List4

List4 is an open-source library that provides immutable lists for functional programming. The project emphasizes four core operations—map, filter, reduce, and zip—as a basis for concise, chainable data processing without mutating input data. Implementations exist for several languages, including JavaScript, Python, Java, and Rust, aiming for a consistent API across different ecosystems.

Data structure and API: List4 uses an immutable singly linked list with structural sharing, allowing new lists to be created from existing ones without altering the original. Core operations include cons (prepending), head, tail, and isEmpty, alongside higher-order routines map, filter, reduce, and zip. Transformations produce new lists that share unmodified parts of the structure, benefiting memory efficiency in streaming pipelines.

Usage example: List4.of(1, 2, 3, 4).map(n => n * 2).filter(n => n > 4).toArray() shows a typical pipeline. The library provides adapters to convert to native arrays or collections and includes utilities for constructors, error handling, and simple integration with streams.

History and reception: Originating in the late 2010s as a compact portable abstraction, List4 has developed a modest ecosystem of language bindings and community extensions. It is used in education to illustrate immutable data structures and in lightweight projects requiring predictable memory usage. Critics point to overlap with other persistent structures; supporters highlight readability and interoperability across languages.

See also: Immutable data structure, Persistent data structure, Functional programming, Linked list.