Make null values loop-able.

[hs-lsong]

When the loop collection contains null, the loop var will be searching the outer scope of the value instead of just use the null value. For example,

context.put("number", -1);
context.put("the_list", Lists.newArrayList(1L, 2L, null, null, null));
String template = "{% for number in the_list %} {{ number }} {% endfor %}";

When rendering the above for tag, because the 3rd and later values are null, the loop var is number, and it will find the value of it from outside of the forTag scope. It will result in the following

Expected :" 1  2  null  null  null "
Actual   :" 1  2  -1  -1  -1 "

With this special NullValue instance, the loopVar value will stay inside the loop.

[boulter]

Perhaps this should be put in a feature because I could see someone relying on this "default" functionality, intentionally or not.

[jasmith-hs]

We use https://github.com/HubSpot/jinjava/blob/0c47c7f14e6d02b378dc1c625319e6ed9adb090f/src/main/java/com/hubspot/jinjava/LegacyOverrides.java to override the legacy Jinjava behaviour so this could be a good place for this to reside
withNullableLoopValues()

[jasmith-hs]

Just having this have the String value of null wouldn't be sufficient.
Likely you'd want to unwrap the NullValue -> null inside of com.hubspot.jinjava.util.ScopeMap#get(Object)

Creating some construct such as:

record ScopeMap.ScopeValue(Object value) {}

And always unwrapping those values when accessing them inside of a ScopeMap seems like it would do the trick

[hs-lsong]

Can you elaborate why we need to unwrap the NullValue -> null? I don't think we need anywhere to use the null value.

[jasmith-hs]

null != "null"
Here's a quick example:

{% set foo = [0, 1, None, 2] %}
{% for i in foo %}
{% if i %}
 i is {{ i }}
{% endif %}
{% endfor %}

The expected jinja output is:

 i is 1
 i is 2

If treating null as "null", the output would be:

i is 1
i is null
i is 2