[antlr-interest] learning pitfall....

[Ric Klaren]

On Tue, Apr 02, 2002 at 08:24:07PM +1000, Lloyd Dupont wrote:
> But if I add this production in the grammar:
> // ------- added in java.g, in JavaRecognizer class ----------
> script : (statement | importDefinition) *  ;
> //---------------------------------------------------------------------

The problem is that this rule can be empty (zero or more times something).
Which will not work (antlr needs lookahead to decide things if a rule can
be empty there's nothing to decide with...) Rewriting to something like

calling_rule: yyy (script)? xxx ;
script: (statement | importDefinition)+ ;

Will solve the problem.

> By the way, reading anything I could about nondeterminism I see an example with a token set like that :
> -------------
> ('b' | )
> --------------
> I hardly figure out what this could mean ! could you explain ?

Its a 'b' or nothing or in other words an optional 'b'. Or alternatively (
'b' )? Sometimes its covenient to write an optional alternative like this
in order to add specific action code for the empty alternative.

Hope this helps,

Ric
-- 
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    ---- Ric Klaren ----- klaren at cs.utwente.nl ----- +31 53 4893722  ----
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