There’s an interesting little function in R called on.exit(). It can be used
in your own function to perform some side effect. For example, in addition to
returning a value, the following function uses on.exit() to also print two messages.
Note what happens if we remove add=TRUE from the second on.exit() usage.
We see “first” isn’t printed anymore.
To learn more about on.exit(), refer to Hadley’s book “Advanced R.” You can buy it from Amazon
or read it online for FREE.