add fp2 sample

csharp-fp2/Application/AccountApplication.cs

@@ -0,0 +1,46 @@
+using CsharpFp2.Domain;
+using CsharpFp2.Infrastructure;
+
+namespace CsharpFp2.Application;
+
+// delegate type is optional, but nice to illustrate
+public delegate WithdrawResult WithdrawMoney(Guid accountId, decimal amount);
+
+public static class AccountApplication
+{
+    public static WithdrawMoney CreateWithdrawMoney(
+        LoadAccount loadAccount,
+        SaveAccount saveAccount
+    )
+    {
+        return (accountId, amount) =>
+        {
+            var account =
+                loadAccount(accountId) ?? throw new InvalidOperationException("Account not found.");
+
+            Console.WriteLine(
+                $"[App] Account loaded. Current balance: {account.Balance.Amount:0.00}"
+            );
+            Console.WriteLine($"[App] Executing withdrawal of {amount:0.00}...");
+
+            switch (AccountDomain.Withdraw(account, new Money(amount)))
+            {
+                case WithdrawResult.Success s:
+                {
+                    Console.WriteLine(
+                        $"[App] Withdrawal applied. New balance: {s.Account.Balance.Amount:0.00}"
+                    );
+                    saveAccount(s.Account);
+                    Console.WriteLine("[App] Account persisted.");
+                    return s;
+                }
+
+                case var other:
+                {
+                    Console.Error.WriteLine($"[App Error] Withdrawal failed. Error: {other}");
+                    return other;
+                }
+            }
+        };
+    }
+}

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),
+            }
+        );
+    }
+}

csharp-fp2/Domain/Money.cs

@@ -0,0 +1,8 @@
+namespace CsharpFp2.Domain;
+
+public sealed record Money(decimal Amount)
+{
+    public Money Add(Money other) => new(Amount + other.Amount);
+
+    public Money Subtract(Money other) => new(Amount - other.Amount);
+}

csharp-fp2/Infrastructure/InMemoryAccount.cs

@@ -0,0 +1,41 @@
+using CsharpFp2.Domain;
+
+namespace CsharpFp2.Infrastructure;
+
+// If we don't like working with generic delegates directly, we can
+// create custom named delegate types.
+public delegate Account? LoadAccount(Guid id);
+
+public delegate void SaveAccount(Account accunt);
+
+// If we don't want to use tuples, or you really miss the idea of a combined "interface",
+// we can create a named container
+//   public sealed record AccountPersistence(LoadAccount Load, SaveAccount Save);
+
+public static class InMemoryAccount
+{
+    public static (LoadAccount, SaveAccount) Create()
+    {
+        Dictionary<Guid, Account> store = new();
+
+        Account? GetById(Guid id)
+        {
+            var found = store.TryGetValue(id, out var account);
+            Console.WriteLine(
+                found ? $"[Repo] Loaded account {id}" : $"[Repo] Account {id} not found"
+            );
+
+            return found ? account : null;
+        }
+
+        void Save(Account account)
+        {
+            store[account.Id] = account;
+            Console.WriteLine(
+                $"[Repo] Saved account {account.Id} with balance {account.Balance.Amount:0.00}"
+            );
+        }
+
+        return (GetById, Save);
+    }
+}

csharp-fp2/Program.cs

@@ -0,0 +1,18 @@
+using CsharpFp2.Application;
+using CsharpFp2.Domain;
+using CsharpFp2.Infrastructure;
+
+Console.WriteLine("[csharp-fp2] Starting withdraw money demo...");
+
+var (loadAccount, saveAccount) = InMemoryAccount.Create();
+var withdrawMoney = AccountApplication.CreateWithdrawMoney(loadAccount, saveAccount);
+
+var accountId = Guid.NewGuid();
+Console.WriteLine($"[csharp-fp2] Seeding account {accountId} with opening balance 200.00");
+saveAccount(new Account(accountId, new Money(200m)));
+
+decimal amount = 100m;
+Console.WriteLine($"[csharp-fp2] Executing withdrawal {amount:0.00} from account {accountId}");
+withdrawMoney(accountId, amount);
+
+Console.WriteLine("[csharp-fp2] Demo completed.");

csharp-fp2/csharp-fp2.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <RootNamespace>CsharpFp2</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+</Project>