What Is Haskell Data Type?


Scott Campbell

The Haskell programming language is known for its strong static typing system, which is based on the concept of data types. In Haskell, every expression has a type associated with it, and this type determines what operations can be performed on that expression. Understanding Haskell data types is crucial for writing correct and efficient programs.

What are Data Types?

Data types are a way to classify and categorize values in programming languages. They define the range of values that a variable can hold and the operations that can be performed on those values. In Haskell, data types play a central role in ensuring type safety and preventing runtime errors.

Primitive Data Types

Haskell provides several primitive data types that represent basic values:

  • Int: This data type represents integers, both positive and negative.
  • Float: This data type represents single-precision floating-point numbers.
  • Double: This data type represents double-precision floating-point numbers.
  • Char: This data type represents individual characters.
  • Bool: This data type represents boolean values (True or False).

User-defined Data Types

In addition to primitive data types, Haskell allows you to define your own custom data types using the ‘data’ keyword. These user-defined data types can have multiple constructors and fields, providing a powerful way to model complex real-world entities.

The syntax for defining a new data type in Haskell is as follows:

data TypeName = Constructor1 FieldType1 | Constructor2 FieldType2 | ..

The ‘TypeName’ is the name of the new data type, and each ‘Constructor’ represents a different way of constructing a value of that type. Each constructor can have one or more fields, each with its own type.

Type Inference

Haskell has a powerful type inference system that can often determine the types of expressions without explicit type annotations. This makes Haskell code concise and expressive while still maintaining strong static typing.

For example, if you write the following code:

x = 42

Haskell will infer that the type of ‘x’ is ‘Int’ because it is assigned an integer value.

Type Safety

One of the key benefits of Haskell’s strong static typing system is type safety. The compiler checks that all operations are performed on values of compatible types, preventing many common programming errors at compile time.

For example, if you try to add an ‘Int’ and a ‘Bool’, the compiler will raise a type error because these types are not compatible for addition.


Data types are fundamental to Haskell programming. They provide a way to classify and categorize values, ensuring type safety and preventing runtime errors. Understanding Haskell data types is essential for writing correct and efficient programs in this powerful language.

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