Extend enumerations types in F#

Question

Another question related to F# feature called "Type extensions".

It seems impossible to extend enumerations in F#. I use C# Extensions Methods a lot for extending enums: add range validation logic, method that returns string representation etc.

Unfortunately it seems possible extend only discriminated union but impossible to extend simple enumerations:

1. Intrinsic extension

// CustomEnum.fs
module CustomEnumModule

type CustomEnum = 
    | Value1 = 1
    | Value2 = 2

// Trying to split definition of the enum
type CustomEnum with 
    | Value3 = 3

Error: "error FS0010: Unexpected symbol '|' in member definition"

2. Optional extension

// CustomEnumEx.fs
open CustomEnumModule

type CustomEnum with
    member public x.PrintValue() =
        printfn "%A" x

Error: "error FS0896: Enumerations cannot have members"

It seems weird for me because (1) we can treat simple enumerations as a special case of full-featured discriminated union and we can extend discriminated unions and (2) extending .NET enums is a good way to add some features (including FP-features) to existing infrastructure.

Is this behavior intentional or this is simple bug in implementation?

P.S. Unfortunately F# Spec is silent in this regard, or at least I can't find any proofs for one or another behavior there.

Answer 1

It is possible to create a module with the same name as a type which is similar to extending the type:

type CustomEnum = Value1 = 1 | Value2 = 2

[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module CustomEnum =
    let Print = function
    | CustomEnum.Value1 -> "One"
    | CustomEnum.Value2 -> "Two"
    | _ -> invalidArg "" ""

let value = CustomEnum.Value1

let s = CustomEnum.Print value