Question

I know I must be missing something really obvious here. B.GetInstance().Call() generates the error: Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved.

I’m using v1.9.9.9.

type A() =
    member x.Call() = B.GetInstance().Call()

and B() =
    static member GetInstance() = new B()
    member x.Call() = ()

I just discovered that this works: (B.GetInstance() :> B).Call()

Any idea why the cast is necessary?

Answer 1

Frequently when you’ve got a recursive set of methods whose types to infer, F# needs help. A more pleasant alternative would be to annotate the definition of B.GetInstance:

type A() =
  member x.Call() = B.GetInstance().Call()

and B() =
  static member GetInstance() : B = new B()
  member x.Call() = ()

I believe that the reason you run into this problem is that F# tries to solve all inferred types on all methods in A and B simultaneously (because they are defined as mutually recursive types), and this leads to problems, but perhaps someone from the F# team will weigh in.