如何在Antlr中使用访问者创建自定义AST

Question

I want to use the visitor pattern to walk the ParserRuleContext and build an AST using predefined nodes.

In my grammar I have the following rule:

expr
    : loop
    | block

I would like to produce an AST that has the following structure:

    exprNode
      / 
loopNode

The BaseVisitor generated by Antlr has the following method in it:

public T visitExpr(MyParser.ExprContext ctx) { return visitChildren(ctx); }

The ASTBuilder overrides this method and creates a exprNode . To this exprNode I want to append either a loopNode or a blockNode by calling my overrode versions of visitLoop and visitBlock . The problem I have is that I don't know the identity of the expr , as I only obtained exprContext from the parent. How do I check which invocation of the expr rule was used?

Answer 1

Don't override visitExpr . Just override visitLoop and visitBlock .

The auto-generated visitExpr will call the appropriate one of those two (via visitChildren ) and return whatever visitLoop or visitBlock returned. So as long as you override those methods to return the proper result, visitExpr will also return the proper result automatically.

Answer 2

Labeling the rule alts provides convenience contexts for determining identity:

expr
    : loop   #loopExpr
    | block  #blockExpr
    ;

The generated parser will now contain LoopExprContext extends ExprContext and BlockExprContext extends ExprContext classes with appropriately behaved visitor enter and exit methods.