How to return true value for null list of records

Question

I just want to understand why list of records(0 items) is not returning null/true in below scenarios. 
 
 I have problem returning the List of Empty Records to be empty/null. 
 Any thoughts?? 
 
 We always practiced using null vs 0 vs "" vs {} depending on the type of data. However here {} for set of array records is not working to identify as null. Of course same behavior with CDT's as well.

Sanchit Gupta (Xebia) · Answer

This is the known behaviour of a!isnull() and a!isnullOrEmpty() also, as it probably uses length() function to check if a value is null which would return 1 for a null CDT or record. Y ou can consider it as a best practice to use the below code to check null values for CDTs and Records specially. 
 {
 a!isNullOrEmpty({}),
 cast(type!Map,{}) = a!map(),
 cast('recordType!{aa4ce38f-1a9d-4827-af51-e2ccb80a2c8b}AS Vehicle',{}) = ri!value,
 cast(type!{urn:com:appian:types:PA}PA_AA_VEHICLE,{}) = ri!cdt
}

Mike Schmitt · Answer

The primitive isnull() function is a bit finnicky with respect to what it considers "null" - in Appian, arrays are a totally different data type from single values, and an empty array has never technically counted as null according to isnull(). In older days we were encouraged to set up a separate expression rule like "rule!apn_isBlank()" which would do a more robust test for null / blank / empty, returning a more intiutive return when fed (for example) an empty list of query result, or other sorts of potentially-empty values. These days however, we have a!isNullOrEmpty() which accomplishes at least (i'll estimate) 80 - 90% of the corner-case handling that we formerly needed to use a specialty expression rule for (there remains a particular corner case, which I don't remember off the top of my head).

How to return true value for null list of records

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