F# Delegates

In this article, you will explore the concept of F# delegates and learn how to use them to create more advanced and flexible functionality.

F# delegates make it possible for you to define functions as first-class values, meaning you can work with them as if they were first-class values.

A function is treated like any other type of data, whether it is an integer, a string, or an array, by being treated as a value. This means that a function can be passed as an argument to another function, returned from another function, or stored in a data structure.

Using F# delegate means encapsulating a method or function into a value that can be passed around or invoked later.

The delegation mechanism is a flexible and powerful way to create higher-order functions, which are often referred to as multi-purpose functions because they can be both inputs or outputs of other functions.

Table Of Content-

F# Delegates:
- Declaring Delegates In F#:

Declaring Delegates In F#

Delegate declarations define which methods can be referenced by the delegate, and which methods can be called by the delegate.

There is a possibility of a delegate referring to a method whose signature has the same as that of the delegate itself.

Delegate declarations have the following syntax:

type delegate-typename = delegate of type1 -> type2

Below is an example of declaring delegate:

// Delegate1 works with tuple arguments.
type Delegate1 = delegate of (int * int) -> int

// Delegate2 works with curried arguments.
type Delegate2 = delegate of int * int -> int

There are two delegates that can both be used to refer to any method with two int parameters and which returns an int type variable.

This is what the syntax looks like:

type1 is a representation of the argument type(s).
type2 is a representation of the return type.

Note:

There is an automatic conversion between the argument types.
The delegate can be attached to the function value, as well as to static or instance methods in the function.
A F# function value may be directly passed to a delegate constructor as an argument.
For static methods, the delegate is called by referencing the class name and the method name in the delegate call. An instance method refers to the instance of the object and the method of the object and is referred to as its name.
Calling the encapsulated function is achieved through the Invoke method of the delegate type.
Moreover, when the Invoke method name is referenced without the parentheses, delegates can also be passed as function values.

Here is an example of the delegate that Microsoft provides in this example, so that we can gain a better understanding:

Example:

type Test() = static member add(a : int, b : int) = a + b static member mul (a : int) (b : int) = a * bmember x.Add(a : int, b : int) = a + b member x.Mul (a : int) (b : int) = a * b// Delegate1 works with tuple arguments. type Delegate1 = delegate of (int * int) -> int // Delegate2 works with curried arguments. type Delegate2 = delegate of int * int -> intlet InvokeDelegate1 (dlg: Delegate1) (a: int) (b: int) = dlg.Invoke(a, b) let InvokeDelegate2 (dlg: Delegate2) (a: int) (b: int) = dlg.Invoke(a, b)// For static methods, use the class name, the dot operator, and the // name of the static method. let del1 : Delegate1 = new Delegate1( Test.add ) let del2 : Delegate2 = new Delegate2( Test.mul ) let test = Test() // For instance methods, use the instance value name, the dot operator, and the instance method name. let del3 : Delegate1 = new Delegate1( test.Add ) let del4 : Delegate2 = new Delegate2( test.Mul )for (a, b) in [ (50, 100); (20, 5) ] do printfn "%d + %d = %d" a b (InvokeDelegate1 del1 a b) printfn "%d * %d = %d" a b (InvokeDelegate2 del2 a b) printfn "%d + %d = %d" a b (InvokeDelegate1 del3 a b) printfn "%d * %d = %d" a b (InvokeDelegate2 del4 a b)

And the output will be as:

50 + 100 = 150
50 * 100 = 5000
50 + 100 = 150
50 * 100 = 5000
20 + 5 = 25
20 * 5 = 100
20 + 5 = 25
20 * 5 = 100

Example Explanation

We wrote this F# code that defines a Test class with both static and instance methods and delegates to invoke those methods. Here’s a breakdown of the code:

The Test class contains static methods add and mul for addition and multiplication of two integers, respectively. Additionally, it has instance methods Add and Mul that perform the same operations but are called on an instance of the Test class.
We defined two delegate types called Delegate1 and Delegate2, which specify the function signatures they can represent. Delegate1 takes a tuple of two integers as arguments and returns an integer, while Delegate2 takes two integer arguments and returns an integer.
We wrote two functions, InvokeDelegate1 and InvokeDelegate2, that take delegate instances along with two integer arguments, and then invoke the delegate instances using the Invoke method with those integer arguments.
We created delegate instances del1, del2, del3, and del4, which represent static and instance methods of the Test class by using the new keyword.
We also created an instance of the Test class called test.
Finally, we used a for loop to iterate over a list of tuples containing (a, b) pairs. We called the InvokeDelegate1 and InvokeDelegate2 functions with del1, del2, del3, and del4 delegates, passing a and b as arguments. We printed the results of these method invocations using the printfn statement.

Conclusion

Delegates are a powerful language feature in F# that allow you to represent functions as data. They are particularly useful in event handling and functional programming, where higher-order functions can take delegates as arguments. Understanding delegates in F# will help you write more expressive and functional code.

We value your feedback.