How do I pretty-print productions and line numbers, using ANTLR4? - pretty-print

Question

I'm trying to write a piece of code that will take an ANTLR4 parser and use it to generate ASTs for inputs similar to the ones given by the -tree option on grun (misc.TestRig). However, I'd additionally like for the output to include all the line number/offset information.
For example, instead of printing
(add (int 5) '+' (int 6))
I'd like to get
(add (int 5 [line 3, offset 6:7]) '+' (int 6 [line 3, offset 8:9]) [line 3, offset 5:10])
Or something similar.
There aren't a tremendous number of visitor examples for ANTLR4 yet, but I am pretty sure I can do most of this by copying the default implementation for toStringTree (used by grun). However, I do not see any information about the line numbers or offsets.
I expected to be able to write super simple code like this:
String visit(ParseTree t) {
return "(" + t.productionName + t.visitChildren() + t.lineNumber + ")";
}
but it doesn't seem to be this simple. I'm guessing I should be able to get line number information from the parser, but I haven't figured out how to do so. How can I grab this line number/offset information in my traversal?
To fill in the few blanks in the solution below, I used:
List<String> ruleNames = Arrays.asList(parser.getRuleNames());
parser.setBuildParseTree(true);
ParserRuleContext prc = parser.program();
ParseTree tree = prc;
to get the tree and the ruleNames. program is the name for the top production in my grammar.

Answer 1

The Trees.toStringTree method can be implemented using a ParseTreeListener. The following listener produces exactly the same output as Trees.toStringTree.
public class TreePrinterListener implements ParseTreeListener {
private final List<String> ruleNames;
private final StringBuilder builder = new StringBuilder();
public TreePrinterListener(Parser parser) {
this.ruleNames = Arrays.asList(parser.getRuleNames());
}
public TreePrinterListener(List<String> ruleNames) {
this.ruleNames = ruleNames;
}
#Override
public void visitTerminal(TerminalNode node) {
if (builder.length() > 0) {
builder.append(' ');
}
builder.append(Utils.escapeWhitespace(Trees.getNodeText(node, ruleNames), false));
}
#Override
public void visitErrorNode(ErrorNode node) {
if (builder.length() > 0) {
builder.append(' ');
}
builder.append(Utils.escapeWhitespace(Trees.getNodeText(node, ruleNames), false));
}
#Override
public void enterEveryRule(ParserRuleContext ctx) {
if (builder.length() > 0) {
builder.append(' ');
}
if (ctx.getChildCount() > 0) {
builder.append('(');
}
int ruleIndex = ctx.getRuleIndex();
String ruleName;
if (ruleIndex >= 0 && ruleIndex < ruleNames.size()) {
ruleName = ruleNames.get(ruleIndex);
}
else {
ruleName = Integer.toString(ruleIndex);
}
builder.append(ruleName);
}
#Override
public void exitEveryRule(ParserRuleContext ctx) {
if (ctx.getChildCount() > 0) {
builder.append(')');
}
}
#Override
public String toString() {
return builder.toString();
}
}
The class can be used as follows:
List<String> ruleNames = ...;
ParseTree tree = ...;
TreePrinterListener listener = new TreePrinterListener(ruleNames);
ParseTreeWalker.DEFAULT.walk(listener, tree);
String formatted = listener.toString();
The class can be modified to produce the information in your output by updating the exitEveryRule method:
#Override
public void exitEveryRule(ParserRuleContext ctx) {
if (ctx.getChildCount() > 0) {
Token positionToken = ctx.getStart();
if (positionToken != null) {
builder.append(" [line ");
builder.append(positionToken.getLine());
builder.append(", offset ");
builder.append(positionToken.getStartIndex());
builder.append(':');
builder.append(positionToken.getStopIndex());
builder.append("])");
}
else {
builder.append(')');
}
}
}