Printing F# discriminated union

Have you tried printfn "%A" ast? The %A specifier takes into consideration the StructuredFormatDisplayAttribute^[MSDN], if present.

To convert a discriminated union into a string, you should use sprintf "%A" ast instead of ast.ToString().

If you want Enum.GetName, you can use Microsoft.FSharp.Reflection namespace. See What is the Enum.GetName equivalent for F# union member?.