Declarative Programming
Declarative programming describes what the result should be rather than specifying the step-by-step procedure to compute it. SQL, HTML, CSS, configuration languages, and logic programming are declarative: you state constraints or desired outcomes and let the runtime determine execution.
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Intro
Declarative Programming
Declarative programming describes the result you want. You express facts, rules, relationships, constraints, or a desired state. An engine chooses how to produce that result.
Imperative programming describes a sequence of operations. You control more of the path from the starting state to the result. The distinction is about where control lives, not which syntax a language uses.
You already meet declarative programming in common tools. A SQL query describes a result set. A CSS rule describes which properties apply to matching elements. A Make rule connects a target to its prerequisites. A Terraform configuration describes managed infrastructure. A Kubernetes configuration describes objects that a control system should maintain.
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Sources
- https://www.haskell.org/onlinereport/haskell2010/
Supports
- Haskell expressions, function definitions, pattern bindings, types, and modules
- Expression-led functional programming as a declarative form
- https://www.postgresql.org/docs/current/tutorial-select.html
Supports
- SQL SELECT lists, table lists, Boolean qualifications, sorting, and duplicate removal
- Queries that specify result properties without fixing a physical access path
- https://www.postgresql.org/docs/current/using-explain.html
Supports
- Query plans, plan nodes, cost estimates, scan strategies, and runtime inspection
- Execution cost depending on engine plans and data access rather than query length
- https://www.w3.org/TR/css-cascade-5/
Supports
- CSS declarations, origins, importance, cascade layers, specificity, scope, and order
- Rule resolution that assigns property values to elements
- https://www.gnu.org/software/make/manual/make.html#Rule-Introduction
Supports
- Make rules composed of targets, prerequisites, and recipes
- Engine decisions about which targets require rebuilding
- https://developer.hashicorp.com/terraform/language
Supports
- Terraform as a declarative language for resources, expressions, and dependencies
- Operation ordering from implicit and explicit relationships rather than file order
- https://developer.hashicorp.com/terraform/intro
Supports
- Desired infrastructure state, resource graphs, state tracking, and change selection
- Version-controlled and reusable infrastructure configuration
- https://developer.hashicorp.com/terraform/cli/commands/plan
Supports
- Previewing proposed infrastructure changes before applying them
- Comparing configuration with prior and remote object state
- https://kubernetes.io/docs/tasks/manage-kubernetes-objects/declarative-config/
Supports
- Declarative creation and update of Kubernetes objects from configuration files
- Diff, apply, live configuration, merge behavior, and management tradeoffs
- https://kubernetes.io/docs/concepts/architecture/controller/
Supports
- Control loops that compare desired and current cluster state
- Repeated action that moves observed state toward desired state