A cycling Automaton in Elm. This Automaton produces values off a non-empty list. When the list is exhausted, it starts again. The input signal is just used as a source of events - its values are ignored.

Cycling.elm

-- non-empty list
data NonEmpty x = NonEmpty x [x]

-- neFromList :: [x] -> NonEmpty x
neFromList list = case list of []     -> Maybe.Nothing
                               (x:xs) -> Maybe.Just (NonEmpty x xs)

-- neFromListOr :: NonEmpty x -> [x] -> NonEmpty x
neFromListOr other qs = maybeOr other (neFromList xs)

-- maybeOr :: a -> Maybe a -> a
maybeOr other = Maybe.maybe other id

-- init'' :: s -> (s -> (o,s)) -> Automaton i o
init'' i f = Automaton.init' i (\_ x -> f x)

-- cycler :: NonEmpty a -> Automaton z a
cycler list = init'' list (\(NonEmpty x xs) -> (x, maybeOr list (neFromList xs)))

list = NonEmpty 1 [4, 9, 12]

main = asText <~ (Automaton.run (cycler list) (every second))