How Do You Create a Data Structure in Haskell?


Scott Campbell

Creating a Data Structure in Haskell

Haskell is a powerful functional programming language that offers a wide range of data structures to handle complex problems efficiently. In this tutorial, we will explore the process of creating a data structure in Haskell, step by step.

Step 1: Defining the Data Type

To begin with, we need to define the data type that represents our desired structure. This can be done using the data keyword followed by the name of our type and its constructors. Constructors are essentially functions that create values of our type.


data MyData = Constructor1 Int String
            | Constructor2 Bool
            | Constructor3 Char

In this example, we have defined a data type called MyData with three constructors: Constructor1, Constructor2, and Constructor3. Each constructor takes different arguments.

Step 2: Implementing Functions for the Data Type

Once we have defined our data type, we can implement various functions to manipulate it. These functions can perform operations like accessing values, modifying values, or creating new instances of our data type.


getValue :: MyData -> Int
getValue (Constructor1 x _) = x

modifyValue :: MyData -> Int -> MyData
modifyValue (Constructor1 _ y) newX = Constructor1 newX y

createInstance :: Int -> String -> MyData
createInstance x y = Constructor1 x y

In this example, we have implemented three functions: getValue, modifyValue, and createInstance. The getValue function extracts the integer value from an instance of MyData.

The modifyValue function takes an instance of MyData and a new integer value, and returns a modified instance with the updated value. The createInstance function creates a new instance of MyData with the given integer and string arguments.

Step 3: Using the Data Structure

Now that we have defined our data type and implemented functions for it, we can start using our data structure in other parts of our Haskell program.


main :: IO ()
main = do
    let instance1 = createInstance 10 "Hello"
    let instance2 = modifyValue instance1 20
    putStrLn $ "Value of instance2: " ++ show (getValue instance2)

In this example, we create an instance of MyData using the createInstance function. We then modify its value using the modifyValue function. Finally, we print the updated value using the $, show, and putStrLn functions.

Tips for Creating Data Structures in Haskell:

  • Pure Functions: When working with data structures in Haskell, it is recommended to use pure functions that don’t have side effects. Pure functions ensure referential transparency and make your code easier to reason about.
  • Type Safety: Haskell’s strong type system allows you to create data structures with well-defined types.

    Leveraging this feature ensures type safety throughout your program.

  • Patter Matching: Pattern matching is a powerful technique in Haskell that allows you to handle different cases of your data structure efficiently. Utilize pattern matching to simplify your code and handle different scenarios effectively.


Creating a data structure in Haskell involves defining a data type with constructors and implementing functions to manipulate the data. By following best practices like using pure functions, leveraging the type system, and utilizing pattern matching, you can create robust and efficient data structures in Haskell. Happy coding!

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