# Erlang- Creating a function which only adds/subtracts positive integers

I want to create a function which allows me to input a list, and then add all of the positive numbers from the list together, leaving any that are negative.

([3,1,-1]) For example only 3 and 1 would be added from this list.

Would I right in thinking the function should be recursive and have a guard?

sum_positives(List) ->

sum_positives([]) -> 0;