Defining Variable in R Package Environment

demo.R

progress_print <- function(){
  L <- list(init = function(x) {M <<- x}, step = function() NULL, term = function() NULL)
  itr <- 0
  M <- 1
  L$step <- function(){
    itr <<- itr + 1
    cat(paste0("Job ", itr, " out of ", M, "\n"))
  }
  return(L)
}

# this will give error
assign("progress_print", value = progress_print, envir = environment(llply))

# so we have to do like this
environment(progress_print) <- environment(llply)

# here is one usage of above
ldply(seq(10), function(i) data.frame(x = rnorm(1), y = rnorm(1)), .progress = "print")