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# Iterators
Tomo supports using functions as iterable objects. This allows you to write
arbitrary iteration behavior, such as using a polling-based API, and write
regular loops or comprehensions over that API.
For example, the `Path.each_line()` API method returns a function that
successively gets one line from a file at a time until the file is exhausted:
```tomo
(./test.txt).write("
line one
line two
line three
")
iter := (./test.txt).each_line()
assert iter() == "line one"
assert iter() == "line two"
assert iter() == "line three"
assert iter() == none
for line in (./test.txt).each_line()
pass
```
You can write your own iterator methods this way. For example, this iterator
iterates over prime numbers up to a given limit:
```tomo
func primes_up_to(limit:Int)
n := 2
return func()
if n > limit
return !Int
while not n.is_prime()
n += 1
n += 1
return (n - 1)?
assert [p for p in primes_up_to(11)] == [2, 3, 5, 7, 11]
```
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