Using get() with replacement functions

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To understand why this doesn’t work, you need to understand what colnames<- does. Like every function in that looks like it’s modifying an object, it’s actually modifying a copy, so conceptually colnames(x) <- y gets expanded to:

copy <- x
colnames(copy) <- y
x <- copy

which can be written a little more compactly if you call the replacement operator in the usual way:

x <- `colnames<-`(x, y)

So your example becomes

get("x") <- `colnames<-`(get("x"), y)

The right side is valid R, but the command as a whole is not, because you can’t assign something to the result of a function:

x <- 1
get("x") <- 2
# Error in get("x") <- 2 : 
#  target of assignment expands to non-language object

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