11.3 Case study: make your own function operator

urls <- c(
  "adv-r" = "https://adv-r.hadley.nz", 
  "r4ds" = "http://r4ds.had.co.nz/"
  # and many many more
)
path <- paste(tempdir(), names(urls), ".html")

walk2(urls, path, download.file, quiet = TRUE)

Here we make a function operator that add a little delay in reading each page:

delay_by <- function(f, amount) {
  force(f)
  force(amount)
  
  function(...) {
    Sys.sleep(amount)
    f(...)
  }
}
system.time(runif(100))
#>    user  system elapsed 
#>       0       0       0

system.time(delay_by(runif, 0.1)(100))
#>    user  system elapsed 
#>     0.0     0.0     0.1

And another to add a dot after nth invocation:

dot_every <- function(f, n) {
  force(f)
  force(n)
  
  i <- 0
  function(...) {
    i <<- i + 1
    if (i %% n == 0) cat(".")
    f(...)
  }
}

walk(1:100, dot_every(runif, 10))
#> ..........

Can now use both of these function operators to express our desired result:

walk2(
  urls, path, 
  download.file %>% dot_every(10) %>% delay_by(0.1), 
  quiet = TRUE
)