To be more precise, I believe we're looking to catch an expression where evaluating it to head-normal form demands itself already evaluated in head-normal form.  In your example, there is no loop because `length a` can be evaluated without evaluating any of the items in `a`.  But you get an infinite loop if you make cons strict in the item:

    {-# LANGUAGE BangPatterns #-}
    import Prelude hiding (length)

    -- | List whose cons is strict in the value.
    data SList a = Nil | Cons !a (SList a)
        deriving Show

    infixr 5 `Cons`

    length :: SList a -> Int
    length xs0 = go 0 xs0
      where
        go !n Nil = n
        go !n (Cons _ xs) = go (n+1) xs

    a :: SList Int
    a = (length a * 2) `Cons` (length a * 3) `Cons` (length a * 4) `Cons` Nil

    main :: IO ()
    main = print a