammar contains at least one tag. + Your YYSTYPE need not be a macro; it can be a typedef. + This change is for compatibility with other Yacc implementations, + and is required by POSIX. * Locations columns and lines start at 1. In accordance with the GNU Coding Standards and Emacs. +* You may now declare per-type and default %destructor's and %printer's: + + For example: + + %union { char *string; } + %token STRING1 + %token STRING2 + %type string1 + %type string2 + %union { char character; } + %token CHR + %type chr + %destructor { free ($$); } %symbol-default + %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1 + %destructor { } + + guarantees that, when the parser discards any user-defined symbol that has a + semantic type tag other than `', it passes its semantic value to + `free'. However, when the parser discards a `STRING1' or a `string1', it + also prints its line number to `stdout'. It performs only the second + `%destructor' in this case, so it invokes `free' only once. + + [Although we failed to mention this here in the 2.3a release, the default + %destructor's and %printer's were experimental, and they were rewritten in + future versions.] + * Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with `-y', `--yacc', or `%yacc'), Bison no longer generates #define statements for associating token numbers with token names. Removing the #define statements @@ -19,7 +186,7 @@ Changes in version 2.3+: `%{ ... %}' syntax. To generate the pre-prologue, Bison concatenates all prologue blocks that you've declared before the first %union. To generate the post-prologue, Bison concatenates all prologue blocks that you've - declared after the first %union. + declared after the first %union. Previous releases of Bison inserted the pre-prologue into both the header file and the code file in all cases except for LALR(1) parsers in C. In the @@ -76,6 +243,9 @@ Changes in version 2.3+: If you have multiple occurrences of any one of the above declarations, Bison will concatenate the contents in declaration order. + [Although we failed to mention this here in the 2.3a release, the prologue + alternatives were experimental, and they were rewritten in future versions.] + * The option `--report=look-ahead' has been changed to `--report=lookahead'. The old spelling still works, but is not documented and may be removed in a future release.