ANTLR4 AST Creation - How to create an AstVistor

问题

With the help of this SO question How to create AST with ANTLR4? I was able to create the AST Nodes, but I'm stuck at coding the BuildAstVisitor as depicted in the accepted answer's example.

I have a grammar that starts like this:

mini: (constDecl | varDef | funcDecl | funcDef)* ;

And I can neither assign a label to the block (antlr4 says label X assigned to a block which is not a set), and I have no idea how to visit the next node.

public Expr visitMini(MiniCppParser.MiniContext ctx) {
    return visitConstDecl(ctx.constDecl());
}

I have the following problems with the code above: I don't know how to decide whether it's a constDecl, varDef or any other option and ctx.constDecl() returns a List<ConstDeclContext> whereas I only need one element for the visitConstDecl function.

edit:

More grammar rules:

mini: (constDecl | varDef | funcDecl | funcDef)* ;

//--------------------------------------------------

constDecl:   'const' type ident=ID init ';' ;
init:        '=' ( value=BOOLEAN | sign=('+' | '-')? value=NUMBER ) ;
// ...

//--------------------------------------------------

OP_ADD: '+';
OP_SUB: '-';
OP_MUL: '*';
OP_DIV: '/';
OP_MOD: '%';

BOOLEAN        : 'true' | 'false' ;
NUMBER         : '-'? INT ;
fragment INT   : '0' | [1-9] [0-9]* ;
ID             : [a-zA-Z]+ ;
// ...

I'm still not entirely sure on how to implement the BuildAstVisitor. I now have something along the lines of the following, but it certainly doesn't look right to me...

@Override
public Expr visitMini(MiniCppParser.MiniContext ctx) {
  for (MiniCppParser.ConstDeclContext constDeclCtx : ctx.constDecl()) {
    visit(constDeclCtx);
  }
  return null;
}

@Override
public Expr visitConstDecl(MiniCppParser.ConstDeclContext ctx) {
  visit(ctx.type());
  return visit(ctx.init());
}

回答1:

If you want to get the individual subrules then implement the visitXXX functions for them (visitConstDecl(), visitVarDef() etc.) instead of the visitMini() function. They will only be called if there's really a match for them in the input. Hence you don't need to do any checks for occurences.

来源：https://stackoverflow.com/questions/44003999/antlr4-ast-creation-how-to-create-an-astvistor

标签

java

antlr4

abstract-syntax-tree