Array#cycle
/ Enumerable#cycle
does what you are looking for:
e = [1,2,3].cycle #=> #<Enumerator: [1, 2, 3]:cycle>
e.next #=> 1
e.next #=> 2
e.next #=> 3
e.next #=> 1
e.next #=> 2
(1..3).cycle
returns equivalent values.