declarative-programming

Declarative programming

How do I write declarative code?

• Declarative programming is a paradigm where you describe what you want the program to accomplish, not how to do it

• SQL is a common example of declarative programming

• It allows you to extract and manipulate data using queries, without needing to specify the control flow

• SQL was covered in more detail in Section 8 – SQL

• Declarative code is made up of:

  • Facts – things that are known or assumed to be true

  • Rules – logical relationships between facts

• Unlike imperative programming, where order and control flow are essential, declarative code allows you to define facts and rules in any order, and then write a query that uses them

• In the following examples, a logic-based declarative language (like Prolog) is used

Example: Facts about dog breeds

type(beagle, hound).
type(labrador, retriever).
type(poodle, companion).
type(golden_retriever, retriever).
type(bulldog, companion). size(beagle, medium).
size(labrador, large).
size(poodle, medium).
size(golden_retriever, large).
size(bulldog, medium). shedding(beagle, moderate).
shedding(labrador, heavy).
shedding(poodle, low).
shedding(golden_retriever, heavy).
shedding(bulldog, low).

Interpreting clauses

• Each fact uses the format:

predicate(object, property).

Sample queries

• Use queries to retrieve information from the facts

?- size(poodle, medium).
true. ?- type(X, retriever).
X = labrador ;
X = golden_retriever. ?- shedding(X, low), size(X, medium).
X = poodle ;
X = bulldog.

Key points