Domain-Specific Languages (DSLs)

Background
- Types of DSLs
Examples in Practice
Embedded DSLs
- Clojure (Hiccup for HTML)
- Ruby (RSpec for Testing)
Design Considerations
Resources

Background

A Domain-Specific Language (DSL) is a programming language specialized for a particular application domain. Unlike general-purpose languages, DSLs trade generality for expressiveness in their domain.

Types of DSLs

Type	Description	Examples
External	Standalone syntax and parser	SQL, CSS, Regular expressions
Internal	Embedded in host language	RSpec (Ruby), Gradle (Groovy)
Fluent	Method chaining APIs	jQuery, Builder patterns

Examples in Practice

SQL - Database Queries

SELECT users.name, COUNT(orders.id) as order_count
FROM users
LEFT JOIN orders ON users.id = orders.user_id
WHERE users.active = true
GROUP BY users.id
HAVING order_count > 5
ORDER BY order_count DESC;

Regular Expressions - Pattern Matching

import re

# Match email addresses
pattern = r'^[\w.-]+@[\w.-]+\.\w+$'
re.match(pattern, 'user@example.com')

CSS - Styling

.card {
  display: flex;
  flex-direction: column;
  padding: 1rem;
  border-radius: 8px;
  box-shadow: 0 2px 4px rgba(0, 0, 0, 0.1);
}

Make - Build Automation

build: deps
        @echo "Building..."
        cargo build --release

test: build
        cargo test

deps:
        cargo fetch

Embedded DSLs

Languages with powerful metaprogramming enable internal DSLs:

Clojure (Hiccup for HTML)

[:div {:class "container"}
  [:h1 "Welcome"]
  [:ul
    (for [item items]
      [:li {:key (:id item)} (:name item)])]]

Ruby (RSpec for Testing)

describe Calculator do
  it "adds two numbers" do
    expect(Calculator.add(2, 3)).to eq(5)
  end

  it "multiplies two numbers" do
    expect(Calculator.multiply(3, 4)).to eq(12)
  end
end

Design Considerations

Expressiveness: Minimize boilerplate for domain tasks
Safety: Constrain operations to valid domain operations
Tooling: IDE support, error messages, debugging
Learning curve: Balance power with simplicity