foldr :: (a -> b -> b) -> b -> [a] -> b
foldr _ z []     = z
foldr f z (x:xs) = f x (foldr f z xs)

infixr 5 ++
(++) :: [a] -> [a] -> [a]
[]     ++ ys = ys
(x:xs) ++ ys = x : (xs ++ ys)

concat :: [[a]] -> [a]
concat []       = []
concat (xs:xss) = xs ++ concat xss
The right fold replaces constructors by a supplied step and seed; list concatenation replaces the end of its first argument by its second; flattening concatenates each inner list. In particular,
(1)
The right fold allows different element and accumulator types; the list concatenation operations require one common element type.

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