Question

I’m trying to see if inline can be applied to an implemented method so that the specific type coming in doesn’t have to be spelled out. I’ve done this with one off (Not inherited/implemented) methods, but trying to also do using an interface.

type public IBookInteraction = 
  abstract inline CreateBook : 'a -> MethodResult<BasicBookModel>


type public BookInteraction(?userInteraction) =

  interface IBookInteraction with

  member inline x.CreateBook(bookModel) =
    let userId = (^a : (member UserId : Int32 with get) (bookModel))

    MethodResult<BasicBookModel>()

I’m guessing there’s a way to do this, but it doesn’t work with a generic operator(?) in the interface method signature.

Answer 1

I don’t believe it’s possible to have abstract inline methods. Even if you could, your code wouldn’t work, because your interface definition promises that users can call it with any 'a, but your implementation places a static member constraint on 'a – in a hypothetical world where F# supported abstract inline methods, the declaration of the method on the interface would also need to include the constraint.

In any case, to see why it’s not possible for F# to support abstract inline methods, consider what inline means: the code that you write to implement the method will be essentially copied and pasted into the call site. However, with an abstract method, you don’t know the concrete type that is defining the implementation of the method, so there’s no way to figure out at compile time what code you’re supposed to be inlining!