'base' values may only be used to make direct calls to the base implementations of overridden members

Question

Why can't I call the base implementation of f here:

type Base = 
    abstract f : int -> int -> int
    default this.f (x : int) (y : int) : int = x + y

type Derived = 
    inherit Base
    override this.f (x : int) (y : int) : int = base.f -x -y

The call to base.f elicits this compiler error:

error FS0419: 'base' values may only be used to make direct calls to the base implementations of overridden members

If I change f to take a single argument then it compiles. Presumably this is something to do with curried parameters vs tupled parameters, but the above code looks fine to me.

Answer 1

I believe that the issue is that base can't be captured by a closure - the call has to be made directly. However, overriding a curried function automatically creates a closure since only the first argument is applied immediately. Therefore, even though it looks like you are indeed using the base value to make a direct call to the base implementation of the overridden member, you're actually using the base value within a closure, which is illegal.

Unfortunately, I don't think there's any great way to work around this issue. Generally, you should avoid curried members when possible, but here's one alternative:

type Base = 
    abstract f : int -> (int -> int)
    default this.f (x : int) = fun y -> x + y

type Derived = 
    inherit Base
    override this.f x = 
       let fn = base.f -x
       fun y -> fn -y