demo-ddd-without-oop/csharp-fp3/Domain/Account.cs

using CsharpFp3.Library;
using static CsharpFp3.Library.ResultModule;

namespace CsharpFp3.Domain;

public sealed record Account(Guid Id, Money Balance);

public abstract record AccountError
{
    public sealed record OpeningBalanceMustBeNonNegative : AccountError;

    public sealed record AmountMustBePositive : AccountError;

    public sealed record InsufficientBalance(Money Balance, Money Amount) : AccountError;

    public sealed record AccountNotFound(Guid AccountId) : AccountError;
}

public static class AccountDomain
{
    public static Result<Account, AccountError> Open(Guid id, Money openingBalance) =>
        openingBalance.Amount < 0m
            ? Fail(new AccountError.OpeningBalanceMustBeNonNegative())
            : Ok(new Account(id, openingBalance));

    public static Result<Account, AccountError> Withdraw(Account account, Money amount) =>
        (account.Balance.Amount, amount.Amount) switch
        {
            (_, <= 0m) => Fail(new AccountError.AmountMustBePositive()),

            var (balance, transaction) when balance < transaction => Fail(
                new AccountError.InsufficientBalance(account.Balance, amount)
            ),

            var (balance, transaction) => Ok(
                account with
                {
                    Balance = new Money(balance - transaction),
                }
            ),
        };
}