I'm working on an application where I'm sending output to a console window. If the output is an integer or double I wanted the print function to perform special formatting. For everything else just call .ToString(). Page 115 in Expert F# shows an example of how to do this sort of thing with pattern matching, here is an example (Stripped down for clarity):

let Print (t:obj) =
    let text =
        match t with
        | :? int -> (t :?> int).ToString("#,###")
        | :? double -> (t :?> double).ToString("#,###.0")
        | _ -> t.ToString()
    ...
    Console.Write(text)

Update: Like Don mentioned in the comments there is a much more succinct way to write this using ":? int as i". Thanks Don!

let Print (t:obj) =
    let text =
        match t with
        | :? int as t -> t.ToString("#,###")
        | :? double as t -> t.ToString("#,###.0")
        | _ -> t.ToString()
    ...
    Console.Write(text)

A few points:

1. The compiler complained that I didn't have a type annotation on the parameter. Not sure why this is but book shows that annotating it as an obj does the trick.
2. The :? construct looks at the type of the value.
3. The :?> operator casts the value down to a particular type.