Return a list in dplyr mutate()

The idiomatic way to do this using data.table would be to use the := (assignment by reference) operator. Here's an illustration:

it[, c(paste0("V", 4:5)) := myfun(V2, V3)]

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If you really want a list, why not:

as.list(it[, myfun(V2, V3)])

Alternatively, maybe this is what you want, but why don't you just use the data.table functionality:

it[, c(.SD, myfun(V2, V3))]
#    V1 V2 V3 V4 V5
# 1:  a  1  2  3 -1
# 2:  a  2  3  5 -1
# 3:  b  3  4  7 -1
# 4:  b  4  2  6  2
# 5:  c  5  2  7  3    

Note that if myfun were to name it's output, then the names would show up in the final result columns:

#    V1 V2 V3 new.1 new.2
# 1:  a  1  2     3    -1
# 2:  a  2  3     5    -1
# 3:  b  3  4     7    -1
# 4:  b  4  2     6     2
# 5:  c  5  2     7     3