使用 ANTLR4 解析公式 [英] Parse a formula using ANTLR4

问题描述

我正在尝试使用 ANTLR4 将数学公式解析为 LaTeX 的子集.例如，它应该将 (a+4)/(b*10) 解析为 \frac{a+4}{b\cdot 10}.

I am trying to parse a mathematical formula to a subset of LaTeX using ANTLR4. For example it should parse (a+4)/(b*10) to \frac{a+4}{b\cdot 10}.

我的简单语法创建了一棵树，如下所示:

My simple grammar creates a tree like this:

现在我正在尝试实现解析树侦听器，以在遍历树时以某种方式构造 LaTeX 字符串.在这里，我失败了，因为要构造一个像 \frac{}{} 这样的字符串，它必须以递归方式构建.然而，解析树遍历器会一个接一个地访问树节点(据我所知，以广度优先的方式).

Now I am trying to implement parse tree listeners to somehow construct the LaTeX String while the tree is traversed. Here, I am failing because to construct a String like \frac{}{} it has to be built recursively. The parse tree walker, however, visits one tree node after the other (in a breadth-first way as far as I can tell).

我读过有关解析树访问者的文章，这可能正是我所需要的.但我无法找到一些如何应用这些访问者的示例.

I've read about parse tree visitors that might be what I need. But I wasn't able to find some examples how these visitors are applied.

您能否举例说明在这种特殊情况下如何使用解析树侦听器/访问器?您认为我首先将 ANTLR 用于解析器的方法有意义吗?

Could you provide an example how parse tree listeners/visitors can be used in this particular case? Do you think my approach to use ANTLR for the parser makes sense in the first place?

推荐答案

您可以通过实现 ParseTreeVisitor 接口来创建解析树遍历器.为方便使用，可以在编译语法时指定 Antlr 生成基本访问者(在 Antlrworks 中，Run->Generate Recognizer->Next->Generate Visitor->Next->Finish).基本访问者将被称为 MyGrammarBaseVisitor.请注意，访问者有一个泛型类型 T，每个访问方法都应返回该类型.我建议使用 Void 进行手动操作，或者使用 String 以便在代码生成过程中使用.

You can create a parse tree walker by implementing the ParseTreeVisitor interface. For ease of use, you can specify Antlr to generate a base visitor when compiling the grammar (in Antlrworks, Run->Generate Recognizer->Next->Generate Visitor->Next->Finish). The base visitor will be called MyGrammarBaseVisitor. Note that the visitor has a generic type T, which every single visit method should return. I recommend using Void for manual manipulation or String for ease of use during code generation.

在扩展基本访问者之后(我假设我们在这里处理String)，您需要覆盖访问方法.这些方法以您拥有的语法规则命名.这些方法中的每一个都将接收一个 ParserContext ctx 参数，您可以使用它来访问子规则和/或获取终端值.例如，您可以这样做:

After you extend the base visitor (I'll assume here we're dealing with String), you need to override the visit methods. These methods are named after the grammar rules you have. Each of these methods will receive a ParserContext ctx parameter, which you use to visit child rules and/or get terminal values. For example, you could do this:

class MyVisitor extends MyGrammarBaseVisitor<String> {
    @Override
    public String visitMultiplicative(MyGrammarParser.MultiplicativeContext ctx) {
        if (ctx.opMult().getText().equals("/")) return "\\frac{" + visit(ctx.expr(0)) + "}{" + visit(ctx.expr(1)) + "}";
        else return visit(ctx.expr(0)) + "\\cdot " + visit(ctx.expr(1));
    }
    // visit methods for other rules...
}

我假设你的 multiplicative 规则看起来像 multiplicative: expr opMult expr;opMult: '*' |'/'; 您可以在 The Definitive Antlr 中找到更多信息4 参考.您还可以在 Antlr 文档 中找到更多信息和示例.

I'm assuming your multiplicative rule looks like multiplicative: expr opMult expr; opMult: '*' | '/'; You can find more information in The Definitive Antlr 4 Reference. You may also find more information and examples in the Antlr documentation.

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