The R programming language has been lazy for over twenty-five years. This
paper presents a review of the design and implementation of call-by-need in
R, and a data-driven study of how generations of programmers have put
laziness to use in their code. We analyze 16,707 packages and
observe the creation of 270.9 B promises. Our data suggests that there is
little supporting evidence to assert that programmers use laziness to avoid
unnecessary computation or to operate over infinite data structures. For the
most part R code appears to have been written without reliance on, and in
many cases even knowledge of, delayed argument evaluation. The only significant
exception is a small number of packages which leverage call-by-need for
meta-programming.