ML-Yacc Tiger Parser减少/减少错误

Question

我正在经历在Appel的“ML中的现代编译器实现”一书中生成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算子的规则引起的减少 - 减少冲突。这里是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 

我已经定义与UNARY低于零下更高的优先级，并且在使用%prec我的规则明确设置它。当然，当我删除任一规则时，冲突消失，但语法会错误地解析MINUS标志。

我无法诊断此错误 - 任何想法？

Answer 1

疯狂的猜测：你的规则之一可能允许exp为空吗？如果是这样，那么这会在exp是可选的任何地方产生歧义 - 例如在- exp之前。

Answer 2

作为接受的答案（他/她是对的）的后续 - 在序列的生产中出现了一个错误，允许exp转到epsilon。

这里是问题的代码位（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*) 

这里是校正后的代码：

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*)