Deklarative

Deklarative refers to a programming paradigm that emphasizes describing the desired outcome of a computation rather than the steps required to produce it. In declarative programming, the programmer specifies what should be computed, and the underlying system determines how to perform the computation.

Core principles of declarative programming include minimizing or eliminating side effects and mutable state, using high-level abstractions such as constraints, rules, or pure functions, and focusing on the result rather than the control flow. This often involves referential transparency and a separation between the what and the how, enabling reasoning about code at a higher level.

Common domains and languages illustrate declarative style. SQL queries declare what data is needed, not how to fetch it. HTML and XML declare document structure rather than rendering details. Prolog and other logic programming languages express rules and facts to derive conclusions. Functional programming languages like Haskell emphasize pure functions and immutability, contributing to a declarative feel. Configuration and infrastructure tools, such as Terraform or Ansible, describe the desired system state declaratively.

Advantages of declarative approaches include easier reasoning about code, opportunities for automatic optimization and parallel execution, and clearer articulation of intentions. Limitations can include less direct control over performance characteristics, potential trade-offs between expressiveness and efficiency, and debugging challenges when behavior is not immediately tied to explicit steps. In practice, many systems blend declarative and imperative elements to balance clarity with control.

Historically, the declarative paradigm gained prominence in the 1970s and 1980s through work on functional and logic programming, influencing a wide range of modern languages and tools that favor describing outcomes over explicit procedures.