How do you define this statically resolved type parameter?

Question

given

open System
open System.Windows
open System.Windows.Input
open System.ComponentModel

type RelayCommand (canExecute:(obj -> bool), action:(obj -> unit)) =
    let event = new DelegateEvent<EventHandler>()
    interface ICommand with
        [<CLIEvent>]
        member x.CanExecuteChanged = event.Publish
        member x.CanExecute arg = canExecute(arg)
        member x.Execute arg = action(arg)
    member x.CheckCanExecute (sender:obj) (eventArgs:EventArgs) = event.Trigger([| sender;eventArgs  |])

how do I write a statically resolved type parameterized function that can satisfy a call to CheckCanExecute ?

while this function works it doesn't help me learn statically resolved type parameter syntax

let checkCanExecute (c:RelayCommand) = c.CheckCanExecute (box this) (EventArgs())

I expected this to work

let checkCanExecute (e:^a) = (^a: (member CheckCanExecute: sender:obj -> EventArgs -> unit ) (e, (box me),(EventArgs())))

but at the callsite checkCanExecute addCommand

I get method or object constructor 'CheckCanExecute' not found (when using the 2nd definition, the 1st compiles just fine)

how do I define a class let binding (or member binding if that is a better way to get the job done) that uses Statically Resolved Type Parameters to be able to call the method on anything that has the matching method signature?

Answer 1

I think some of the difficulty here is caused by the fact that CheckCanExecute is defined as a curried function. For members, it is probably better to go with a tupled function (the way curried functions are compiled is tricky and it might be confusing the statically resolved constraint).

If you change the RelayCommand member as follows:

 member x.CheckCanExecute (sender:obj, eventArgs:EventArgs) =  
   event.Trigger([| sender;eventArgs  |])

And make your checkCanExecute an inline function that requires tupled function:

let inline checkCanExecute (e:^a) = 
  (^a: (member CheckCanExecute: obj * EventArgs -> unit ) (e, box me,(EventArgs())))

Then the following type checks:

checkCanExecute me