Array with no items but length still 0

Question

Hello, again. 
 
 This is a strange one.... and I'm on version 24.1.281.0 
 I have two expression rules... one generates some error messages (or none) and the other deals with the errors. However, in the scenario where the first expression rule generates no errors, the second one still thinks there are errors. 
 
 Expression 1: (called SH_ErrorGenerator) 
 a!localVariables(
 local!stringArray: {"a", "b"},
 local!error1: "", /* value from another expression rule normally */
 local!error2: "", /* value from another expression rule normally */
 local!error3: {""}, /* value from another expression rule normally */
 local!errors: {
 local!error1,
 local!error2,
 local!error3
 },
 local!allErrors: cast( /* make sure this is list of string not list of variant */
 typeof(local!stringArray),
 reject(a!isNullOrEmpty, a!flatten(local!errors))
 ),
 local!anyErrors: and(not(isnull(local!allErrors)), length(local!allErrors) > 0),
 {
 errors: local!allErrors,
 anyErrors: local!anyErrors
 }
) 
 
 Expression 2: 
 a!localVariables(
 local!result: rule!SH_ErrorGenerator(),
 local!errors: local!result.errors,
 local!anyErrors: and(not(isnull(local!errors)), length(local!errors) > 0),
 local!anyErrors
) 
 
 Here is the result I see. 
 
 result.anyErrors (calculated by SH_ErrorGenerator) is false (as it should be), but local!anyErrors (calculated by second rule) says true. And even the local variables screen says there are 0 items in the array. 
 
 Any idea what's going on here? 
 
 Thank you.

Shubham Aware · Accepted Answer

Appian treats lists with empty strings as having length, but they cannot be indexed. Use a!isNullOrEmpty() to check for real errors instead of length().

Harsha Sharma · Answer

The expression for local!anyErrors in expression 2 checks if error is not null as well as its length. Though the local!errors is having 0 items yet as the function length("") returns 1. So length will return true. Similarly behavior of isnull() can be understood from below. 
 not(isnull({""})) returns true not(isnull("")) returns false 
 length("") /*returns true*/
not(isnull({""})) /*returns true with a List of empty string*/
not(isnull("")) /*returns false with empty string*/ 
 Given the behavior and working of isnull() and length() the value for local!anyErrors in expression 2 seems fine. To properly check and correct the behavior as per your requirement I suggest you to either replace the local!anyErrors definition with a!isNotNullOrEmpty(local!errors) or simply use local!result.anyErrors. 
 local!anyErrors: a!isNotNullOrEmpty(local!errors) or,
local!anyErrors: local!result.anyErrors /* Either of these*/

Stefan Helzle · Answer

In this blog post, I try to cover how to check for empty data. 
 https://appian.rocks/2022/10/17/the-magic-of-null-checking/

Array with no items but length still > 0

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