match with typeof in fsharp

Question

The first match works, but not the second one. Is there any way to match without declaring variables, beside using a chain of if/elif ?

(Note that I use the value elem, while I match the variable t)

  let t = typeof<string>
  match propType with
  | t               ->  elem.GetValueAsString() :> obj
  | typeof<string>  ->  elem.GetValueAsString() :> obj

Answer 1

Your first pattern actually doesn't match typeof<string>. It binds propType to a new value t shadowing the previous t which is equals to typeof<string>.

Since typeof<string> is not a literal, the second pattern doesn't work as well (although it is a redundant pattern in your example). You have to use when guard as follows:

  match propType with
  | t when t = typeof<string> -> elem.GetValueAsString() :> obj
  | t ->  elem.GetValueAsString() :> obj

Answer 2

If you're trying to match against the type of a value, you can use the :? operator

Example :

let testMatch (toMatch:obj) = match toMatch with
                        | :? string as s -> s.Split([|';'|]).[0]
                        | :? int as i -> (i+1).ToString()
                        | _ -> String.Empty