Iteratees

Iteratees are a design pattern for incremental, backpressure-aware processing of streams in functional programming. They describe a consumer of an input stream that can process elements as they arrive and communicate with the source to control the flow.

An iteratee runs in a monadic context and can be fed input step by step by an enumerator (the producer). It may respond with Continue, asking for more input, or with Done, providing a result. In many formulations, the iteratee’s state is kept in a purely functional form, enabling safe and predictable composition.

Core components include the iteratee, the enumerator, and the enumeratee. The iteratee consumes input, the enumerator supplies input, and the enumeratee is a transformer that maps one input stream to another, enabling modular pipeline construction without materializing intermediate data structures.

Benefits include bounded memory usage for large data streams, predictable resource management, and the ability to compose complex processing pipelines without building intermediate collections. The pattern also addresses concerns with lazy I/O by making evaluation and data flow explicit, which can reduce space leaks and improve error handling.

History and usage: the iteratee pattern emerged in the functional programming community as a solution for safe and efficient streaming I/O. It has been implemented in Haskell and has influenced streaming libraries in other languages, including Scala, where variants appeared in frameworks that support incremental processing and backpressure. Related concepts include enumeratees, enumerators, and alternative streaming abstractions such as pipes and conduits.

See also: Pipes, Conduit, Enumeratee, Enumerators, streaming I/O.