try and write chsarp in an fsharp style

csharpchallenge.cs

using System;


// Trying to replicate https://github.com/ijrussell/FSharpForTheMasses/blob/main/IntroductionToFSharp/4-demo-ddd-3.fsx
namespace csharpchallenge
{
  record EligibleRegisteredCustomer(string Name, string Email) : Customer;
  record RegisteredCustomer(string CustomerName, string? Email) : Customer;
  record UnregisteredCustomer(string Name) : Customer;

  abstract record Customer;

  class Program
  {
    static decimal CalculateOrderTotal(Customer customer, decimal spend)
    {
      var discount =
          customer switch
          {
            EligibleRegisteredCustomer _ when spend >= 100M => spend * 0.1m,
            _ => 0m
          };
      return spend - discount;
    }


    static void Main(string[] args)
    {

      var john = new EligibleRegisteredCustomer("John", "john@test.org");
      var mary = new EligibleRegisteredCustomer("Mary", "mary@test.org");
      var richard = new RegisteredCustomer("Richard", null);
      var sarah = new UnregisteredCustomer("Sarah");

      var assertJohn = CalculateOrderTotal(john, 100.0m) == 90.0m;
      var assertMary = CalculateOrderTotal(mary, 99.0m) == 99.0m;
      var assertRichard = CalculateOrderTotal(richard, 100.0M) == 100.0m;
      var assertSarah = CalculateOrderTotal(sarah, 100.0M) == 100.0m;

      Console.WriteLine(assertJohn);
      Console.WriteLine(assertMary);
      Console.WriteLine(assertRichard);
      Console.WriteLine(assertSarah);

    }
  }
}