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cex: update NEWS for 3.7

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* NEWS: Update to the current style of cex display.
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Akim Demaille
2020-07-21 07:41:06 +02:00
parent 7d5474e979
commit 431774d1f6
2 changed files with 116 additions and 71 deletions
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NEWS
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@@ -2,39 +2,6 @@ GNU Bison NEWS
* Noteworthy changes in release ?.? (????-??-??) [?]
* Noteworthy changes in release 3.6.93 (2020-07-20) [beta]
** Bug fixes
Portability issues.
* Noteworthy changes in release 3.6.92 (2020-07-19) [beta]
Changes in the display of counterexamples.
** Documentation
*** Examples
The bistromathic demonstrates %param and how to quote sources in the error
messages:
> 123 456
1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
1 | 123 456
| ^~~
* Noteworthy changes in release 3.6.91 (2020-07-09) [beta]
** Bug fixes
Portability issues.
* Noteworthy changes in release 3.6.90 (2020-07-04) [beta]
** Deprecated features
The YYPRINT macro, which works only with yacc.c and only for tokens, was
@@ -51,52 +18,114 @@ Changes in the display of counterexamples.
Contributed by Vincent Imbimbo.
When given `--report=counterexamples` or `-Wcounterexamples`, bison will
now output counterexamples for conflicts in the grammar. These are
strings in the grammar which can be parsed in two ways due to the
conflict. For example:
When given `-Wcounterexamples`/`-Wcex`, bison will now output
counterexamples for conflicts in the grammar.
Shift/reduce conflict on token "/":
Example exp "+" exp • "/" exp
**** Unifying Counterexamples
Unifying counterexamples are strings which can be parsed in two ways due
to the conflict. For example on a grammar that contains the usual
"dangling else" ambiguity:
$ bison else.y
else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
else.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
$ bison else.y -Wcex
else.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
else.y: warning: shift/reduce conflict on token "else" [-Wcounterexamples]
Example: "if" exp "then" "if" exp "then" exp • "else" exp
Shift derivation
exp
↳ exp "+" exp
↳ exp • "/" exp
Example exp "+" exp • "/" exp
↳ "if" exp "then" exp
↳ "if" exp "then" exp • "else" exp
Example: "if" exp "then" "if" exp "then" exp • "else" exp
Reduce derivation
exp
↳ exp "/" exp
↳ exp "+" exp •
↳ "if" exp "then" exp "else" exp
↳ "if" exp "then" exp •
When Bison is installed with text styling enabled, the example is actually
shown twice, with colors highlighting the ambiguity.
When text styling is enabled, colors are used in the examples and the
derivations to highlight the structure of both analyses. In this case,
This is a shift/reduce conflict caused by none of the operators having
precedence, so the example can be parsed in the two ways shown. When
bison cannot find an example that can be derived in two ways, it instead
generates two examples that are the same up until the dot:
"if" exp "then" [ "if" exp "then" exp • ] "else" exp
First example expr • ID ',' ID $end
Shift derivation
$accept
↳ s $end
↳ a ID
↳ expr
↳ expr • ID ','
Second example expr • ID $end
Reduce derivation
$accept
↳ s $end
↳ a ID
↳ expr •
vs.
"if" exp "then" [ "if" exp "then" exp • "else" exp ]
The counterexamples are "focused", in two different ways. First, they do
not clutter the output with all the derivations from the start symbol,
rather they start on the "conflicted nonterminal". They go straight to the
point. Second, they don't "expand" nonterminal symbols uselessly.
**** Nonunifying Counterexamples
In the case of the dangling else, Bison found an example that can be
parsed in two ways (therefore proving that the grammar is ambiguous).
When it cannot find such an example, it instead generates two examples
that are the same up until the dot:
$ bison foo.y
foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
foo.y: note: rerun with option '-Wcounterexamples' to generate conflict counterexamples
foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
4 | a: expr
| ^~~~
$ bison -Wcex foo.y
foo.y: warning: 1 shift/reduce conflict [-Wconflicts-sr]
foo.y: warning: shift/reduce conflict on token ID [-Wcounterexamples]
First example: expr • ID ',' ID $end
Shift derivation
$accept
↳ s $end
↳ a ID
↳ expr
↳ expr • ID ','
Second example: expr • ID $end
Reduce derivation
$accept
↳ s $end
↳ a ID
↳ expr •
foo.y:4.4-7: warning: rule useless in parser due to conflicts [-Wother]
4 | a: expr
| ^~~~
In these cases, the parser usually doesn't have enough lookahead to
differentiate the two given examples.
The counterexamples are "focused" in two different ways. First, they do
not clutter the output with all the derivations from the start symbol,
rather they start on the "conflicted nonterminal". They go straight to the
point. Second, they don't "expand" nonterminal symbols uselessly.
**** Reports
Counterexamples are also included in the report when given
`--report=counterexamples`/`-rcex` (or `--report=all`), with more
technical details:
State 7
1 exp: "if" exp "then" exp • [$end, "then", "else"]
2 | "if" exp "then" exp • "else" exp
"else" shift, and go to state 8
"else" [reduce using rule 1 (exp)]
$default reduce using rule 1 (exp)
shift/reduce conflict on token "else":
1 exp: "if" exp "then" exp •
2 exp: "if" exp "then" exp • "else" exp
Example: "if" exp "then" "if" exp "then" exp • "else" exp
Shift derivation
exp
↳ "if" exp "then" exp
↳ "if" exp "then" exp • "else" exp
Example: "if" exp "then" "if" exp "then" exp • "else" exp
Reduce derivation
exp
↳ "if" exp "then" exp "else" exp
↳ "if" exp "then" exp •
*** File prefix mapping
@@ -110,6 +139,11 @@ Changes in the display of counterexamples.
** Changes
*** Diagnostics
When text styling is enabled and the terminal supports it, the warnings
now include hyperlinks to the documentation.
*** Relocatable installation
When installed to be relocatable (via `configure --enable-relocatable`),
@@ -150,6 +184,18 @@ Changes in the display of counterexamples.
Now the parser state can be examined when parsing is finished. The parser
state is reset when starting a new parse.
** Documentation
*** Examples
The bistromathic demonstrates %param and how to quote sources in the error
messages:
> 123 456
1.5-7: syntax error: expected end of file or + or - or * or / or ^ before number
1 | 123 456
| ^~~
** Bug fixes
*** Include the generated header (yacc.c)
@@ -4415,7 +4461,7 @@ LocalWords: yysymbol yytnamerr yyreport ctx ARGMAX yysyntax stderr LPAREN
LocalWords: symrec yypcontext TOKENMAX yyexpected YYEMPTY yypstate YYEOF
LocalWords: autocompletion bistromathic submessages Cayuela lexcalc hoc
LocalWords: yytoken YYUNDEF YYerror basename Automake's UTF ifdef ffile
LocalWords: gotos readline Imbimbo Wcounterexamples
LocalWords: gotos readline Imbimbo Wcounterexamples Wcex Nonunifying rcex
Local Variables:
ispell-dictionary: "american"