Why is this left-recursive and how do I fix it?

Question

I'm learning ANTLR4 and I'm confused at one point. For a Java-like language, I'm trying to add rules for constructs like member chaining, something like that:

expr1.MethodCall(expr2).MethodCall(expr3);

I'm getting an error, saying that two of my rules are mutually left-recursive:

expression
    : literal
    | variableReference
    | LPAREN expression RPAREN
    | statementExpression
    | memberAccess
    ;

memberAccess: expression DOT (methodCall | fieldReference);

I thought I understood why the above rule combination is considered left-recursive: because memberAccess is a candidate of expression and memberAccess starts with an expression.

However, my understanding broke down when I saw (by looking at the Java example) that if I just move the contents of memberAccess to expression, I got no errors from ANTLR4 (even though it still doesn't parse what I want, seems to fall into a loop):

expression
    : literal
    | variableReference
    | LPAREN expression RPAREN
    | statementExpression
    | expression DOT (methodCall | fieldReference)
    ;

Why is the first example left-recursive but the second isn't?
And what do I have to do to actually parse the initial line?

score 1 · Accepted Answer · answered Apr 01 '15 at 16:12

1

The second is left-recursive but not mutually left recursive. ANTLR4 can eliminate left-recursive rules with an inbuilt algorithm. It cannot eliminate mutually left recursive rules. There probably exists an algorithm, but this would hardly preserve actions and semantic predicates.

answered Apr 01 '15 at 16:12

CoronA

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I see. But what's the way to parse the example I've given? Everything I try seems to be mutually left-recursive, hang (infinite loop, maybe?) or parse incorrectly. – Theodoros Chatzigiannakis Apr 01 '15 at 16:23
Have you tried to reorder the alternatives with the second solution? What is the rule code of statementExpression. Is it clearly disjunct from the last line? Yet I cannot imagine which grammar lets the parser hang ... (ANTLR consumes at least one character per step, so unless an error occurs, is should reach the end and stop). – CoronA Apr 01 '15 at 16:29
I have tried reoreding and expanding as much as I could. I can't say for sure that I have tried all the permutations, but the tree of my test rig doesn't come up at all whenever I include an immediate left-recursive rule. So having anything like `expression DOT IDENTIFIER` as an alternative in `expression` doesn't generate anything. If I remove it, I get an output (but not what I want, of course). – Theodoros Chatzigiannakis Apr 03 '15 at 14:46

score 0 · Answer 2 · answered Apr 03 '15 at 15:21

For some reason, ANTLRWorks 2 was not responding when my grammar had left-recursion, causing me to (erroneously) believe that my grammar was wrong.

Compiling and testing from commandline revealed that the version with immediate left-recursion did, in fact, compile and parse correctly.

(I'm leaving this here in case anyone else is confused by the behavior of the IDE.)

Why is this left-recursive and how do I fix it?

2 Answers2