andThen

andThen is a functional programming concept that denotes the sequential application of two functions. If f maps A to B and g maps B to C, then the expression f andThen g represents a new function from A to C defined by applying f to an input and then applying g to the result. This is a form of function composition and is commonly contrasted with compose, which applies the functions in the opposite order (for example, g andThen f is not the same as f compose g unless the functions commute).

Mathematically, if f: A -> B and g: B -> C, then (f andThen g)(x) = g(f(x)). The operation is associative, so (f andThen g) andThen h equals f andThen (g andThen h). Function composition like this preserves the input-output behavior while creating new, reusable pipelines of transformations.

In programming languages, andThen appears as a method or operator on function types. In Java, the Function interface exposes andThen, so you can create a composed function by chaining two or more single-argument functions. For example, if addOne maps x to x + 1 and timesTwo maps x to x * 2, then addOne.andThen(timesTwo) applied to 3 yields 8. In Scala, Function1 instances provide an andThen method to achieve the same effect, enabling concise pipelines of simple transformations.

Common uses include building readable data processing pipelines, transforming input through multiple steps, and composing small, reusable functions to form more complex behavior without writing explicit intermediate variables. andThen emphasizes the order of application, which is critical for correct results and clear code semantics. See also: compose.