concatMap
concatMap is a higher-order function used in functional programming to map a function over a collection where the function itself returns a collection, and then concatenate the results into a single collection. It combines mapping and flattening in one step, effectively performing a one-level join after the map. For lists, the typical signature is concatMap :: (a -> [b]) -> [a] -> [b], and it can be defined as concatMap f xs = concat (map f xs) or equivalently as join . map f. This means: apply f to each element of xs to obtain a list of lists, then concatenate those lists in order.
In the list monad, concatMap corresponds to the list-specific form of bind: it is the same as
Other languages use similar constructs with varying names: Scala uses flatMap, JavaScript uses Array.prototype.flatMap, and ReactiveX
Performance and memory: concatMap processes elements sequentially and produces a flattened result without creating a nested