ActivePatterns

let (|Int|_|) str = 
    match Int32.TryParse(str) with
    | (true,int) -> Some int
    | _ -> None

let (|Bool|_|) str =
    match Boolean.TryParse(str) with
    | (true,bool) -> Some bool
    | _ -> None

let (|DeNull|_|) arg =
    match arg with
    | null -> None
    | a -> Some a

let (|Date|_|) str =
    match DateTime.TryParse(str) with
    | (true,date) -> Some date
    | _ -> None

let whatAmI =
    function
    | DeNull str ->
        match str with
        | Int _ -> printfn "I'm an Int"
        | Bool _ -> printfn "I'm a Bool"
        | Date _ -> printfn "I'm a DateTime"
        | _ -> printfn "I'm something which I don't know, but definitely no dumb null"
    | _ -> printfn "I'm a dumb null from another planet"

That's what I love F# and ActivePatterns for:
It makes my code readable and succinct.