flatMap

FlatMap is a higher-order function used in many programming languages to map each element of a collection to another collection and then flatten the result into a single collection. It is often described as the combination of map and flatten, or as the monadic bind in functional programming.

In practice, flatMap takes a function f and a collection C. For each element x in C, f(x) yields an iterable or collection; the results are concatenated in order into one sequence.

Relation to map and flatten: map applies f to each element producing a collection of collections; flatten merges them. flatMap performs both steps in one pass.

Examples: In JavaScript, [1,2,3].flatMap(x => [x, x*2]) yields [1, 2, 2, 4, 3, 6]. In Scala, List(1,2).flatMap(n => List(n, -n)) yields List(1, -1, 2, -2).

Common variations: in Java streams, stream.flatMap returns a Stream; in Kotlin and Groovy, flatMap is common; in Swift flatMap has evolved with optional handling, and compactMap is used for certain cases.

Performance considerations: flatMap may allocate intermediate collections and may be less cache-friendly than specialized flattening; however, modern runtimes optimize common patterns.

See also: map, flatten, and monadic bind.