插入符号前缀而不是antlr中的后缀 [英] caret prefix instead of postfix in antlr

问题描述

我知道插入符号后缀在antlr中的含义(即，使root生效)，但是插入符号何时是前缀，如我一直在阅读的以下语法中那样(该语法是全新的，并且由学习antlr的新团队完成)....

I know what the caret postfix means in antlr(ie. make root) but what about when the caret is the prefix as in the following grammar I have been reading(this grammar is brand new and done by a new team learning antlr)....

selectClause
    : SELECT resultList -> ^(SELECT_CLAUSE resultList) 
    ;


fromClause
    : FROM tableList -> ^(FROM_CLAUSE tableList) 
    ;

另外，我知道=>的意思，但是->呢? ->代表什么?

Also, I know what => means but what about the -> ? What does -> imply?

谢谢， 院长

推荐答案

^用作内联树运算符，指示某个令牌应成为树的根.

The ^ is used as an inline tree operator, indicating a certain token should become the root of the tree.

例如，规则:

p : A B^ C;

创建以下AST:

  B
 / \
A   C

还有一种使用重写规则创建AST的方法. 重写规则放置在解析器规则的替代项之后(或右侧).您用开头的重写规则，后跟要包含在AST中的规则/令牌.

There's another way to create an AST which is using a rewrite rule. A rewrite rule is placed after (or at the right of) an alternative of a parser rule. You start a rewrite rule with an "arrow", ->, followed by the rules/tokens you want to be in the AST.

遵循上一条规则:

p : A B C;

，并且您想要反转令牌，但保持ASST为扁平"状态(无根节点).可以使用以下重写规则

and you want to reverse the tokens, but keep the ASST "flat" (no root node). THis can be done using the following rewrite rule:

p : A B C -> C B A;

如果要创建类似于p : A B^ C;的AST，请使用^( ... )开始重写规则，其中括号内的第一个标记/规则将成为根节点.所以规则:

And if you want to create an AST similar to p : A B^ C;, you start your rewrite rule with ^( ... ) where the first token/rule inside the parenthesis will become the root node. So the rule:

p : A B C -> ^(B A C);

产生与p : A B^ C;相同的AST.

• 树结构
• 如何输出使用ANTLR构建的AST?

• Tree construction
• How to output the AST built using ANTLR?

这篇关于插入符号前缀而不是antlr中的后缀的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！