ANTLR4-使用Visitor将语法树转换为AST

Question

我的任务是实现一个访问者，该访问者将把以下计算算术表达式的语法转换为AST:

grammar SmallC;

program :   exp
;
exp :   lexp ((op = '>' | op = '<' |op = '>=' | op = '<=' | op = '!=' | op = '==') lexp)?
;
lexp :  term ((op = '+'|op = '-') term)*
;
term:   factor((op = '%'| op = '*'| op = '/') factor)*
;
factor  :   Number
;
Number  : [0-9]+
;    

目前，ANTLR为表达式"3*6"创建了以下树：

程序->exp->lexp->term->（因子*因子）->Number->（3*6）

我想要的是这样的东西：

程序->乘法（3,6）

我试图通过实现一个访问者函数来开始这项工作，该函数将遍历树并输出一些数据结构，但失败得很惨。以下是我的课程：

Main.Java

import org.antlr.v4.runtime.*;
public class Main {
public static void main(String[] args) {
ANTLRInputStream input = new ANTLRInputStream("3*6");
SmallCLexer lexer = new SmallCLexer(null);
lexer.setInputStream(input);
CommonTokenStream tokens = new CommonTokenStream(lexer);
tokens.fill();
SmallCParser parser = new SmallCParser(null);
parser.setBuildParseTree(true);
parser.setTokenStream(tokens);
ParserRuleContext tree = parser.program();
MyVisitor visitor = new MyVisitor();
visitor.visit(tree);
}
}

MyVisitor.java

import org.antlr.v4.runtime.*;

public class MyVisitor extends SmallCBaseVisitor<SmallCNode> {

@Override public SmallCNode visitExp(SmallCParser.ExpContext ctx)  {
    if ( ctx.lexp().size() == 2) {
        SmallCLexp lhs = (SmallCLexp) ctx.lexp(0).accept(this);
        SmallCLexp rhs = (SmallCLexp) ctx.lexp(1).accept(this);
        String op = ctx.op.getText();
        return new SmallCExp(lhs,rhs,op);
    } else {
        visitLexp(ctx.lexp(0));
    }

}
@Override public SmallCNode visitLexp(SmallCParser.LexpContext ctx) {
    if ( ctx.term().size() == 2) {
        SmallCTerm lhs = (SmallCTerm) ctx.term(0).accept(this);
        SmallCTerm rhs = (SmallCTerm) ctx.term(1).accept(this);
        String op = ctx.op.getText();
        return new SmallCLexp(lhs,rhs,op);
    } else {
        visitTerm(ctx.term(0));
    }

}
@Override public SmallCNode visitTerm(SmallCParser.TermContext ctx) {
    if (ctx.factor().size() == 2) {
        SmallCFactor lhs = (SmallCFactor) ctx.factor(0).accept(this);
        SmallCFactor rhs = (SmallCFactor) ctx.factor(1).accept(this);
        String op = ctx.op.getText();
        return new SmallCTerm(lhs,rhs,op);
    } else {
        visitFactor(ctx.factor(0));
    }

}
@Override public SmallCNode visitFactor(SmallCParser.FactorContext ctx) {
        String fc = ctx.getText();
        return new SmallCFactor(fc);
}

下面是我的节点类：

public class SmallCNode {
}

public class SmallCExp extends SmallCNode{
    SmallCLexp lhs;
    SmallCLexp rhs;
    String op;
    public SmallCExp(SmallCLexp lhs, SmallCLexp rhs, String op) {
        super();
        this.lhs = lhs;
        this.rhs = rhs;
        this.op = op;
    }
}
public class SmallCLexp extends SmallCNode {
    SmallCTerm lhs;
    SmallCTerm rhs;
    String op;
    public SmallCLexp(SmallCTerm lhs, SmallCTerm rhs, String op) {
        super();
        this.lhs = lhs;
        this.rhs = rhs;
        this.op = op;
    }
}
public class SmallCTerm extends SmallCNode{
    SmallCFactor lhs;
    SmallCFactor rhs;
    String op;
    public SmallCTerm(SmallCFactor lhs, SmallCFactor rhs, String op) {
        super();
        this.lhs = lhs;
        this.rhs = rhs;
        this.op = op;
}
}
public class SmallCFactor extends SmallCNode {
    String factor;
    public SmallCFactor(String factor) {
        super();
        this.factor = factor;
}

}

当我运行Main.java时，我会收到以下错误：

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    This method must return a result of type SmallCNode
    at MyVisitor.visitExp(MyVisitor.java:8)
    at MyVisitor.visitExp(MyVisitor.java:1)
    at SmallCParser$ExpContext.accept(SmallCParser.java:151)
    at org.antlr.v4.runtime.tree.AbstractParseTreeVisitor.visitChildren(AbstractParseTreeVisitor.java:70)
    at SmallCBaseVisitor.visitProgram(SmallCBaseVisitor.java:20)
    at SmallCParser$ProgramContext.accept(SmallCParser.java:103)
    at org.antlr.v4.runtime.tree.AbstractParseTreeVisitor.visit(AbstractParseTreeVisitor.java:42)
    at Main.main(Main.java:20)

现在，我对这件事还很陌生，我意识到我可能错过了很多。我不知道如何从这里继续，如果我能得到一份我缺少的东西的清单，我将不胜感激：）

Answer 1

代码中的许多else块不返回SmallCNode，它们必须返回。

不应该是：

@Override public SmallCNode visitTerm(SmallCParser.TermContext ctx) {
if (ctx.factor().size() == 2) {
    SmallCFactor lhs = (SmallCFactor) ctx.factor(0).accept(this);
    SmallCFactor rhs = (SmallCFactor) ctx.factor(1).accept(this);
    String op = ctx.op.getText();
    return new SmallCTerm(lhs,rhs,op);
} else {
    visitFactor(ctx.factor(0));
}

但是：

@Override public SmallCNode visitTerm(SmallCParser.TermContext ctx) {
if (ctx.factor().size() == 2) {
    SmallCFactor lhs = (SmallCFactor) ctx.factor(0).accept(this);
    SmallCFactor rhs = (SmallCFactor) ctx.factor(1).accept(this);
    String op = ctx.op.getText();
    return new SmallCTerm(lhs,rhs,op);
} else {
    return visitFactor(ctx.factor(0));
}