Haskell concatMap function

The higher-order function with type (a -> [b]) -> [a] -> [b] and value concat (map f xs). It replaces each input element by a list and concatenates those lists in input order. A direct definition is concatMap f = foldr (\x rest -> f x ++ rest) []. This formulation processes an input element's produced list once; it need not repeatedly append an already accumulated output prefix.