# Lazy Stream Programming in Prolog

## TL;DR

This paper introduces lazy stream generators in Prolog, providing a unified interface for handling finite and infinite data sequences incrementally, with a flexible API and algebraic operations for stream manipulation.

## Contribution

It presents a novel Prolog extension with lazy stream generators, combining an API, algebra, and interoperation with logic engines for efficient data processing.

## Key findings

- Unified Prolog interface for stream processing
- Supports finite and infinite sequences incrementally
- Enables interoperation between different stream types

## Abstract

In recent years, stream processing has become a prominent approach for incrementally handling large amounts of data, with special support and libraries in many programming languages. Unfortunately, support in Prolog has so far been lacking and most existing approaches are ad-hoc. To remedy this situation, we present lazy stream generators as a unified Prolog interface for stateful computations on both finite and infinite sequences of data that are produced incrementally through I/O and/or algorithmically. We expose stream generators to the application programmer in two ways: 1) through an abstract sequence manipulation API, convenient for defining custom generators, and 2) as idiomatic lazy lists, compatible with many existing list predicates. We define an algebra of stream generator operations that extends Prolog via an embedded language interpreter, provides a compact notation for composing generators and supports moving between the two isomorphic representations. As a special instance, we introduce answer stream generators that encapsulate the work of coroutining first-class logic engines and support interoperation between forward recursive AND-streams and backtracking-generated OR-streams. Keywords: lazy stream generators, lazy lists, first-class logic engines, stream combinators, AND-stream / OR-stream interoperation, Prolog extensions