编译器构造-在Java中使用cup的Parser.cup

Question

我试图用Java做一个编译器，我使用CUP为语法生成语法。

我有这个Parser.cup

/* Terminals (tokens returned by the scanner). */
terminal PLUS, MINUS;
terminal TIMES, DIV, DIVINT;
terminal LPAREN, RPAREN;
terminal EXP, MOD;
terminal String NUMERIC;
terminal SEMICOLON;
terminal CLOSE_BLOCK;
terminal OPEN_BLOCK;
/* Non terminals */
non terminal expr_list;
non terminal Expression expr;
non terminal Statement statement;
non terminal ListUtil<Statement> stmList;
non terminal ExpressionStatement statementPart;
non terminal BlockStatement block;
/* Precedences */
precedence left OPEN_BLOCK, CLOSE_BLOCK;
precedence left PLUS, MINUS;
precedence left TIMES, DIV, DIVINT, MOD;
precedence left EXP;
start with expr_list;
/* The grammar */
statement ::=
     block:b
    {: RESULT = b; :}
    | statementPart:s
    {: RESULT = s; :}
;
block ::=
    OPEN_BLOCK stmList:s CLOSE_BLOCK
    {: RESULT = new BlockStatement(s); :}
;   
stmList ::= 
    statementPart:s
    {:RESULT = new ListUtil<Statement>(s);:}
    | stmList:stml statementPart:s
    {: RESULT = stml.append(s); :}
| 
    {: RESULT = new ListUtil<Statement>(); :}
;
statementPart ::= expr:e 
    {: 
         RESULT = new ExpressionStatement(e);
    :} 
          SEMICOLON              
;
expr  ::= NUMERIC:n
  {:
    RESULT = new ResultExpression(n);
  :}
| expr:l PLUS expr:r
  {:
    RESULT = new PlusExpression(l, r);
  :}
| expr:l MINUS expr:r
  {:
    RESULT = new MinusExpression(l, r);
  :}
| expr:l TIMES expr:r
  {:
    RESULT = new TimesExpression(l, r);
  :}
| expr:l DIV expr:r
  {:
    RESULT = new DivExpression(l, r);
  :}
| expr:l DIVINT expr:r
  {:
    RESULT = new DivintExpression(l, r);
  :}
| expr:l EXP expr:r
  {:
    RESULT = new ExpExpression(l, r);
  :}
| expr:l MOD expr:r
  {:
    RESULT = new ModExpression(l, r);
  :}
| LPAREN expr:e RPAREN
  {:
    RESULT = e;
  :}
;

当我尝试生成Parser.java时，Eclipse返回以下警告：

  [cup] Warning : *** Production "expr ::= LPAREN expr RPAREN " never reduced
  [cup] Warning : *** Production "expr ::= expr MOD expr " never reduced
  [cup] Warning : *** Production "expr ::= expr EXP expr " never reduced
  [cup] Warning : *** Production "expr ::= expr DIVINT expr " never reduced
  [cup] Warning : *** Production "expr ::= expr DIV expr " never reduced
  [cup] Warning : *** Production "expr ::= expr TIMES expr " never reduced
  [cup] Warning : *** Production "expr ::= expr MINUS expr " never reduced
  [cup] Warning : *** Production "expr ::= expr PLUS expr " never reduced
  [cup] Warning : *** Production "expr ::= NUMERIC " never reduced
  [cup] Warning : *** Production "statementPart ::= expr NT$0 SEMICOLON " never reduced
  [cup] Warning : *** Production "NT$0 ::= " never reduced
  [cup] Warning : *** Production "stmList ::= " never reduced
  [cup] Warning : *** Production "stmList ::= stmList statementPart " never reduced
  [cup] Warning : *** Production "stmList ::= statementPart " never reduced
  [cup] Warning : *** Production "block ::= OPEN_BLOCK stmList CLOSE_BLOCK " never reduced
  [cup] Warning : *** Production "statement ::= statementPart " never reduced
  [cup] Warning : *** Production "statement ::= block " never reduced

我想删除此警告，有帮助吗？

Answer 1

您必须更改行的开头

像这样的东西可以工作：

以语句开头；