WillNess

http://stackoverflow.com/questions/20568276/implement-insert-in-haskell-with-foldr
  /20570385#20570385
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You need a [paramorphism](http://stackoverflow.com/a/13317563/849891) for that:

    para  :: (a -> [a] -> b -> b) -> b -> [a] -> b
    foldr :: (a ->        b -> b) -> b -> [a] -> b
    
    para  c n (x : xs) = c x xs (para c n xs)