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'
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.
Posting as an alternative that's not really better, just different:
As of F# 6, you can also annotate the return type and the compiler will infer what you mean:
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 : InterfaceA =
{ x=self.x+another.Value }