Concatenate values in a column

[#1]

Seems like this should be simple, but I cannot find a clear example.  I would like to concatinate the values in columns of a specific name("ROW" and "COL") and create a new column with the result in it.  Example below. 

ROW    COL     CELL

   5          1         5_1

   3          2         3_2

  12         -1       12_-1

best shot at code so far:

column("ROW") << Data type(Character);
column("COL") << Data type(Character);

//a = {column("ROW"), column("COL")};  
ConExpr = concat items({column("ROW"), column("COL")} , "_"); 

//Add "CELL" column = concat col(ROW) "_" col(COL)
dt << new column("CELL",Character, "Nominal", Format("Best", 12), formula(eval(Expr(ConExpr))));

my results:

CELL

   _

   _

Any help in understanding why my code evaluates to that and the correct implementation would be greatly appreciated. 

[#2 · Accepted solution]

Here is all you need to do:

dt << new column("CELL", character,
     formula(:ROW || "_" || :COL));