flatMapandThen

FlatMapandThen is a conceptual pattern in functional programming that describes chaining a monadic flatMap (bind) operation with a subsequent transformation in a single step. The idea merges the monadic flattening step with a post-processing function, producing a concise pipeline that handles both the nested structure and the final value transformation.

Formally, given a monad M with flatMap and map, and functions f: T -> M<U> and g: U -> V, flatMapandThen yields M<V> as m.flatMap(t => f(t).map(g)). This expresses two ideas at once: first apply f to produce a nested monad, then map the inner value with g to obtain the final result, all within the same chaining construct. If the language or library provides a direct andThen-like operator, the pattern can be implemented as a single composition step; otherwise it is typically written as a short combination of flatMap and map.

Examples help illustrate the concept. In Java, with Optional, if m is Optional<T>, f: T -> Optional<U>, and g: U -> V, then m.flatMap(t -> f.apply(t).map(g)) yields Optional<V>. In JavaScript, with Promises, if p is Promise<T>, f: T -> Promise<U>, and g: U -> V, the equivalent pattern is p.then(t => f(t).then(g)), yielding Promise<V>.

Related ideas include map and flatMap as basic monadic operations, and Kleisli composition, which generalizes function composition when the intermediate step returns a monad. FlatMapandThen is a practical naming idea for expressing a common chaining pattern: perform a flatMap to flatten the inner structure, then apply a subsequent pure function to the inner result.