[stringtemplate-interest] difficulty using format and null options together

[John Snyders]

When you look at the implementation of all the options (format, null 
etc.) in ASTExpr.java it seems very natural to handle the null option 
first before format. The code checks for a null value a few steps before 
it has a string to render.
It is documented that the null option is applied first then format then 
separator.

In the case where the format option is transforming characters like 
changing case I think it makes sense to apply null first. I can see that 
in your case you clearly don't want to quote the NULL. The trouble is 
that you need some out of band value to pass to format. In general you 
don't have one and null is really your only 100% safe option. If you 
change the format renderer to check for the string NULL and don't quote 
it then there is a remote but possible chance that one of the non-null 
strings could have the value "NULL" in which case it would not get 
quoted when it should.

Changing ASTExpr so that it passes null to the format renderer could be 
tricky and in addition I don't think that existing renderers ever expect 
to be called with null.

I wonder if the following would work.
When the null option is used the string value is wrapped in a class such 
as NullDefault. This simple class acts much like a string and toString 
just renders the wrapped string. So because of the way ST default 
rendering works it should just call toString and get the intended 
default value. When the NullDefault object is passed to the format 
renderer it will be able to distinguish it from a normal string. This 
could still have some backward compatibility problems depending on if 
people wrote robust renderers that called toString on objects they 
didn't recognize.

Terence, what do you think? Is this worth doing or will it cause problems?
-John

Mark Wharton wrote:
> Hello,
>
> I'm having difficulty using format and null options together in generating an SQL statement with quoted values and NULL keyword. Here's a sample:
>
> 	String ts = "$values; format=\"quote\", null=\"NULL\", separator=\", \"$";
> 	StringTemplate t = new StringTemplate(ts);
> 	t.registerRenderer(String.class, new ValueRenderer());
> 	List values = new ArrayList();
> 	values.add("x");
> 	values.add("y");
> 	values.add(null);
> 	values.add("z");
> 	values.add(null);
> 	t.setAttribute("values", values);
> 	System.out.println("t: "+t.toString());
>
> Actual Result...
>
> 	t: "x", "y", "NULL", "z", "NULL"
>
> Desired Result...
>
> 	t: "x", "y", NULL, "z", NULL
>
> It looks like the null option is applied before the format option and consequently the NULL becomes quoted because it is a string when the renderer processes it. I don't see how it is possible to have non quoted NULL in the output for null values in the list when the format option is also used in the same expression.
>
> Thanks,
>
> Mark
>
> Mark Wharton
> + 8190 9834 2559
> contact at moonbase.com.au
>
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