[antlr-interest] Error 206 Recursion woes

Kyle Ferrio kferrio at gmail.com
Tue Apr 10 22:27:26 PDT 2012


Recursion (both in ANTLR grammars and in the target runtimes) are topics of
interest to me.  So I looked at what you wrote.  It appears that your
partial grammar spec includes some rules which are not referenced and, more
notably, omits some necessary rules.  Consequently, whatever errors I might
derive from your grammar may not actually be interesting to you.  So I took
a different view.

I dashed out two grammars -- not identical but substantially similar to
each other and remotely similar to yours -- one for antlr v3 and one for
artlr v4 [1].  I've attached the grammars with the caveat that neither is a
model to emulate on the whole.  These are toys, absolutely not
production-worthy.  But thee toys do highlight a few details which you
might consider, such as:

 - It is useful to ponder what tokens a parser really needs and hence which
lexer rules should be mere fragments.
 - Using whitespace as a token may come to tears in your grammar.  And WS
does not appear to be semantic in your language.
 - If all/some WS is truly semantic in your language (as in Python or R)
you might do well to filter before lexing, viz. "form fits function."

Both toy grammars -- each processed with and run against its respective
version of antlr -- parsed the following input as expected without error:

*   ${${${${${${${${${${${${${${${${Test15}}}}}}}}}}}}}}}}*

(I used the attached Main.java for antlr v3.4 and
org.antlr.v4.runtime.misc.TestRig for antlr v4.)

I hope you get something out of this even if it does not answer your
question directly.  Sorry to post and run, but checking email may be spotty
for me the rest of the week.

Good luck!


[1] I probably could have written one grammar for both versions of the
antlr tool, but already I've been spoiled by how tolerant Honey Badger (aka
antlr v4) is of my misteps.  And it's been a long day.  :)

On Tue, Apr 10, 2012 at 7:19 PM, Tangeleno <tangeleno at gmail.com> wrote:

> The error is "Ln17:7 Alternative 1: after matching input such as '$'
> '{''$' '{''$' '{''$' '{''$' '{' the decision cannot predict what comes
> next due to recursion overflow to id from accessor and to object data
> from id and to accessor from objectData"
>  From what I can gather from the book pg 281 is the closest I can find
> to this problem it should be a warning and not an error. The problem is
> coming about when I add the rule objectData as an alternative to the
> param rule, which I really don't understand as because it's complaining
> about id accessor and objectData...
> So my question is what am I doing wrong and what can I do to fix it
> while still allowing nested objectDatas?
> http://privatepaste.com/6611be2380 is an example of what can happen in
> this silly language and 6 levels is by no means the cap...
> objectData
>     :    '$' '{'accessor objectMember*'}'
>     ;
> objectMember
>     :    ('.'|':') accessor;
> accessor:    id indexer* typeCast*;
> id    :    (ID|objectData);
> indexer    :    '[' commaParams ']'
>     ;
> commaParams
>     :    param (',' param)*
>     ;
> param    :    objectData|number|string
>     ;
> spaceParams
>     :    param (WS param)* NewLine
>     ;
> typeCast:    '(' id ')';
> number    :    INT|FLOAT;
> string    :    STRING|.*;
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