Why does this happen when a user-defined R function does not return a value?

Question

In the function shown below, there is no return. However, after executing it, I can confirm that the value entered d normally.

There is no return. Any suggestions in this regard will be appreciated.

Code

#installed plotly, dplyr
accumulate_by <- function(dat, var) {
  var <- lazyeval::f_eval(var, dat)
  lvls <- plotly:::getLevels(var)
  dats <- lapply(seq_along(lvls), function(x) {
  cbind(dat[var %in% lvls[seq(1, x)], ], frame = lvls[[x]])
  })
dplyr::bind_rows(dats)
}


d <- txhousing %>%
  filter(year > 2005, city %in% c("Abilene", "Bay Area")) %>%
  accumulate_by(~date)

Answer 1

In the function, the last assignment is creating 'dats' which is returned with bind_rows(dats) We don't need an explicit return statement. Suppose, if there are two objects to be returned, we can place it in a list

---

In some languages like python, for memory efficiency, generators are used which will yield instead of creating the whole output in memory i.e. Consider two functions in python

def get_square(n):
    result = []
    for x in range(n):
        result.append(x**2)
return result

When we run it

get_square(4)
#[0, 1, 4, 9]

The same function can be written as a generator. Instead of returning anything,

def get_square(n):
    for x in range(n):
        yield(x**2)

Running the function

get_square(4) 
#<generator object get_square at 0x0000015240C2F9E8> 

By casting with list, we get the same output

list(get_square(4))
#[0, 1, 4, 9]