add fp3 sample

csharp-fp3/Domain/Account.cs

@@ -0,0 +1,44 @@
+using CsharpFp3.Library;
+
+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
+            ? Result<Account, AccountError>.Fail(new AccountError.OpeningBalanceMustBeNonNegative())
+            : Result<Account, AccountError>.Ok(new Account(id, openingBalance));
+
+    public static Result<Account, AccountError> Withdraw(Account account, Money amount) =>
+        (account.Balance.Amount, amount.Amount) switch
+        {
+            (_, <= 0m) => Result<Account, AccountError>.Fail(
+                new AccountError.AmountMustBePositive()
+            ),
+
+            var (balance, transaction) when balance < transaction => Result<
+                Account,
+                AccountError
+            >.Fail(new AccountError.InsufficientBalance(account.Balance, amount)),
+
+            var (balance, transaction) => Result<Account, AccountError>.Ok(
+                account with
+                {
+                    Balance = new Money(balance - transaction),
+                }
+            ),
+        };
+}

csharp-fp3/Domain/Money.cs

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+namespace CsharpFp3.Domain;
+
+public sealed record Money(decimal Amount);