Add type constraint to derived type F# (This code is not sufficiently generic)

Question

I have this interface:

type IMovingFunc< 'T > =
    abstract member Add : 'T -> unit

Now I want to create a generic type that implements Add function and uses (+) operator:

type MovingSum< ^T >(initial : ^T) =
    let mutable sum = initial

    interface IMovingFunc< ^T> with
        member inline this.Add x = sum <- sum + x

Unfortunately, I'm getting this error:

This code is not sufficiently generic. The type variable ^T when ^T : (static member ( + ) : ^T * ^T -> ^T) could not be generalized because it would escape its scope.

I've tried adding type constraint to MovingSum but it didn't help:

type MovingSum< ^T when ^T : (static member ( + ) : ^T * ^T -> ^T)>(initial : ^T) 

Could you please tell me how to fix this or is there other way to achieve this?

Answer 1

Tomas's answer is good. However, for completeness: it is possible to use statically resolved type parameters on types:

type MovingSum< ^T when ^T : (static member (+) : ^T * ^T -> ^T)>(initial : ^T) =
    let mutable sum = initial

    member inline this.Add(x:^T) = sum <- sum + x

let m = MovingSum(1) in m.Add(3) // works!

But note that:

1. Member constraints need to be included where the parameter is declared.
2. I get a few spurious warnings here, indicating that maybe this language feature is not fully-baked...
3. This doesn't help you anyway, since interface members can't be inlined - given a value of an interface type, the compiler doesn't have any way to determine the runtime type that implements the interface, and so it's impossible to know what implementation to inline.