sum non NA elements only, but if all NA then return NA

Question

I think I already got really good answers on the comments, but I will rephrase the question for future reference.

I am trying to sum by groups using data.table. The

Answer 1

Using sum_ from hablar

library(hablar)
A[, as.numeric(sum_(col2)), .(col1)]
#   col1 V1
#1:    A NA
#2:    B  5
#3:    C  4