Why is := allowed as an infix operator?

Question

I have come across the popular data.table package and one thing in particular intrigued me. It has an in-place assignment operator

:=

Answer 1

It's not just a colon operator but rather := is a single operator formed by the colon and equal sign (just as the combination of "<" and "-" forms the assignment operator in base R). The := operator is an infix function that is defined to be part of the evaluation of the "j" argument inside the [.data.table function. It creates or assigns a value to a column designated by its LHS argument using the result of evaluating its RHS.

Answer 2

It is something that the base R parser recognizes and seems to parse as a left assign (at least in terms or order of operations and such). See the C source code for more details.

as.list(parse(text="a:=3")[[1]])
# [[1]]
# `:=`
# 
# [[2]]
# a
# 
# [[3]]
# [1] 3

As far as I can tell it's undocumented (as far as base R is concerned). But it is a function/operator you can change the behavior of

`:=`<-function(a,b) {a+b}
3 := 7
# [1] 10

As you can see there really isn't anything special about the ":" part itself. It just happens to be the start of a compound token.