F# Generics: A Comprehensive Guide for Developers

In this article, you will learn about F# generics concepts and how they can make your code more flexible and reusable.

F# generics are a powerful feature that allow you to write code that can work with different types, without having to rewrite the code for each specific type. By using generics, you can create functions and classes that can be used with a wide range of input types, making your code more modular and easier to maintain.

During this session, we will be covering the basics of generics, including generic functions and generic types, and we will also see how these can be used in order to write code that is more efficient and scalable.

What are F# Generics?

Generics are a way to create reusable code that can work with different types.

F# generics are implemented using type parameters, which allow you to define a function or class that can work with any type.

A generic type in F# can be used to define functions and data types that are compatible with several types of data, allowing your code to become more generic and less focused by using generics.

Using F# Generics

Generics provide you with the flexibility of defining the data types for the elements in a class or a method before the elements are actually used in the program, allowing you to delay specifying these data types. This means that you can write a class or method that can work with any type of data without having to learn any specific data type.

To use generics in F#, you can define a function or class with a type parameter.

The type parameter is represented using single quotes, and it can be any valid F# type.


In order to write a generic construct, the following syntax needs to be followed:

// Explicitly generic function.
let function-name<type-parameters> parameter-list =

// Explicitly generic method.
[ static ] member object-identifier.method-name<type-parameters> parameter-list [ return-type ] =method-body

// Explicitly generic class, record, interface, structure,
// or discriminated union.
type type-name<type-parameters> type-definition

Below are some examples:


//Generic Function let printFunc<'T> a b = printfn "%A, %A" a bprintFunc<float> 42.0 6.0

There is also the possibility of making a generic function by using the one quotation mark syntax as follows:


//Generic Function let printFunction (a: 'a) (b: 'a) = printfn "%A, %A" a bprintFunction 42.0 6.0
If you are calling generic methods or functions, it might not be necessary for you to specify the type argument when you use generic functions.

If there is any ambiguity in the type argument, you can provide it in angle brackets, just as we did in our first example.When there are more than one type of arguments, you separate them with commas.

Below is another example of swapping 2 elements in an array by defining their indexes:


let swapArrayElements<'a> (arr: 'a array) (index1: int) (index2: int) = let temp = arr.[index1] arr.[index1] <- arr.[index2] arr.[index2] <- temp printf "%A" arr swapArrayElements [| 4; 2; 7; 3; 9 |] 2 4

Example Explanation

We have provided a generic function called swapArrayElements that takes in three parameters. The first parameter is an input array of type ‘a array, where ‘a is a type parameter that represents the type of elements in the array. The second and third parameters are integers representing the indices of the two elements that need to be swapped.

Within the function, we use a temporary variable called temp to store the value of the element at the first index. Then, we assign the value of the element at the second index to the element at the first index, effectively swapping their positions. Finally, we assign the value of temp to the element at the second index, completing the swap.

To provide feedback on the swap, we have included a printf statement that prints the updated array after the swap. It’s important to note that this isn’t a standard implementation of a swap function, as it prints the updated array instead of returning it. In most cases, a swap function would return the updated array rather than printing it.

F# Generic Classes

It is also possible to write generic classes in the same way as generic functions.

This can be demonstrated in the following example:


// generic class type genericClass<'a> (x: 'a) = do printfn "%A" xlet genericString = new genericClass<string>("John") let genericSqeuence = genericClass( seq { for i in 1 ..2.. 8 -> (i, i*i) } )

Benefits of F# Generics

Using generics in F# offers several benefits, including:

  • With generics, you can write code that is reusable across multiple types, reducing the amount of code you need to write.
  • F# generics provide type safety, ensuring that you do not pass the wrong data type to a function or class.
  • Generics in F# can improve the performance of your code by reducing the need for boxing and unboxing.
  • By using generics, you can write more maintainable code, reducing the likelihood of errors and making it easier to modify and extend your code.


F# generics are a powerful feature that allows you to write reusable, type-safe, and efficient code. By using F# generics, you can improve the maintainability and scalability of your code, making it easier to develop and maintain complex applications. As you become more familiar with F# and its features, you will find that generics are an essential tool for building high-quality functional programs.

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