enumeratee

Enumeratee is a concept from early functional streaming libraries, most notably in the Play Framework’s Iteratees model. It refers to a composable transformer that sits between an Enumerator (the producer of data) and an Iteratee (the consumer of data). An Enumeratee transforms a stream of input values of type A into a stream of output values of type B, while coordinating with the consumer to advance processing.

In this model, data flows as: Enumerator -> Enumeratee -> Iteratee. The Enumeratee may be stateless, applying a simple transformation such as map or filter, or stateful, buffering data, framing input, or aggregating it. Crucially, an Enumeratee can also change the element type or alter the framing of the data (for example, turning a stream of raw bytes into lines or JSON objects) as part of a processing pipeline. Enumeratees are designed to be composable, allowing multiple transformers to be chained together to form complex streaming workflows.

Common operations provided by Enumeratee libraries include mapping, filtering, taking or dropping elements, and more advanced transformations like grouping or buffering. Through composition, developers can construct pipelines that perform decoding, parsing, or validation incrementally as data arrives, rather than loading entire streams into memory.

While Enumeratees were central to the original Iteratees approach, many modern Scala and JVM projects have migrated to alternative streaming abstractions such as Akka Streams. Nevertheless, Enumeratee remains a useful historical example of a composable, incremental approach to streaming data processing in a functional setting.