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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()
>> iter()
= "line one" : Text?
>> iter()
= "line two" : Text?
>> iter()
= "line three" : Text?
>> iter()
= NONE : Text?
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)?
>> [p for p in primes_up_to(11)]
= [2, 3, 5, 7, 11]
```
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