Note that the expressions in the 'scanner form are delayed, i.e. they are not evaluated until a 'car or 'cdr is performed on your scanner, and they are evaluated only once.
edit: an important note: scanners have the exact read semantics of lists. So simply zapping cdr at a scanner will not advance the scanner, it will only advance the place you are zapping.
There's no need to use 'peekc or similar: all you need is to use stuff like 'cadr, 'caddr.
Because scanners have the exact read semantics of lists, you can use such things as 'each, 'map, etc. Just don't write using scar, scdr, or sref.
If you wanted to emulate lists, you can do something like:
(def my-cons (a d)
(scanner 'car a
'cdr d))
Of course, since a and d are just var references, there's little point in delaying their execution.
edit2: Here's how you might make a generator:
(def generator (f v)
(scanner 'car v
'cdr (generator f (f v))))
(= b (generator [+ _ 1] 0))
(car b)
=> 0
(cadr b)
=> 1
(cadr:cdr b)
=> 2
'map, 'keep, and a few other functions become lazy when applied on scanners, so you can use an infinite-series generator on them safely
