The idea is to make a cycle type disjoint from the other types, only interface is standardized. Scheme implementations are free to use any underlying structure to achieve it.
(make-cycle list)
(cycle->list cycle)
(cycle? obj)
cycle? returns #t if obj is a cycle, and otherwise returns #f.
(cycle-length cycle)
(cycle-front cycle)
Returns the element in front of the given cycle.
(cycle-remove-front cycle)
cycle-remove-front returns a newly allocated cycle obtained from cycle where the front element have been removed.
(cycle-rotate cycle k)
cycle-rotate returns a cycle obtained from cycle by a rotation of k, which is an integer.
(cycle-insert cycle obj)
Returns a newly allocated cycle where obj is put in front in cycle.
(cycle-map proc cycle1 cycle2 ...)
All cycles must have the same length and proc must be a procedure taking as many arguments as there are cycles in parameter and returning a single value. cycle-map applies proc to the elements of the cycle(s) and returns a newly allocated cycle of the corresponding result.
The cycle composed of elements 1, 2 and 3 with 1 in front can be written as following:
#cycle(1 2 3)