CsharpClasses.cs

 	public sealed class ValidateResult : Result<bool, string>
	{
	}
  
 public abstract class Result<TValue, TError>
    {
        public bool IsOk => this is Ok;
        public sealed class Ok : Result<TValue, TError>
        {
            public TValue Value { get; }
            internal Ok(TValue value) => Value = value;
        }

        public sealed class Err : Result<TValue, TError>
        {
            public TError Error { get; }
            internal Err(TError error) => Error = error;
        }

        public static Result<TValue, TError> NewErr(TError error) => new Err(error);
        public static Result<TValue, TError> NewOk(TValue value) => new Ok(value);
    }

UsingFromFsharp.fsx

        match result with
        | :? ValidateResult.Err as e -> //<= The type 'Err' is not defined.
            log(e)
        | _ -> ()  

Fixed by ->
`
match result with
| :? Result<bool, string>.Err as e -> log ("[ERR]: " + e.Error)
| :? Result<bool, string>.Ok as o -> log ("[OKE]: " + o.Value.ToString())

   `