Question

Is there any way to pattern match on discriminated union functions, e.g.:-

type Test =   | A of string   | B of int   | C of char  let DefaultTest t =   match t with   | A(_) -> A(null)   | B(_) -> B(0)   | C(_) -> C('\u0000')  let a = A |> DefaultTest 

Obviously this code isn’t valid F# as DefaultTest accepts one parameter of type Test rather than ‘a -> Test. Is there any way of achieving this without specifying a value for the discriminated union?

What I’m after, ultimately, is a function which inputs a function of type ‘a -> Test and outputs Test(default value of ‘a).

Answer 1

I am not clear what you are after, but does this help?

type Foo =     | A of int     | B of string  let CallWithDefault f =     let x = Unchecked.defaultof<_>     f x  let defaultA = CallWithDefault A     let defaultB = CallWithDefault B     printfn '(%A) (%A)' defaultA defaultB