issue with reject(fn!isnull, 'array of arrays') function on a data generated from an expression rule

Question

Hi Community, 
 I have a strange problem with the reject function. I am getting an error when I work with the data generated with my expression rule. If I do a copy & paste of the data generated, the function does not give me an error but I am not getting what I expected. 
 A further description of the problem (as maybe I am just taking the wrong approach), I have on CDT on which one of the values is the id's of another CDT. I wanted to count how many times the second CDT is used, so I can have a graphical representation in a form of a pie chart. My expression rule gives returns the data in a way that is easy to work on graphs, like the pie chart below. 
 When I call the expression rule, I get the data like this: 
 
 {{}, {}, {}, {}, {}, {}, {}, {}, {"Outbound", 1}, {}, {"Mudcakes", 2}, {"Happy Industries", 2}, {}, {}} 
 
 Which means that "Outbound" appears 1, "Mudcakes" 2 and "Happy Industries" 2 
 On the pie chart, the values "{}" appears like [Series 1], [Series 2] and so on. I want to get rid of those empty arrays. 
 Then, I try to use the function reject, like below 
 reject (fn!isnull, rule!APP_myExpresionRule ), it troughs me an error

Expression evaluation error at function 'reject': Incorrect number of parameters for function; expected 1 parameters, but found 0 parameters

Now, when I copy the data generated with rule!APP_myExpresionRule, 
 reject(fn!isnull,{{}, {}, {}, {}, {}, {}, {}, {}, {"Outbound", 1}, {}, {"Mudcakes", 2}, {"Happy Industries", 2}, {}, {}}) 
 I get this: 
 {"Outbound", 1, "Mudcakes", 2, "Happy Industries", 2} 
 
 which is not what I am looking for, as I would expect to have something more like this: 
 i 
 {{"Outbound", 1},{"Mudcakes", 2}, {"Happy Industries", 2}} 
 
 Below the pie chart as it looks now:

Any suggestions? 
 
 Thanks! 
 Manuel

Mike Schmitt · Accepted Answer

It appears that Reject is flattening the resulting array prior to evaluating. This isn't much of a surprise to me, since Appian's ability to handle array-of-arrays in most cases is pretty spotty (and slightly unpredictable). I would suggest you consider making your expression rule pass back dictionary data instead of {text, integer} arrays, because then you could pretty easily weed out blank/empty entries from your return list without needing to bend over backwards to avoid the array flattening that's happening here.