CODEWITHSUNDEEP
antlr4
Eugene Strizhok
Eugene Strizhok

Reputation: 1188

mutually left-recursive rules in ANTLR4

I don't understand how ANTLR4 being able to use direct let-recursion actually help us. Ok, you can write a grammar that will recognize a language, alright. But you rarely stop here. Usually you want to do something with parsed tree. And having to collapse several rules in one just to get a direct left-recursion does not help here.

An example. Say I want to define + and - operators and want them to have the same priority. In GOLD I would write something like this:

expression
    : binaryExpression
    | unaryExpression
    ;

binaryExpression 
    : plusExpression
    | minusExpression
    ;

plusExpression
    : expression '+' unaryExpression

minusExpression
    : expression '-' unaryExpression

Surely in ANTLR4 I can do

expression
    : expression '+' unaryExpression
    | expression '-' unaryExpression
    | unaryExpression
    ;

But when in generated visitor I will only have methods for expression and unaryExpression. How am I supposed to know what to do when I visit ExpressionContext? Is it an addition or substruction?

Upvotes: 3

Views: 1228

Answers (1)

Eugene Strizhok
Eugene Strizhok

Reputation: 1188

I found an answer here.

One can "label" alternatives like this:

expression
    : expression '+' unaryExpression # plusExpr
    | expression '-' unaryExpression # minusExpr
    | unaryExpression                # unaryExpr
    ;

After doing so you will find corresponding methods generated in base visitor.

Upvotes: 2

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