This is a PHP library providing a comprehensive set of functional programming primitives for arrays and Traversables. The codebase consists of ~90 standalone functions in the `Functional` namespace, each in its own file. Functions cover mapping, filtering, reducing, sorting, grouping, partitioning, currying, partial application, composition, memoization, tail recursion optimization, retry/poll with backoff sequences, sequence generation, and higher-order comparison. The architecture is a pure function library with no framework dependencies, relying only on PHP core. Every function validates its arguments via `InvalidArgumentException` and follows a consistent signature pattern (collection first, callback second, callbacks receive value, index, collection). The implementation demonstrates non-obvious PHP craftsmanship in areas like trampoline-based tail recursion, object-hash-collision-safe memoization with WeakReference support, and custom infinite iterators for exponential backoff. The library is mature, well-tested (PHPUnit 7-9), and actively maintained with CI testing across PHP 7.1 to 8.5.