ibreak: Create an iterator that can be told to stop
In itertools: Iterator Tools

Create an iterator that iterates over another iterator until a specified function returns FALSE. This can be useful for breaking out of a foreach loop, for example.

1	ibreak(iterable, finished)

`iterable`	Iterable to iterate over.
`finished`	Function that returns a logical value. The iterator stops when this function returns `FALSE`.

# See how high we can count in a tenth of a second
mkfinished <- function(time) {
  starttime <- proc.time()[3]
  function() proc.time()[3] > starttime + time
}
length(as.list(ibreak(icount(), mkfinished(0.1))))