What Is a Data Type Haskell?


Heather Bennett

In Haskell, a data type is a way to classify and organize different types of values. It provides a set of rules and constraints that define the behavior and operations that can be performed on those values. Understanding data types is fundamental to writing functional programs in Haskell.

What are Data Types?

Data types in Haskell are used to represent different kinds of values and provide a set of operations that can be performed on them. They define the structure, properties, and behavior of values within a program.

Basic Data Types

Haskell provides several built-in basic data types:

  • Int: Represents integer values, both positive and negative.
  • Double: Represents floating-point numbers with decimal precision.
  • Char: Represents individual characters.
  • Bool: Represents boolean values, either True or False.

These basic data types allow us to represent simple values in our programs. However, Haskell also allows us to define our own custom data types using algebraic data types (ADTs).

Algebraic Data Types (ADTs)

In Haskell, ADTs are used to define complex data structures by combining existing data types. They allow us to create structured values with multiple components or variants.

To define an ADT, we use the data keyword followed by the name of the type and its constructors. Constructors represent different ways in which a value of that type can be created.

An example of an ADT in Haskell is the List type, which represents a collection of elements:

data List a = Empty | Cons a (List a)

In this example, the List type has two constructors: Empty represents an empty list, and Cons represents a list with an element followed by another list.

Type Inference

Haskell is a statically typed language, which means that every expression and function has a specific type. However, Haskell’s powerful type inference system allows us to omit explicit type annotations in many cases.

The compiler can often infer the types of expressions based on their usage and context. This reduces the need for explicit type declarations and makes the code more concise and readable.


Data types are essential in Haskell programming as they provide structure, organization, and behavior to values within a program. Understanding different data types helps us write more robust and expressive code.

In this article, we explored basic data types such as Int, Double, Char, and Bool. We also learned about algebraic data types (ADTs) which allow us to define custom structured values. Finally, we discussed Haskell’s powerful type inference system that eliminates the need for explicit type annotations in many cases.

By mastering data types in Haskell, you will have a solid foundation for writing functional programs that are both efficient and maintainable.

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