What is the meaning of `sum [] = 0`?

Question

sum :: [Int] -> Int
sum [] = 0
sum (x:xs) = x + sum xs

I do not understand what the purpose of the line sum [] = 0 is. I found this piece of code in a textbook (it did not go into detail as to what this line does.)

Answer 1

sum [] = 0 is the edge condition of the recursion.

Suppose we have sum [1, 4, 6, 8] . The list can be rewritten as 1:4:6:8:[] . The empty list is the last 'element'. The calculation is as follow (1 + ( 4 + (6 + ( 8 + 0)))) . When all the elements of the list have been traversed, what remains is the empty list. Adds zero to the calculated result and ends the iteration.