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C# prototype of Decider pattern. F# version:
namespace EsBankAccount.Account;
using Events = IReadOnlyCollection<IEvent>;
public record Transaction(decimal Amount, DateTime Date);
// events
public interface IEvent { } // used to mimic a discriminated union
public record Deposited(Transaction Transaction) : IEvent;
public record Withdrawn(Transaction Transaction) : IEvent;
public record Closed(DateTime Date) : IEvent;
// commands
public interface ICommand { }
public record Deposit(decimal Amount, DateTime Date) : ICommand;
public record Withdraw(decimal Amount, DateTime Date) : ICommand;
public record Close(DateTime Date) : ICommand;
public record State(decimal Balance, bool IsClosed)
public static readonly State Initial = new(0, false);
public static class Decider
// handle state
private static State Evolve(State state, IEvent @event) =>
@event switch
Deposited deposited => state with { Balance = state.Balance + deposited.Transaction.Amount },
Withdrawn withdrawn => state with { Balance = state.Balance - withdrawn.Transaction.Amount },
Closed _ => state with { IsClosed = true },
_ => state
public static State Fold(this IEnumerable<IEvent> history, State state) =>
history.Aggregate(state, Evolve);
public static State Fold(this IEnumerable<IEvent> history) =>
public static bool IsTerminal(this State state) => state.IsClosed;
// handle commands
public static Events Decide(this State state, ICommand command) =>
command switch
Deposit c => Deposit(c),
Withdraw c => Withdraw(c),
Close c => Close(state, c),
_ => throw new NotImplementedException()
private static Events Deposit(Deposit c) =>
new Deposited(new(c.Amount, c.Date)).Singleton();
private static Events Withdraw(Withdraw c) =>
new Withdrawn(new(c.Amount, c.Date)).Singleton();
private static Events Close(State state, Close c)
var events = new List<IEvent>();
if (state.Balance > 0)
events.Add(new Withdrawn(new(state.Balance, c.Date)));
events.Add(new Closed(c.Date));
return events;
// helpers
public static Events Singleton(this IEvent e) => new IEvent[1] { e };
namespace EsBankAccount.Tests;
using EsBankAccount.Account;
public class AccountShould
public void Make_a_deposit() =>
.Decide(new Deposit(5, DateTime.MinValue))
.Equal(new Deposited(new(5, DateTime.MinValue)).Singleton());
public void Make_a_withdrawal() =>
.Decide(new Withdraw(5, DateTime.MinValue))
.Equal(new Withdrawn(new(5, DateTime.MinValue)).Singleton());
public void Close_the_account_and_withdraw_the_remaining_amount() =>
new IEvent[]
new Deposited(new(5, DateTime.MinValue)),
new Deposited(new(5, DateTime.MinValue))
.Decide(new Close(DateTime.MinValue))
.Equal(new IEvent[]
new Withdrawn(new(10, DateTime.MinValue)),
new Closed(DateTime.MinValue)
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