How to enumerate an enum/type in F#

Question

I've got an enumeration type defined like so:

type tags = 
    | ART  = 0
    | N    = 1
    | V    = 2 
    | P    = 3
    | NULL = 4

is there a way to do a for ... in tags do ?

This is the error that I'm getting:

The value, constructor, namespace or type tags is not defined

Answer 1

Use Enum.GetValues:

let allTags = Enum.GetValues(typeof<tags>)

Answer 2

How about:

let enumToList<'a> = (Enum.GetValues(typeof<'a>) :?> ('a [])) |> Array.toList

This has the advantage of providing a strongly typed list

To use just do:

let tagList = enumToList<tags>

Answer 3

Here is a complete example that prints information about any discriminated union. It shows how to get cases of the discriminated union and also how to get the fields (in case you needed them). The function prints type declaration of the given discriminated union:

open System
open Microsoft.FSharp.Reflection

let printUnionInfo (typ:Type) = 
  printfn "type %s =" typ.Name
  // For all discriminated union cases
  for case in FSharpType.GetUnionCases(typ) do
    printf "  | %s" case.Name
    let flds = case.GetFields()
    // If there are any fields, print field infos
    if flds.Length > 0 then 
      // Concatenate names of types of the fields
      let args = String.concat " * " [ for fld in flds -> fld.PropertyType.Name ] 
      printf " of %s" args
    printfn ""    

// Example
printUnionInfo(typeof<option<int>>)

Answer 4

To make it an enum you need to explicitly give values to each case, otherwise it's a union type:

type tags = 
    | ART = 0
    | N = 1
    | V = 2
    | P = 3
    | NULL= 4
let allTags = System.Enum.GetValues(typeof<tags>)

Answer 5

Robert's right about how to generate an actual enum and get its cases. If you have a true union type, you can get the cases via the Microsoft.FSharp.Reflection.FSharpType.GetUnionCases function.

Answer 6

You can use Enum.GetValues, which returns an Array of objects that you then have to downcast to integer values. (Note: I'm using Mono's F# implementation; maybe things are different with .NET.)

Here are some functions I wrote to get a list of all enumeration values and to get the min and max values:

open System

module EnumUtil =

    /// Return all values for an enumeration type
    let EnumValues (enumType : Type) : int list =
        let values = Enum.GetValues enumType
        let lb = values.GetLowerBound 0
        let ub = values.GetUpperBound 0
        [lb .. ub] |> List.map (fun i -> values.GetValue i :?> int) 

    /// Return minimum and maximum values for an enumeration type
    let EnumValueRange (enumType : Type) : int * int =
        let values = EnumValues enumType
        (List.min values), (List.max values)

Answer 7

In .Net 5 there is a generic overload of Enum.GetValues which eliminates the need for casting.

Enum.GetValues<T>()

Answer 8

type Options = 
    | Exit          = 0
    | CreateAccount = 1

Console.WriteLine()
Console.WriteLine("Choose an option:")
let allOptions = Enum.GetValues(typeof<Options>)
for option in allOptions do
    if (option <> null) then
        Console.WriteLine(sprintf "%d: %s" (option :?> int) (option.ToString()))
let optionChosen = System.Console.ReadLine()