A generator variant of "i:j".

generator_prototype.R

## These are generic functions for extracting and assigning values to the enclosing
## environment of a function. It needs class(f) = "function" to work.

`$.function` = function(f, y) environment(f)[[y]]

`$<-.function` = function(f, y, value) {
  environment(f)[[y]] = value
  invisible(f)
}

## Makes a 'generator' out of the most simple case of the i:j function in R. The 
## enclosing environment is the environment generated by i, so it is disjoint from
## any "interesting" environment.

i = function(input) {
  input_ = deparse(substitute(input))
  input_vec = as.numeric(unlist(strsplit(input_, ":")))
  start = input_vec[1]
  end = input_vec[2]
  
  generator = function() {
    index = generator$index
    if(index > generator$end) stop("Iterator end reached.")
    generator$index = index + 1
    index
  }
  
  class(generator) = c("generator", "function")
  
  generator$index = start
  generator$end = end
  generator
}

gen = i(1:4)
gen$end 
gen()
gen()
gen()
gen()
gen()

## The next step is to make replicate, sapply, etc. compatible with such generators. 
## This can be very when running simulations, say apply(replicate(N, rnorm(N)), 1, mean) 
## when N is large.

## Here's an incomplete variant of sapply. Ideally it should return a generator itself?

gen = i(1:100)

isapply = function(generator, fun) {
  length = generator$end - generator$start
  list_ = rep(NA, length)
  for(i in 1:length) list_[i] = fun(generator())
  list_
}

isapply(gen, function(x) x^2)