let predList =
  [(predicate1, Base.method1);
  (predicate2, method2);
  (predicate4; Derived.method4)
  ...
]

 while ... do
     let input = ReadInput
     if IsBase(input)
         then
             let method = List.Find matchingPredicate predList
             let baseObj = ChooseBaseObject(input)
             baseObj.method  // ????
         else
             let method = List.Find matchingPredicate predList
             let derivedObj = ChooseDerivedObject(input)
             derivedObj.method  // ????

let wrap_method1 (x :Base) = x.method1()
let wrap_method2 (x :Base) =
   match x with
   | :? Derived as xd -> xd.method2()
   | _ -> x.method2()
let wrap_method4 (x :Base) =
   match x with
   | :? Derived as xd -> xd.method4()
   | _ -> ()

// use
let predList =
    [(predicate1, wrap_method1);
    (predicate2, wrap_method2);
    ... ]

while ... do
    let input = ReadInput
    let method = List.Find matchingPredicate predList
    method input