Where Is Data Type in Haskell?


Larry Thompson

In Haskell, data types play a crucial role in defining and manipulating values. They allow us to specify the structure and behavior of variables in our programs. Unlike languages like C or Java, where data types are explicitly declared, Haskell uses a type inference system to automatically determine the type of an expression.

Basic Data Types

Haskell provides several built-in basic data types that represent commonly used values:

  • Int: Represents integer values.
  • Float: Represents floating-point numbers.
  • Bool: Represents boolean values (either True or False).
  • Char: Represents single characters.
  • String: Represents sequences of characters.

These basic data types can be combined and manipulated using various operators and functions provided by Haskell’s standard library. For example, we can perform arithmetic operations on integers and floating-point numbers, compare boolean values using logical operators, and manipulate strings using string manipulation functions.

User-Defined Data Types

In addition to the basic data types, Haskell also allows us to define our own custom data types using the data keyword. This is one of the key features that sets Haskell apart from other programming languages.

To define a new data type, we use the following syntax:

data TypeName = Constructor1 | Constructor2 | ..

The TypeName is the name we give to our new data type, while the Constructors are the possible values it can take. Each constructor can optionally have parameters that allow us to provide additional information when creating values of this type.

For example, let’s say we want to define a data type called Shape that represents different geometric shapes:

data Shape = Circle Float | Rectangle Float Float

In this example, the Shape data type has two constructors: Circle and Rectangle. The Circle constructor takes a single parameter, which represents the radius of the circle. The Rectangle constructor takes two parameters, which represent the width and height of the rectangle.

We can now create values of type Shape by using the defined constructors:

myCircle :: Shape
myCircle = Circle 5.0

myRectangle :: Shape
myRectangle = Rectangle 3.0 4.0

We can also pattern match on these constructors to perform different operations based on the shape:

area :: Shape -> Float
area (Circle r) = pi * r * r
area (Rectangle w h) = w * h

Type Signatures

In Haskell, it is considered good practice to provide explicit type signatures for functions. A type signature specifies the types of the function’s arguments and return value.

To specify a type signature, we use the :: operator. For example:

area :: Shape -> Float

This type signature states that area is a function that takes a value of type Shape as an argument and returns a value of type Float.

Type Inference

One of Haskell’s most powerful features is its ability to automatically infer types without explicit declarations. This is made possible by its strong static typing system.

For example, if we define a function add that takes two integer arguments and returns their sum:

add x y = x + y

Haskell will automatically infer that the type of add is:

add :: Int -> Int -> Int

This means that add takes two integer arguments and returns an integer value.


In Haskell, data types are an essential part of the language’s expressive power. They allow us to define our own custom types, pattern match on them, and provide type safety through strong static typing and type inference. Understanding how to work with data types is crucial for writing robust and reliable Haskell programs.

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