问题描述
我正在尝试使用Bison用玩具语言实现try-catch-finally表达式。
另一件事是,受Scala grammar的启发,try-catch-finally中的项目最终是一个表达式,而不是一个块语句。
这里是grammar.y
:
%code top {
#include <cstdio>
}
%union {
int n;
Ast *ast;
}
%code requires {
class Ast;
int yylex(void);
void yyerror(const char *msg);
}
%token<n> NUM
%token<n> PLUS '+'
%token<n> MINUS '-'
%token<n> TIMES '*'
%token<n> DIVIDE '/'
%token<n> SEMICOLON ';'
%token<n> NEWLINE '\n'
%token<n> IF "if"
%token<n> ELSE "else"
%token<n> TRY "try"
%token<n> CATCH "catch"
%token<n> FINALLY "finally"
%token<n> LPAREN '('
%token<n> RPAREN ')'
%type<ast> prog expr primaryExpr
/* grammar precedence */
%nonassoc "try_catch" /* lower than finally */
%nonassoc "try_catch_finally"
/* operator precedence is higher than grammar precedence (try-catch-finally) */
%left PLUS MINUS
%left TIMES DIVIDE
%start prog
%%
prog : expr
;
expr : "try" expr "catch" expr %prec "try_catch" { $$ = nullptr; }
| "try" expr "catch" expr "finally" expr %prec "try_catch_finally" { $$ = nullptr; }
| primaryExpr
;
primaryExpr : NUM { $$ = nullptr; }
| primaryExpr '+' NUM { $$ = nullptr; }
| primaryExpr '-' NUM { $$ = nullptr; }
| primaryExpr '*' NUM { $$ = nullptr; }
| primaryExpr '/' NUM { $$ = nullptr; }
;
%%
void yyerror(const char *msg) {
fprintf(stderr,"%s\n",msg);
}
生成文件:bison --debug --verbose -Wcounterexamples -o grammar.tab.cpp --defines=grammar.tab.h grammar.y
,我们有一个grammar.output
文件,具有移位/减少冲突:
Terminals unused in grammar
PLUS
MINUS
TIMES
DIVIDE
SEMICOLON
';'
NEWLINE
'\n'
"if"
"else"
LPAREN
'('
RPAREN
')'
State 17 conflicts: 1 shift/reduce
Grammar
0 $accept: prog $end
1 prog: expr
2 expr: "try" expr "catch" expr
3 | "try" expr "catch" expr "finally" expr
4 | primaryExpr
5 primaryExpr: NUM
6 | primaryExpr '+' NUM
7 | primaryExpr '-' NUM
8 | primaryExpr '*' NUM
9 | primaryExpr '/' NUM
Terminals,with rules where they appear
$end (0) 0
'\n' <n> (10)
'(' <n> (40)
')' <n> (41)
'*' <n> (42) 8
'+' <n> (43) 6
'-' <n> (45) 7
'/' <n> (47) 9
';' <n> (59)
error (256)
NUM <n> (258) 5 6 7 8 9
PLUS <n> (259)
MINUS <n> (260)
TIMES <n> (261)
DIVIDE <n> (262)
SEMICOLON <n> (263)
NEWLINE <n> (264)
"if" <n> (265)
"else" <n> (266)
"try" <n> (267) 2 3
"catch" <n> (268) 2 3
"finally" <n> (269) 3
LPAREN <n> (270)
RPAREN <n> (271)
"try_catch" (272)
"try_catch_finally" (273)
Nonterminals,with rules where they appear
$accept (27)
on left: 0
prog <ast> (28)
on left: 1
on right: 0
expr <ast> (29)
on left: 2 3 4
on right: 1 2 3
primaryExpr <ast> (30)
on left: 5 6 7 8 9
on right: 4 6 7 8 9
State 0
0 $accept: • prog $end
NUM shift,and go to state 1
"try" shift,and go to state 2
prog go to state 3
expr go to state 4
primaryExpr go to state 5
State 1
5 primaryExpr: NUM •
$default reduce using rule 5 (primaryExpr)
State 2
2 expr: "try" • expr "catch" expr
3 | "try" • expr "catch" expr "finally" expr
NUM shift,and go to state 2
expr go to state 6
primaryExpr go to state 5
State 3
0 $accept: prog • $end
$end shift,and go to state 7
State 4
1 prog: expr •
$default reduce using rule 1 (prog)
State 5
4 expr: primaryExpr •
6 primaryExpr: primaryExpr • '+' NUM
7 | primaryExpr • '-' NUM
8 | primaryExpr • '*' NUM
9 | primaryExpr • '/' NUM
'+' shift,and go to state 8
'-' shift,and go to state 9
'*' shift,and go to state 10
'/' shift,and go to state 11
$default reduce using rule 4 (expr)
State 6
2 expr: "try" expr • "catch" expr
3 | "try" expr • "catch" expr "finally" expr
"catch" shift,and go to state 12
State 7
0 $accept: prog $end •
$default accept
State 8
6 primaryExpr: primaryExpr '+' • NUM
NUM shift,and go to state 13
State 9
7 primaryExpr: primaryExpr '-' • NUM
NUM shift,and go to state 14
State 10
8 primaryExpr: primaryExpr '*' • NUM
NUM shift,and go to state 15
State 11
9 primaryExpr: primaryExpr '/' • NUM
NUM shift,and go to state 16
State 12
2 expr: "try" expr "catch" • expr
3 | "try" expr "catch" • expr "finally" expr
NUM shift,and go to state 2
expr go to state 17
primaryExpr go to state 5
State 13
6 primaryExpr: primaryExpr '+' NUM •
$default reduce using rule 6 (primaryExpr)
State 14
7 primaryExpr: primaryExpr '-' NUM •
$default reduce using rule 7 (primaryExpr)
State 15
8 primaryExpr: primaryExpr '*' NUM •
$default reduce using rule 8 (primaryExpr)
State 16
9 primaryExpr: primaryExpr '/' NUM •
$default reduce using rule 9 (primaryExpr)
State 17
2 expr: "try" expr "catch" expr •
3 | "try" expr "catch" expr • "finally" expr
"finally" shift,and go to state 18
"finally" [reduce using rule 2 (expr)]
$default reduce using rule 2 (expr)
shift/reduce conflict on token "finally":
2 expr: "try" expr "catch" expr •
3 expr: "try" expr "catch" expr • "finally" expr
Example: "try" expr "catch" "try" expr "catch" expr • "finally" expr
Shift derivation
expr
↳ "try" expr "catch" expr
↳ "try" expr "catch" expr • "finally" expr
Reduce derivation
expr
↳ "try" expr "catch" expr "finally" expr
↳ "try" expr "catch" expr •
State 18
3 expr: "try" expr "catch" expr "finally" • expr
NUM shift,and go to state 2
expr go to state 19
primaryExpr go to state 5
State 19
3 expr: "try" expr "catch" expr "finally" expr •
$default reduce using rule 3 (expr)
让我们关注冲突部分:
State 17
2 expr: "try" expr "catch" expr •
3 | "try" expr "catch" expr • "finally" expr
"finally" shift,and go to state 18
"finally" [reduce using rule 2 (expr)]
$default reduce using rule 2 (expr)
shift/reduce conflict on token "finally":
2 expr: "try" expr "catch" expr •
3 expr: "try" expr "catch" expr • "finally" expr
Example: "try" expr "catch" "try" expr "catch" expr • "finally" expr
Shift derivation
expr
↳ "try" expr "catch" expr
↳ "try" expr "catch" expr • "finally" expr
Reduce derivation
expr
↳ "try" expr "catch" expr "finally" expr
↳ "try" expr "catch" expr •
对于"try" expr "catch" "try" expr "catch" expr "finally" expr
,在默认的reduce中,"finally"
绑定到第一个"try"
而不是第二个"try"
。我认为与Java / Scala行为并不相同。
我尝试使用%prec
来调整优先级来解决它,但是失败了。
我应该如何解决这个问题?
解决方法
如注释中所示,finally
语句中的可选try – catch – finally
子句所产生的移位减少冲突与{{1}中的可选else
子句完全相同}}声明,即所谓的“悬而未决”。
由于“最终悬空”与“其他悬空”是相同的问题,因此我们可以预期解决方案是相同的。在解决方案中,最简单的方法是使用优先级声明,其中最简单的是
if – then – else
将这些标记(因此,最后一个终端是这些标记之一的生产)声明为%right "if" "else" "catch" "finally"
意味着,当涉及这些标记之一的冲突发生时, shift 操作应被选中。由于这是野牛的默认冲突解决方案(请参见注释2),因此优先级声明的唯一作用是抑制有关冲突的警告消息。
在编辑过的问题中,过度设计的解决方案也将起作用,尽管我提醒您不要不必要地使用%right
。 [注1]但是,添加评论还不够:
%nonassoc
您实际上还需要添加声明
%nonassoc "try_catch" /* lower than finally */
上面显示的优先级解决方案具有以下优点:它是独立的。它不仅不依赖于其他优先级声明,还不依赖于%right finally
声明,这也很容易被意外省略。
尽管它与解决问题的方式并不特别相关,但值得注意的是您误解了野牛的报告输出。野牛报告说,17号州的州过渡如下:
%prec
应如下所示:
-
当
"finally" shift,and go to state 18 "finally" [reduce using rule 2 (expr)] $default reduce using rule 2 (expr)
为超前标记时, 移动 超前标记并进入状态18。 -
针对超前令牌
"finally"
的操作也存在冲突: 减少 到"finally"
使用规则2。此操作已消除通过冲突解决算法[注2]。 (野牛将操作放在括号中(expr
表示该操作已通过解决冲突消除了。) -
对于所有其他先行标记(
[reduce using rule 2 (expr)]
),使用规则2将 减少 减少为$default
。
请注意,Bison不报告优先级声明消除的解析动作。那些被默默地丢弃了。
注释
-
如果要在不指定关联性的情况下声明优先级关系,请使用
expr
。与%precedence
不同,它不会默默地隐藏语法错误。 -
默认的冲突解决算法是:
- 如果有换档操作,请使用它。 (最多只能有一个转变。)
- 如果没有移位动作,请使用规则编号最小的减少动作;也就是说,在语法文件中排在第一位的那个。