My "tying the knot", Haskell

My "tying the knot"

--ghc 8.0.1 /opt/ghc/8.0.1/lib/ghc-8.0.0.20160127/

-- This program converts a list into a ring.
-- Basically a less confusing "tying-the-knot" example
-- See https://wiki.haskell.org/Tying_the_Knot

data Node a = Node { prev :: Node a, value :: a, next :: Node a }

x = Node z 1 y
y = Node x 2 z
z = Node y 3 x

since :: Node a -> [Node a]
since node = iterate next node

xAndAfter = since x

listToRingWithBeginEnd
    :: [a]     -- lst
    -> Node a  -- begin
    -> Node a  -- end
    -> (Node a, Node a)  -- (nextOfBegin, prevOfEnd)
listToRingWithBeginEnd [] begin end = (end, begin)
listToRingWithBeginEnd (n:ns) begin end =
    let
        hd = Node begin n noh
        (noh, poe) = listToRingWithBeginEnd ns hd end
    in
        (hd, poe)

listToRing :: [a] -> Node a
listToRing [] = error "list cannot be empty"
listToRing (n:ns) =
    let
        hd = Node poh n noh
        (noh, poh) = listToRingWithBeginEnd ns hd hd
        --(hd, tl) = listToRingWithBeginEnd (n:ns) tl hd -- see https://wiki.haskell.org/Tying_the_Knot
    in
        hd

myRingHdr = listToRing [1,2,3,4,5]
myRingInfLst = since myRingHdr

main = do
    print $ map value $ take 20 $ myRingInfLst


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