[stringtemplate-interest] Problem with null actual parameters (in anonymous templates)

[Iztok Kavkler]

I'm sorry, there was a slight mistake in the example, the '?' was
missing (so the pl could not be empty).

expr: id=IDENTIFIER '(' (pl+=expr (',' pl+=expr)*)? ')'
->    template(id={$id.text}, par={$pl})
    << <id>(<par; separator=", ">) >> ;

The reason for the infinite recursion is as follows: when processing
	f1(f2(), f3())
the first instance of the template gets paramaters
1)	id = "f1",  par = [f2(), f3()]
because expr rule is recursive, template is invoked recursively for both
parameters, first with
2)	id = "f2",  par = null
at this point, the engine will think that "par" is a missing parameter
and will check the enclosing instance, which is 1), obtaining
	par = [f2(), f3()]
and so on to infinity.


> Wow. Yes, I had not thought of this. sometimes you want to say "empty"
> not missing. I'm not sure I see the infinite loop though. the only way
> to get an infinite loop is if they template is ultimately and enclosing
> template of itself.
> 
> Ter
> On Mar 18, 2009, at 9:10 AM, Iztok Kavkler wrote:
> 
>> If an actual parameter to a template without formal attributes is
>> specified but its value is null, the attribute will be looked-up in
>> enclosing instances. This will almost always break recursive invocations
>> of templates (they are common in grammars with output=template), because
>> it creates cycles in the list of enclosing instances.
>>
>> For example, the following snippet should parse and print back nested
>> functions with any number of parameters, like " f1(f2(), f3()) ", but it
>> actually dies with stack owerflow (or it diagnoses the cycle when lint
>> is on):
>>
>> expr: id=IDENTIFIER '(' (pl+=expr (',' pl+=expr)*) ')'
>> ->    template(id={$id.text}, par={$pl})
>>     << <id>(<par; separator=", ">) >> ;
>>
>> The solution is simple: when checking for attributes th function
>> StringTemplate.get should return null if the attribute is present but
>> its value is null (some modifications to setAttribute are also
>> necessary). The following patch to StringTemplate.java does the trick:
>>
>> 486c486
>> <         if ( value==null || name==null ) {
>> ---
>>>         if ( name==null ) {
>> 510a511,516
>>>        
>>>         // null value should be added only if the attribute does not
>>> yet exist
>>>         if ( value==null ) {
>>>            return;
>>>         }
>>>        
>> 772a779,783
>>>         // there is an actual argument, but it is null
>>>         if ( o==null && self.attributes!=null &&
>> self.attributes.containsKey(attribute) ) {
>>>            return o;
>>>         }
>>>
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>