let inline tryParse s =
    let mutable tmp = Unchecked.defaultof<_>
    if ( ^a : (static member TryParse : string * byref< ^a> -> bool ) (s, &tmp) )
    then Some tmp
    else None

let (|Int|_|) s : int option = tryParse s
let (|Int64|_|) s : int64 option = tryParse s
let (|Int16|_|) s : int16 option = tryParse s
let (|Guid|_|) s : Guid option = tryParse s

let s = "F5C2A6F4-9578-43AA-9D12-B166BBB33737"

match s with
| Int i -> printfn "Int %i" i
| Int64 d -> printfn "Int64 %d" d
| Guid g -> printfn "Guid \"%A\"" g
| _ -> printfn "don't what it is: %s" s