You mostly don't need types in F#, although types often add readability, especially for function signatures (IMHO).

F# will even compile

    let add a b = a + b
    add 1 2;;

as

    val add : a:int -> b:int -> int
    val it : int = 3

or

    let add a b = a + b
    add 1ul 2ul;;

as

    val add : a:uint32 -> b:uint32 -> uint32
    val it : uint32 = 3u

So it will even infer type from the first usage of the function.