Jonas Notes

Domain Specific Language

2 min read

Definition

A Domain-Specific Language (DSL) is a programming or specification language that is tailored to a specific problem domain.
Unlike general-purpose languages (e.g., Python, Java, C++), a DSL is optimized for expressiveness and efficiency within its niche.

Types of DSLs

  • External DSLs → Standalone languages with their own syntax, parser, and tooling.
    • Example: SQL, HTML, CSS.
  • Internal DSLs (Embedded DSLs) → DSLs built inside a general-purpose language, reusing its syntax and runtime.
    • Example: LINQ in C#, RSpec in Ruby.

Examples

🗄 SQL (Structured Query Language) – External DSL

SELECT name, age 
FROM users 
WHERE age > 30;
  • SQL is a DSL for database querying and manipulation.
  • Highly expressive in the database domain, but useless outside it.

🎨 CSS – External DSL

body {
  background-color: black;
  color: white;
}
  • CSS is a DSL for styling web pages.
  • Optimized for design, not general programming.

🐍 RSpec (Ruby) – Internal DSL

describe Calculator do
  it "adds numbers" do
    expect(Calculator.add(2, 3)).to eq(5)
  end
end
  • RSpec is an internal DSL in Ruby for writing readable tests.
  • Expresses test logic in a human-like way.

🟨 LINQ (C#) – Internal DSL

var result = from user in users
             where user.Age > 30
             select user.Name;
  • LINQ is an embedded DSL in C# for querying data collections.
  • It borrows ideas from SQL but integrates into the language.

Benefits

  • High expressiveness in a narrow domain.
  • Easier for domain experts (not just programmers) to use.
  • Reduces boilerplate code.

Drawbacks

  • Limited outside the target domain.
  • Requires learning an extra language/syntax.
  • External DSLs need parsers and tooling support.