ML-Yacc Tiger 解析器 Reduce/Reduce 错误

Question

我正在完成 Appel "Modern Compiler Implementation in ML" 书中生成 Tiger Parser 的 Ch3 编程练习。我的 tiger.grm 文件是 here。我试图诊断的错误是由一元和二元减号运算符的规则引起的减少-减少冲突。这是 yacc 错误：

error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on OR
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on AND
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on GE
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on GT
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on LE
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on LT
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on NEQ
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on EQ
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on DIVIDE
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on TIMES
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on MINUS
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on PLUS
error:  state 128: reduce/reduce conflict between rule 48 and rule 46 on RPAREN

state 128:

    boolean : exp . AND exp 
    boolean : exp . OR exp 
    arithmetic : MINUS exp .  (reduce by rule 46)
    arithmetic : exp . PLUS exp 
    arithmetic : exp . MINUS exp 
    arithmetic : exp MINUS exp .  (reduce by rule 48)
    arithmetic : exp . DIVIDE exp 
    arithmetic : exp . TIMES exp 
    comparison : exp . EQ exp 
    comparison : exp . NEQ exp 
    comparison : exp . GT exp 
    comparison : exp . LT exp 
    comparison : exp . LE exp 
    comparison : exp . GE exp

我定义的 UNARY 优先级高于 MINUS，并使用 %prec 在我的规则中明确设置它。当然，当我删除任何一条规则时，冲突都会消失，但语法会错误地解析减号。

我无法诊断出这个错误 - 有什么想法吗？

Answer 1

大胆猜测：是否有可能您的规则之一允许 exp 为空？如果是这样，那么在 exp 是可选的任何地方都会产生歧义——比如在 - exp.

之前

Answer 2

作为已接受答案的后续行动（he/she 是正确的）- 在制作允许 exp 进入 epsilon 的序列时出现错误。

这是有问题的代码（见最后一行）：

sequence : LPAREN exp_sequence RPAREN ()
exp_sequence : (*epsilon*) ()
         | exp seq     ()

seq : (*epsilon*)                () (*an exp sequence can be empty*)
    | SEMICOLON exp exp_sequence () (*exps separated by semicolon*)

这是更正后的代码：

sequence : LPAREN exp_sequence RPAREN ()
exp_sequence : (*epsilon*) ()
             | exp seq     ()

seq : (*epsilon*)                () (*an exp sequence can be empty*)
    | SEMICOLON exp seq () (*exps separated by semicolon*)

ML-Yacc Tiger 解析器 Reduce/Reduce 错误

ML-Yacc Tiger Parser Reduce/Reduce error

compiler-construction

parsing

sml

tiger

ml-yacc