python - Generator functions in R

Question

Is there a package or language construct in R that facilitates or provides the implementation of "Python-like generators"?

By "Python-like generators" I mean functions that keep state between calls, in R syntax and borrowing the keyword yield from Python will be something like:

iterable.fun <- function(){
  yield list('a','b','c')
}

With yield instead of a return, then calling the function three consecutive times would give:

> iterable.fun()
  'a'
> iterable.fun()
  'b'
> iterable.fun()
  'c'

Edit: I left out an aspect of Python generators that makes them different from iterators. It is that the whole list of objects to iterate on is not built on the first call and then iterated, but each function call creates the one element that will return for that call.

Answer 1

The iterators package has this functionality

library(iterators)
abc <- iter(c('a','b','c'))
nextElem(abc)
## [1] "a"
nextElem(abc)
## [1] "b"
nextElem(abc)
## [1] "c"

Or you could use lambda.r and <<-. This example is modified from

http://cartesianfaith.wordpress.com/2013/01/05/infinite-generators-in-r/

there are more examples in the blog post

library(lambda.r)
seq.gen(start) %as% {
  value <- start - 1L
  function() {
    value <<- value + 1L
    return(value)
  }
}



foo <- seq.gen(1)
foo()
## [1] 1
foo()
## [1] 2
foo()
## [1] 3

note that you could also use a regular function to do this.

seq.gen <-function(start) {
  value <- start - 1L
  function() {
    value <<- value + 1L
    return(value)
  }
}
foo2 <- seq.gen(1)
foo2()
## [1] 1
foo2()
## [1] 2
foo2()
## [1] 3

If you want to select from a possible list, then you could perhaps do so using switch

seq.char(start) %as% {
  value <- start - 1L
  function() {
    value <<- value + 1L
    return(switch(value,'a','b','c'))
  }
}

foo.char <- seq.char(1)
 foo.char()
## [1] "a"
 foo.char()
## [1] "b"
 foo.char()
## [1] "c"