add fp2 sample

csharp-fp2/Domain/Account.cs

@@ -0,0 +1,42 @@
+namespace CsharpFp2.Domain;
+
+public sealed record Account(Guid Id, Money Balance);
+
+public abstract record WithdrawResult
+{
+    public sealed record Success(Account Account) : WithdrawResult;
+
+    public sealed record AmountMustBePositive : WithdrawResult;
+
+    public sealed record InsufficientBalance(Money balance, Money amount) : WithdrawResult;
+}
+
+public static class AccountDomain
+{
+    public static Account Open(Guid id, Money openingBalance)
+    {
+        // Assuming we don't intend to allow invalid open balances to be passed
+        // here, we can view this as a technical (i.e. dev-time) error and
+        // stick to an exception.
+        if (openingBalance.Amount < 0)
+            throw new ArgumentOutOfRangeException(nameof(openingBalance));
+
+        return new Account(id, openingBalance);
+    }
+
+    public static WithdrawResult Withdraw(Account account, Money amount)
+    {
+        if (amount.Amount <= 0)
+            return new WithdrawResult.AmountMustBePositive();
+
+        if (account.Balance.Amount < amount.Amount)
+            return new WithdrawResult.InsufficientBalance(account.Balance, amount);
+
+        return new WithdrawResult.Success(
+            account with
+            {
+                Balance = account.Balance.Subtract(amount),
+            }
+        );
+    }
+}