F# - Delegates

A delegate is a reference type variable that holds the reference to a method. The reference can be changed at runtime. F# delegates are similar to pointers to functions, in C or C++.

Declaring Delegates

Delegate declaration determines the methods that can be referenced by the delegate. A delegate can refer to a method, which have the same signature as that of the delegate.

Syntax for delegate declaration is −

type delegate-typename = delegate of type1 -> type2

For example, consider the delegates −

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

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

Both the delegates can be used to reference any method that has two int parameters and returns an int type variable.

In the syntax −

type1 represents the argument type(s).

type2 represents the return type.

Please note −

The argument types are automatically curried.

Delegates can be attached to function values, and static or instance methods.

F# function values can be passed directly as arguments to delegate constructors.

For a static method the delegate is called by using the name of the class and the method. For an instance method, the name of the object instance and method is used.

The Invoke method on the delegate type calls the encapsulated function.

Also, delegates can be passed as function values by referencing the Invoke method name without the parentheses.

The following example demonstrates the concept −

Example

type Myclass() =
   static member add(a : int, b : int) =
      a + b
   static member sub (a : int) (b : int) =
      a - b
   member x.Add(a : int, b : int) =
      a + b
   member x.Sub(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 -> int

let 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( Myclass.add )
let del2 : Delegate2 = new Delegate2( Myclass.sub )
let mc = Myclass()

// For instance methods, use the instance value name, the dot operator, 
// and the instance method name.

let del3 : Delegate1 = new Delegate1( mc.Add )
let del4 : Delegate2 = new Delegate2( mc.Sub )

for (a, b) in [ (400, 200); (100, 45) ] 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)

400 + 200 = 600
400 - 200 = 200
400 + 200 = 600
400 - 200 = 200
100 + 45 = 145
100 - 45 = 55
100 + 45 = 145
100 - 45 = 55