Understanding toListOf with van Laarhoven traversals

Learn
3

My bad, I had a typo in toListOf's type. K is of course Const. Here’s a running but failing test, trying to prove that toListOf . elements = id on [Int]. It fails with the first non-empty list [0]:

module TinyLens where

import Data.Functor.Const ( Const(Const), getConst )
import Data.Semigroup ( Endo(Endo), appEndo )
import Test.QuickCheck ( quickCheck )

elements :: (Traversable t, Applicative f) => (a -> f b) -> (t a -> f (t b))
elements = traverse

toListOf :: ((a -> Const (Endo [a]) a) -> (s -> Const (Endo [a]) s)) -> (s -> [a])
toListOf t x =  let foo = t $ Const . Endo . map . const
                    bar = getConst $ foo x
                in  appEndo bar []

propSameList :: [Int] -> Bool
propSameList xs = toListOf elements xs == xs

main = quickCheck propSameList

Any ideas? Maybe my choice of foo is wrong?