adding numbers present in array without using sum function

Agree, sum() function should be your go-to for "summing" an array.  

But if you are looking to unnecessarily complicate your code, how about a recursive function to add an array of numbers.

Note with recursive rules, you must save them prior to testing.

/* 
rule!chris_add_recur(
  array: List of Integer
  index: Integer
  total: Integer
)
*/

a!localVariables(
  local!index: if(rule!APN_isEmpty(ri!index),1,ri!index),
  local!total: if(rule!APN_isEmpty(ri!total),0,ri!total),
  
  if(
    local!index>count(ri!array),
    ri!total,
    rule!chris_add_recur(
      array: ri!array,
      index: local!index+1,
      total: index(ri!array,local!index)+local!total
    )
  )
)

Call this rule with only the array input as:

rule!chris_add_recur(
  array: {1,2,3,4}
)

thank you for your answer actually i also agree with sum() but i had this question in my interview 

thank you for your answer actually i also agree with sum() but i had this question in my interview 

you can use reduce() function

but within reduce also we need to use fn!sum as predicate right

instead of sum function, we can write our own rule to add two numbers using (+) sign

Did you get the logic of Chris's code?