An R function return and assignment puzzle

December 29, 2015

(This article was first published on R – Win-Vector Blog, and kindly contributed to R-bloggers)

Here is an R programming puzzle. What does the following code snippet actually do? And ever harder: what does it mean? (See here for some material on the difference between what code does and what code means.)

f <- function() { x <- 5 }

In R version 3.2.3 (2015-12-10) -- "Wooden Christmas-Tree" the code appears to call the function f() and return nothing (nothing is printed). When teaching I often state that you should explicitly use a non-assignment expression as your return value. You should write code such as the following:

f <- function() { x <- 5; x }
## [1] 5

(We are showing an R output as being prefixed with ##.)

But take a look at the this:

f <- function() { x <- 5 }
## [1] 5

It prints! Read further for what is really going on.


What is going on is: in R in the absence of an explicit return() statement functions always return the value of the last statement executed. Also in R assignment is itself a value returning expression (returning the value assigned). So the original function f <- function() { x <- 5 } is in fact returning a 5. We just don’t see it. The 5 returned is “invisible” (see the “return values” section of Advanced R, Hadley Wickham, CRC 2015 for details).

As we said: R assignments return values. So you can return them and you can chain them like so:

a <- b <- c <- 5
## [1] 5

What happens is the assignment x <- 5 returns a value (in this case 5), but that value has an attribute marking it invisible. This is why when you assign a value to a variable in R you don’t see printing as a side effect. For example we don’t see anything printed when we type the following:

x <- 5

We can remove the invisible attribute by adding parenthesis as follows:

( x <- 5 )
## [1] 5

Assignment also strips the invisible attribute, so we can write code like the following:

f <- function() { x <- 5 }
z <- f()
## [1] 5

(We can think of the expression z <- f() as removing the invisible attribute from the 5 stored in the variable z and then returning a new value 5 that is again invisible. So we don’t see any printing during the assignment, but the value stored in z is now visible. Likely all of the visibility notes are stored in a reference handle and not actually in the values to allow efficient re-use of values.)

This is subtle and strange, and one of the reasons it can be hard to first approach R. R has fairly subtle semantics, but that is part of why it is so safe to program in and so powerful to use.

In language design I tend to prefer more transparency (the user reliably seeing something more directly related to what is going on- something vitally important for learning and debugging) and would have opted for assignment not returning a value (another way to suppress needless printing).

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