How to downcast from obj to option?

Question

I have a function that takes a parameter of type object and needs to downcast it to an option.

member s.Bind(x : obj, rest) =
    let         


        

Answer 1

There isn't any nice way to solve this problem currently.

The issue is that you'd need to introduce a new generic type parameter in the pattern matching (when matching against option<'a>), but F# only allows you to define generic type parameters in function declarations. So, your only solution is to use some Reflection tricks. For example, you can define an active pattern that hides this:

let (|SomeObj|_|) =
  let ty = typedefof>
  fun (a:obj) ->
    let aty = a.GetType()
    let v = aty.GetProperty("Value")
    if aty.IsGenericType && aty.GetGenericTypeDefinition() = ty then
      if a = null then None
      else Some(v.GetValue(a, [| |]))
    else None

This will give you None or Some containing obj for any option type:

let bind (x : obj) rest =   
    match x with    
    | SomeObj(x1) -> rest x1
    | _ -> failwith "Invalid type"

bind(Some 1) (fun n -> 10 * (n :?> int))