Is it possible to pass discriminated union tags as arguments?

Question

Is it possible to pass the type of a discriminated union tag to another function so it can use it for pattern matching?

Non working example of what I mean:

Answer 1

Discriminated unions works best if there is no semantic overlap between the cases.

In your example, each case contains the same component with the same meaning, a string indicating "the name of the animal". But that's a semantic overlap! The discriminating union will then force you to make distinctions you don't want to: You don't want to be forced to discriminate between the "name of a pig" and the "name of a cow"; you just want to think of "the name of an animal".

Let's make a type that fits better:

type Animal = Pig  | Cow  | Fish
type Pet = Animal * string

let animals = [(Pig, "Mike"); (Fish, "Eve"); (Pig, "Romeo")

With that type, filtering out non-Pigs is a one-liner:

animals |> List.filter (fst >> (=) Pig) 

If not every animal has a name, use an option type:

type Pet = Animal * string option

You would use a discriminated union for your animals if you knew that, say, every Pig has a name, but no Fish does: those cases have no overlap.