[SOLVED] How to implement an F# interface with a member returing an instance of that interface?

How to implement an F# interface with a member returing an instance of that interface?

Let's say I have the following interface in F#:

type InterfaceA =
 abstract Magic : InterfaceA -> InterfaceA

How can I implement such interface? When I try to do it like this:

type MyTypeA = {x:int} with
 interface InterfaceA with
  member self.Magic another = 
   {x=self.x+another.x}

I get the error: This expression was expected to have type 'InterfaceA' but here has type 'MyTypeA'

Solution

To fix the type error, you need to explicitly cast the returned value to the InterfaceA type - unlike for example C#, F# does not do this automatically:

type InterfaceA =
 abstract Magic : InterfaceA -> InterfaceA
 abstract Value : int

type MyTypeA = 
  {x:int} 
  interface InterfaceA with
    member self.Value = self.x
    member self.Magic another = 
      { x=self.x+another.Value } :> InterfaceA

Note that your code also did not work because another was of type InterfaceA and so it did not have the x field you could access. To fix this, I added a member Value to the interface.