Discriminated Union label to string

Question

given the following Discriminated Union:

type A = B of string | C of int

How can I get the constructor B name?

A.B.ToString()
// would return something like:
val it : string = "FSI_0045+it@109-19"
// when I desire
val it : string = "B"

for example with this type it works:

type D = E | F

D.E.ToString();;
val it : string = "E"

I normally get the string name of an instance of the DU with

let stringFromDU (x: 'a) =
  match FSharpValue.GetUnionFields(x, typeof<'a>) with
  | case, _ -> case.Name

But in this case, I do not have an instance, I just want to serialize the label name.

Answer 1

If you enable the latest language version, e.g. by passing --langversion:preview to FSI or setting

<PropertyGroup>
   <LangVersion>preview</LangVersion>
</PropertyGroup>

in your .fsproj, the following will work:

type A = B of int
let n = nameof A.B

Note: with F# 5 this will be supported out of the box :-)

Answer 2

You're using FSharpValue from FSharp.Reflection namespace in your example. Note that there's another class in that library for handling scenarios where you want to work with types only, FSharpType.

let cases = FSharpType.GetUnionCases(typeof<A>)

Outside of unions, it also provides helpers for other operations on F# native types.