Experiment: Polish eqxweb C# template by using OneOf library to pattern match

equinox-web-csharp/Domain/Aggregate.cs

@@ -2,6 +2,7 @@
 using Equinox.Store;
 using Microsoft.FSharp.Core;
 using Newtonsoft.Json;
+using OneOf;
 using Serilog;
 using System;
 using System.Collections.Generic;
@@ -12,7 +13,7 @@ namespace TodoBackendTemplate
  public static class Aggregate
  {
  /// NB - these types and names reflect the actual storage formats and hence need to be versioned with care
- public abstract class Event
+ public abstract class Event : OneOfBase<Event.Happened, Event.Compacted>
  {
  public class Happened : Event
  {
@@ -43,21 +44,10 @@ public class State
 
  internal State(bool happened) { Happened = happened; }
 
- public static readonly State Initial = new State(false);
-
- static void Evolve(State s, Event x)
- {
- switch (x)
- {
- case Event.Happened e:
- s.Happened = true;
- break;
- case Event.Compacted e:
- s.Happened = e.Happened;
- break;
- default: throw new ArgumentOutOfRangeException(nameof(x), x, "invalid");
- }
- }
+ static void Evolve(State s, Event x) =>
+ x.Match(
+ (Event.Happened _) => s.Happened = true,
+ (Event.Compacted e) => s.Happened = e.Happened);
 
  public static State Fold(State origin, IEnumerable<Event> xs)
  {
@@ -74,22 +64,15 @@ public static State Fold(State origin, IEnumerable<Event> xs)
  }
 
  /// Defines the decision process which maps from the intent of the `Command` to the `Event`s that represent that decision in the Stream 
- public abstract class Command
+ public abstract class Command : OneOfBase<Command.MakeItSo>
  {
  public class MakeItSo : Command
  {
  }
 
- public static IEnumerable<Event> Interpret(State s, Command x)
- {
- switch (x)
- {
- case MakeItSo c:
- if (!s.Happened) yield return new Event.Happened();
- break;
- default: throw new ArgumentOutOfRangeException(nameof(x), x, "invalid");
- }
- }
+ public static Event[] Interpret(State s, Command x) =>
+ x.Match((MakeItSo _) => 
+ s.Happened ? new Event[0] : new Event [] { new Event.Happened()});
  }
 
  class Handler

equinox-web-csharp/Domain/Domain.csproj

@@ -9,6 +9,7 @@
  <PackageReference Include="Equinox" Version="1.0.3" />
  <PackageReference Include="FSharp.Core" Version="4.5.4" />
  <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
+ <PackageReference Include="OneOf" Version="2.1.127" />
  </ItemGroup>
 
 </Project>

equinox-web-csharp/Domain/Todo.cs

@@ -2,6 +2,7 @@
 using Equinox.Store;
 using Microsoft.FSharp.Core;
 using Newtonsoft.Json;
+using OneOf;
 using Serilog;
 using System;
 using System.Collections.Generic;
@@ -14,6 +15,7 @@ public static class Todo
  {
  /// NB - these types and names reflect the actual storage formats and hence need to be versioned with care
  public abstract class Event
+ : OneOfBase<Event.Added, Event.Updated, Event.Deleted, Event.Cleared, Event.Compacted>
  {
  /// Information we retain per Todo List entry
  public abstract class ItemData
@@ -95,37 +97,23 @@ public static State Fold(State origin, IEnumerable<Event> xs)
  var nextId = origin.NextId;
  var items = origin.Items.ToList();
  foreach (var x in xs)
- switch (x)
- {
- case Event.Added e:
- nextId++;
- items.Insert(0, e.Data);
- break;
- case Event.Updated e:
+ x.Switch(
+ (Event.Added e) => { nextId++; items.Insert(0, e.Data); },
+ (Event.Updated e) =>
+ {
  var i = items.FindIndex(item => item.Id == e.Data.Id);
  if (i != -1)
  items[i] = e.Data;
- break;
- case Event.Deleted e:
- items.RemoveAll(item => item.Id == e.Id);
- break;
- case Event.Cleared e:
- nextId = e.NextId;
- items.Clear();
- break;
- case Event.Compacted e:
- nextId = e.NextId;
- items = e.Items.ToList();
- break;
- default:
- throw new ArgumentOutOfRangeException(nameof(x), x, "invalid");
- }
+ },
+ (Event.Deleted e) => items.RemoveAll(item => item.Id == e.Id),
+ (Event.Cleared e) => { nextId = e.NextId; items.Clear(); },
+ (Event.Compacted e) => { nextId = e.NextId; items = e.Items.ToList(); });
  return new State(nextId, items.ToArray());
  }
- 
+
  /// Determines whether a given event represents a checkpoint that implies we don't need to see any preceding events
  public static bool IsOrigin(Event e) => e is Event.Cleared || e is Event.Compacted;
- 
+
  /// Prepares an Event that encodes all relevant aspects of a State such that `evolve` can rehydrate a complete State from it
  public static Event Compact(State state) => new Event.Compacted { NextId = state.NextId, Items = state.Items };
  }
@@ -140,6 +128,7 @@ public class Props
 
  /// Defines the operations a caller can perform on a Todo List
  public abstract class Command
+ : OneOfBase<Command.Add, Command.Update, Command.Delete, Command.Clear>
  {
  /// Create a single item
  public class Add : Command
@@ -166,38 +155,26 @@ public class Clear : Command
  }
 
  /// Defines the decision process which maps from the intent of the `Command` to the `Event`s that represent that decision in the Stream 
- public static IEnumerable<Event> Interpret(State s, Command x)
- {
- switch (x)
- {
- case Add c:
- yield return Make<Event.Added>(s.NextId, c.Props);
- break;
- case Update c:
- var proposed = new {c.Props.Order, c.Props.Title, c.Props.Completed};
+ public static IEnumerable<Event> Interpret(State s, Command x) =>
+ x.Match(
+ (Add c) => new Event[] { Make<Event.Added>(s.NextId, c.Props) },
+ (Update c) =>
+ {
+ var proposed = new { c.Props.Order, c.Props.Title, c.Props.Completed };
 
  bool IsEquivalent(Event.ItemData i) =>
  i.Id == c.Id
- && new {i.Order, i.Title, i.Completed} == proposed;
-
- if (!s.Items.Any(IsEquivalent))
- yield return Make<Event.Updated>(c.Id, c.Props);
- break;
- case Delete c:
- if (s.Items.Any(i => i.Id == c.Id))
- yield return new Event.Deleted {Id = c.Id};
- break;
- case Clear _:
- if (s.Items.Any()) yield return new Event.Cleared {NextId = s.NextId};
- break;
-
- default:
- throw new ArgumentOutOfRangeException(nameof(x), x, "invalid");
- }
+ && new { i.Order, i.Title, i.Completed } == proposed;
 
- T Make<T>(int id, Props value) where T : Event.ItemEvent, new() =>
- new T {Data = {Id = id, Order = value.Order, Title = value.Title, Completed = value.Completed}};
- }
+ if (s.Items.Any(IsEquivalent))
+ return Enumerable.Empty<Event>();
+ return new[] { Make<Event.Updated>(c.Id, c.Props) };
+ },
+ (Delete c) => s.Items.Any(i => i.Id == c.Id) ? new Event[] { new Event.Deleted { Id = c.Id } } : new Event[0],
+ (Clear _) => s.Items.Any() ? new Event[] { new Event.Cleared { NextId = s.NextId } } : new Event[0]);
+
+ static T Make<T>(int id, Props value) where T : Event.ItemEvent, new() =>
+ new T { Data = { Id = id, Order = value.Order, Title = value.Title, Completed = value.Completed } };
  }
 
  /// Defines low level stream operations relevant to the Todo Stream in terms of Command and Events