Ritlists

Ritlists are a family of data structures used to represent ordered collections in a persistent, immutable form. The term is used in some niche programming literature to describe lists that optimize for concatenation and sharing of common suffixes.

Design: Ritlists are built as tree-like structures where elements are stored in small blocks (leaves) and connected by internal nodes that keep metadata such as the total length. This enables efficient operations that would otherwise require copying entire lists. Ritlists emphasize structural sharing, so updating a ritlist produces a new structure that reuses parts of the old one.

Operations: Basic operations include empty, cons (prepend), head, tail, and length. Concatenation (append) is optimized to reuse existing nodes; indexing by position is typically logarithmic in size. Higher-order operations such as map, filter, and fold are implemented in a manner that preserves sharing and avoids materializing intermediate lists where possible.

Performance and use: In practice, ritlists are useful in functional languages and compilers for representing sequences that are grown by many concatenations or in streaming pipelines. They balance memory efficiency with access speed and are related to other persistent structures such as finger trees and ropes.

Origin and notes: The concept appears in several theoretical treatments of persistent data structures; there is no single standard implementation, and variants differ in block size, balancing strategy, and exact complexity guarantees. See also: persistent data structure, finger tree, rope, linked list.