F# Interface

Last Updated : 17 Mar 2025

F# provides Interface type. It provides pure abstraction. It is a collection of abstract methods. A Class which implements interface must provide definition for its all methods.

Syntax:

// Interface declaration:
[ attributes ]
type interface-name =
    [ interface ]     [ inherit base-interface-name ...]
    abstract member1 : [ argument-types1 -> ] return-type1
    abstract member2 : [ argument-types2 -> ] return-type2
    ...
[ end ]

F# Interface Example

In this example we have created an interface and implemented it into a class.

type IEmployee =
   abstract member ShowName : unit -> unit

type Manager(id:int, name:string) =
   interface IEmployee with
      member this.ShowName() = printfn "Id = %d \nName = %s" id name

let manager = new Manager(100,"RajKumar")
//manager.ShowName()    // error: you can't directly access interface abstract method by using class object.
// so, we need to upcast class type object to interface type by using :> operator.
(manager :> IEmployee).ShowName()

Output:

Id = 100
Name = RajKumar

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