Augmenting Observable.Create for F# - f#

Question

I'd like to augment
public static IObservable<TSource> Create<TSource>(
Func<IObserver<TSource>, Action> subscribe)
{...}
For use in F# so that rather than calling with a Function or Action I can just use standard F# types i.e IObserver -> (unit -> unit).
How can I accomplish this?
Edit:
Adding full example. Not sure why obsAction does not work.
open System
open System.Reactive
open System.Reactive.Disposables
open System.Reactive.Linq
type Observable with
static member Create(subscribe) =
Observable.Create(fun observer -> Action(subscribe observer))
let obsDispose (observer:IObserver<_>) =
let timer = new System.Timers.Timer()
timer.Interval <- 1000.00
let handlerTick = new Timers.ElapsedEventHandler(fun sender args -> observer.OnNext("tick"))
let handlerElapse = new Timers.ElapsedEventHandler(fun sender args -> printfn "%A" args.SignalTime)
timer.Elapsed.AddHandler(handlerTick)
timer.Elapsed.AddHandler(handlerElapse)
timer.Start()
Disposable.Empty
let obsAction (observer:IObserver<_>) =
let timer = new System.Timers.Timer()
timer.Interval <- 1000.00
let handlerTick = new Timers.ElapsedEventHandler(fun sender args -> observer.OnNext("tick"))
let handlerElapse = new Timers.ElapsedEventHandler(fun sender args -> printfn "%A" args.SignalTime)
timer.Elapsed.AddHandler(handlerTick)
timer.Elapsed.AddHandler(handlerElapse)
timer.Start()
let action() =
timer.Elapsed.RemoveHandler(handlerTick)
timer.Elapsed.RemoveHandler(handlerElapse)
timer.Dispose()
action
let obsOtherAction (observer:IObserver<_>) =
let timer = new System.Timers.Timer()
timer.Interval <- 1000.00
let handlerTick = new Timers.ElapsedEventHandler(fun sender args -> observer.OnNext("tick"))
let handlerElapse = new Timers.ElapsedEventHandler(fun sender args -> printfn "%A" args.SignalTime)
timer.Elapsed.AddHandler(handlerTick)
timer.Elapsed.AddHandler(handlerElapse)
timer.Start()
new System.Action( fun () ->
timer.Elapsed.RemoveHandler(handlerTick)
timer.Elapsed.RemoveHandler(handlerElapse)
timer.Dispose())
let worksNeverStops = obsDispose |> Observable.Create |> Observable.subscribe(fun time -> printfn "Time: %A" time)
let actionWorks = obsOtherAction |> Observable.Create |> Observable.subscribe(fun time -> printfn "Time: %A" time)
let doesNotWork = obsAction |> Observable.Create |> Observable.subscribe(fun time -> printfn "Time: %A" time)

Answer 1

The problem you're facing is an FP gotcha.
In,
static member Create(subscribe) =
Observable.Create(fun observer -> Action(subscribe observer))
the type of subscribe is IObserver<_> -> unit -> unit.
Now there's a subtle difference between IObserver<_> -> unit -> unit
and IObserver<_> -> Action where Action : unit -> unit. The difference is that the former is curried, and the latter isn't.
When an observer subscribes, subscribe observer returns a method in to which () can be applied to get unit - your subscribe method will never actually be called until the last () is applied - which won't be until it un-subscribes by which point it will already be detached.
You can get over it by forcing it to not be curried:
let action() = ... | let action = (subscribe observer)
Action(action)
Further:
If you check the IL, the equivalent VB (function refs are more clearer in VB) versions for the Invoke for the FastFunc generated for
static member Create(subscribe) =
Observable.Create(fun observer -> Action(subscribe observer))
is:
Friend Function Invoke(ByVal arg As IObserver(Of a)) As Action
Return New Action(AddressOf New Observable-Create-Static#27-1(Of a)(Me.subscribe, arg).Invoke)
End Function
and for:
static member Create(subscribe) =
Observable.Create(fun observer ->
let action = subscribe observer
Action(action))
is:
Friend Function Invoke(ByVal arg As IObserver(Of a)) As Action
Return New Action(AddressOf New Observable-Create-Static#28-1(Me.subscribe.Invoke(arg)).Invoke)
End Function
AddressOf New Closure(Me.subscribe, arg).Invoke -> The subscribe function won't get called until the dispose action is called.
AddressOf New Closure(Me.subscribe.Invoke(arg)).Invoke -> The subscribe function actually gets called and the resulting action is returned as expected.
I hope it is now clear why the second case works, and not the first.