[Haskell-beginners] take until duplicate

I’m looking at how to take from a list until a duplicate is found. e.g. takeUntilDup [1,2,3,4,3] = [1,2,3,4] I found this implementation takeUntilDup = foldr (\x r -> x : takeWhile (/= x) r) [] It works but I can’t see how!!? The accumulated result is built up in r so with input [1,2,3,4,3] then, at the point when r = [1, 2, 3, 4], the fold is about to use the number 3. i.e. it does takeWhile (/=3) [1,2,3,4] which gives [1,2] Please, how does this work? Thanks Mike .