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Advanced Topic 41 of 50 Languages & Compilers

Algebraic Data Types

Types built from sums and products.

How it works

Algebraic data types build new types by combining others in two ways: product types, such as tuples and records, which hold several values together, and sum types, also called tagged unions or variants, which hold one of several alternatives, each possibly carrying data. For example, a shape can be a Circle with a radius or a Rectangle with width and height. Recursive definitions model lists and trees, and pattern matching takes values apart safely.

Step-by-step walkthrough

  1. A product type combines values: a pair holds both fields.
  2. A sum type is a choice of variants.
  3. A Shape is either a Circle or a Rectangle.
  4. The compiler knows all cases, so it can check you handled each.
  5. They model data precisely and make illegal states unrepresentable.

Where it is used

Haskell, OCaml, Rust enums, Swift enums, Kotlin sealed classes and TypeScript discriminated unions use them to model option values, results, abstract syntax trees, state machines and messages.

Common pitfall

Languages without sum types simulate them with inheritance or flags, losing exhaustiveness checking. Deeply nested types become hard to read. Adding a new variant requires updating all matches, which the compiler flags, a benefit. Make illegal states unrepresentable, prefer Option and Result over null and exceptions for expected cases and match exhaustively.

Where it fits in the course

Algebraic Data Types is topic 41 of 50 in Languages & Compilers, rated Advanced. It follows Type Systems. The next topic is Pattern Matching.

Practice and explore

How to study this topic

  1. Read the explanation above and restate the definition in your own words.
  2. Open the interactive lesson and answer its mini quiz.
  3. Read the common pitfall, then write a short example that avoids it.
  4. Check your understanding in the Languages & Compilers quiz, and revisit the topic if you miss a question.

Frequently asked questions

What is Algebraic Data Types?
Types built from sums and products. It belongs to Languages & Compilers and is an advanced-level topic.
Where do people use Algebraic Data Types?
Haskell, OCaml, Rust enums, Swift enums, Kotlin sealed classes and TypeScript discriminated unions use them to model option values, results, abstract syntax trees, state machines and messages.
What is the most common mistake with Algebraic Data Types?
Languages without sum types simulate them with inheritance or flags, losing exhaustiveness checking.
What should I learn before this topic?
Start with Type Systems, the topic just before it in Languages & Compilers.
How can I practice it?
Answer the mini quiz in the interactive lesson, then take the Languages & Compilers quiz.
Is Algebraic Data Types hard to learn?
It is an Advanced topic. Expect to read the explanation more than once and to connect it with earlier topics. Working through a small example by hand helps most.
How long does it take to learn Algebraic Data Types?
Most learners can read the explanation and finish the interactive lesson in under an hour. Fluency comes from applying the idea, so plan a few short practice sessions with the quiz.
Which topics are related to Algebraic Data Types?
Type Systems, Pattern Matching, LLVM are closely connected. The full list is in the related topics section of this page.
What should I study after Algebraic Data Types?
Continue with Pattern Matching, the next topic in Languages & Compilers.

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