Question

I’ve been struggling to get this to compile for about an hour. It must be something stupid. Can you spot it?

in my lib project:

namespace TravelerStuff
open System

type Traveler =         
    abstract GetData : unit -> unit

type public DeltaTraveler() =
    interface Traveler with
         member v.GetData () =

        printf "hello"

and in my console test app:

[<EntryPoint>] let main _ =
    let traveler = new TravelerStuff.DeltaTraveler()
    traveler.GetData // this line won't compile: (The field, constructor or member 'GetData' is not defined)

Answer 1

You need to upcast. F# currently won’t do it for you in this situation.

(traveler :> TravelerStuff.Traveler).GetData()

// open the namespace to reduce typing.

open TravelerStuff
(traveler :> Traveler).GetData()

Snip from F# docs.

In many object-oriented languages,
upcasting is implicit; in F#, the
rules are slightly different.
Upcasting is applied automatically
when you pass arguments to methods on
an object type. However, for let-bound
functions in a module, upcasting is
not automatic, unless the parameter
type is declared as a flexible type.
For more information, see Flexible Types (F#).