Simple generator

counter <- function(low, high, step) {
  current <- low
  function() {
    if (current > high) {
      return(NULL)
    }
    result <- current
    current <<- current + step
    result
  }
}

# One value per call, made on demand
next_number <- counter(3, 9, 2)
repeat {
  number <- next_number()
  if (is.null(number)) break
  cat("number =", number, "\n")
}

# No end condition: it never finishes by itself
powers_of_two <- function() {
  current <- 1
  function() {
    result <- current
    current <<- current * 2
    result
  }
}

# replicate() asks for exactly five
next_power <- powers_of_two()
first_five <- replicate(5, next_power())
# first_five = 1 2 4 8 16
cat("first_five =", first_five, "\n")