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

[Terence Parr]

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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