Question

I have an enumeration in F#, like this:

type Creature =
   | SmallCreature = 0
   | MediumCreature = 1
   | GiantCreature = 2
   | HumongousCreature = 3
   | CreatureOfNondescriptSize = 4

I do not like manually typing out numbers, and I want to easily insert more items in the enumeration later without having to shift the numbers.

I tried this

type Creature =
   | SmallCreature
   | MediumCreature
   | GiantCreature
   | HumongousCreature
   | CreatureOfNondescriptSize

but it caused an error The type 'Creature' is not a CLI enum type later in the program

let input = Int32.Parse(Console.ReadLine())
let output = match EnumOfValue<int, Creature>(input) with // <---Error occurs here
    | Creature.SmallCreature -> "Rat"
    | Creature.MediumCreature -> "Dog"
    | Creature.GiantCreature -> "Elephant"
    | Creature.HumongousCreature -> "Whale"
    | Creature.CreatureOfNondescriptSize -> "Jon Skeet"
    | _ -> "Unacceptably Hideous Monstrosity"

Console.WriteLine(output)
Console.WriteLine()
Console.WriteLine("Press any key to exit...")
Console.Read() |> ignore

How can I define an enumeration without manually assigning number values to each item?

Answer 1

Unfortunately, you can’t. Are the numeric values important? If so, it somewhat escapes the intended use of enums (flags aside). You might consider a class or discriminated union in that case.

Your second example is, in fact, a discriminated union. But your later use of EnumOfValue, which expects an enum, is causing the error.

Another option is to store the enum to number mapping in a dictionary and replace the pattern matching with a dictionary lookup. Then, the numeric value of the enum would be irrelevant.

I agree that manually managing enum values is burdensome. I hope it’s addressed in a future version.