An R function return and assignment puzzle

By John Mount

Image may be NSFW.
Clik here to view.

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 
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 
(We are showing an R output as being prefixed with ##.)
But take a look at the this:
f 
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  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 
What happens is the assignment x  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 
We can remove the invisible attribute by adding parenthesis as follows:
( x 
Assignment also strips the invisible attribute, so we can write code like the following:
f 
(We can  ...read more
Source:: r-bloggers.com