Rework and introduce more attributes

- Update LangVersion to "preview", which allows us
  to use the generic attributes feature from C# 11
- Any attribute implementing ICreateEntityAttribute
  now registers an entity for the marked type
- [Proxy<T>] registers type not owned by the module
- [Add<TEntity>] and [Add<TRelation, TTarget>]
  which will call EntityBuilder.Set(...) on registration
- A number of shorthand attributes for [Add<...>]
- Re-introduce [Singleton]
- IterActionGenerator has been cleaned up a bit ..
- .. and as a result now supports queries, which don't
  by design don't match any (non-sourced) entities
- Add [DependsOn] to modules that were missing them

src/Immersion/Immersion.csproj

@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
+    <LangVersion>preview</LangVersion>
     <TargetFramework>net6.0</TargetFramework>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>

src/Immersion/ObserverTest.cs

@@ -2,14 +2,15 @@ using System;
 using gaemstone.ECS;
 using gaemstone.Flecs;
 using static gaemstone.Bloxel.Components.CoreComponents;
-using static gaemstone.Client.Components.RenderingComponents;
 
 namespace Immersion;
 
 [Module]
+[DependsOn<gaemstone.Bloxel.Components.CoreComponents>]
+[DependsOn<gaemstone.Client.Components.RenderingComponents>]
 public class ObserverTest
 {
-	[Observer(typeof(ObserverEvent.OnSet))]
-	public static void DoObserver(in Chunk chunk, in MeshHandle _)
+	[Observer<ObserverEvent.OnSet>(Expression = "[in] Chunk, [none] (Mesh, *)")]
+	public static void DoObserver(in Chunk chunk)
 		=> Console.WriteLine($"Chunk at {chunk.Position} now has a Mesh!");
 }

src/Immersion/Program.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Diagnostics;
 using System.Threading;
 using gaemstone.Bloxel;
@@ -31,6 +31,8 @@ var window = Window.Create(WindowOptions.Default with {
 window.Initialize();
 window.Center();
 
+// universe.Modules.Register<ObserverTest>();
+
 universe.Modules.Register<gaemstone.Client.Systems.Windowing>();
 game.Set(new Canvas(Silk.NET.OpenGL.ContextSourceExtensions.CreateOpenGL(window)));
 game.Set(new GameWindow(window));

src/gaemstone.Bloxel/Systems/BasicWorldGenerator.cs

@@ -6,6 +6,7 @@ using static gaemstone.Bloxel.Constants;
 namespace gaemstone.Bloxel.Systems;
 
 [Module]
+[DependsOn<gaemstone.Bloxel.Components.CoreComponents>]
 public class BasicWorldGenerator
 {
 	private readonly FastNoiseLite _noise;

src/gaemstone.Bloxel/gaemstone.Bloxel.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
+    <LangVersion>preview</LangVersion>
     <TargetFramework>net6.0</TargetFramework>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>

src/gaemstone.Client/Components/ResourceComponents.cs

@@ -5,18 +5,20 @@ namespace gaemstone.Client.Components;
 [Module]
 public class ResourceComponents
 {
-	// Entities can have for example Texture as a tag, in which case
-	// they're the actual resource holding the data or handle.
-
 	[Tag]
 	public struct Resource {  }
 
+	// Entities can have for example Texture as a tag, in which case
+	// they're the actual resource holding the data or handle.
+    //
 	// Entities can also have a (Texture, $T) pair where $T is a resource,
 	// meaning the entity has that resource assigned as their texture.
 
-	[Tag, Relation]
+	// TODO: Reintroduce IsA when flecs bug is fixed.
+	// https://github.com/SanderMertens/flecs/issues/858
+	[Tag, Relation]//, IsA<Resource>]
 	public struct Texture {  }
 
-	[Tag, Relation]
+	[Tag, Relation]//, IsA<Resource>]
 	public struct Mesh {  }
 }

src/gaemstone.Client/Systems/FreeCameraController.cs

@@ -1,5 +1,4 @@
 using System;
-using gaemstone.Client.Components;
 using gaemstone.ECS;
 using Silk.NET.Input;
 using Silk.NET.Maths;
@@ -10,8 +9,9 @@ using static gaemstone.Components.TransformComponents;
 namespace gaemstone.Client.Systems;
 
 [Module]
-[DependsOn(typeof(CameraComponents))]
-[DependsOn(typeof(Input))]
+[DependsOn<gaemstone.Client.Components.CameraComponents>]
+[DependsOn<gaemstone.Client.Systems.Input>]
+[DependsOn<gaemstone.Components.TransformComponents>]
 public class FreeCameraController
 {
 	[Component]

src/gaemstone.Client/Systems/Input.cs

@@ -10,7 +10,7 @@ using static gaemstone.Client.Systems.Windowing;
 namespace gaemstone.Client.Systems;
 
 [Module]
-[DependsOn(typeof(Windowing))]
+[DependsOn<gaemstone.Client.Systems.Windowing>]
 public class Input
 {
 	[Component]
@@ -36,8 +36,9 @@ public class Input
 		public bool Released;
 	}
 
-	[System(typeof(SystemPhase.OnLoad))]
-	public static void ProcessInput(GameWindow window, RawInput input, TimeSpan delta)
+	[System<SystemPhase.OnLoad>]
+	public static void ProcessInput(TimeSpan delta,
+		GameWindow window, RawInput input)
 	{
 		window.Handle.DoEvents();
 

src/gaemstone.Client/Systems/MeshManager.cs

@@ -11,6 +11,9 @@ using ModelRoot = SharpGLTF.Schema2.ModelRoot;
 namespace gaemstone.Client.Systems;
 
 [Module]
+[DependsOn<gaemstone.Client.Components.RenderingComponents>]
+[DependsOn<gaemstone.Client.Components.ResourceComponents>]
+[DependsOn<gaemstone.Client.Systems.Windowing>]
 public class MeshManager
 {
 	private const uint PositionAttribIndex = 0;

src/gaemstone.Client/Systems/Renderer.cs

@@ -15,10 +15,10 @@ using static gaemstone.Components.TransformComponents;
 namespace gaemstone.Client.Systems;
 
 [Module]
-[DependsOn(typeof(gaemstone.Components.TransformComponents))]
-[DependsOn(typeof(gaemstone.Client.Components.CameraComponents))]
-[DependsOn(typeof(gaemstone.Client.Components.RenderingComponents))]
-[DependsOn(typeof(gaemstone.Client.Systems.Windowing))]
+[DependsOn<gaemstone.Client.Components.CameraComponents>]
+[DependsOn<gaemstone.Client.Components.RenderingComponents>]
+[DependsOn<gaemstone.Client.Systems.Windowing>]
+[DependsOn<gaemstone.Components.TransformComponents>]
 public class Renderer
 	: IModuleInitializer
 {
@@ -54,7 +54,7 @@ public class Renderer
 		_modelMatrixUniform  = GL.GetUniformLocation(_program, "modelMatrix");
 	}
 
-	[System(typeof(SystemPhase.PreStore))]
+	[System<SystemPhase.PreStore>]
 	public void Clear(Canvas canvas)
 	{
 		var GL = canvas.GL;
@@ -64,7 +64,7 @@ public class Renderer
 		GL.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);
 	}
 
-	[System(typeof(SystemPhase.OnStore))]
+	[System<SystemPhase.OnStore>]
 	public void Render(Universe universe, [Game] Canvas canvas,
 		in GlobalTransform cameraTransform, in Camera camera, CameraViewport? viewport)
 	{
@@ -125,7 +125,7 @@ public class Renderer
 		}
 	}
 
-	[System(typeof(SystemPhase.PostFrame))]
+	[System<SystemPhase.PostFrame>]
 	public static void SwapBuffers(GameWindow window)
 		=> window.Handle.SwapBuffers();
 

src/gaemstone.Client/Systems/TextureManager.cs

@@ -1,6 +1,5 @@
 using System;
 using System.IO;
-using gaemstone.Client.Components;
 using gaemstone.ECS;
 using Silk.NET.OpenGL;
 using SixLabors.ImageSharp;
@@ -12,9 +11,9 @@ using Texture = gaemstone.Client.Components.ResourceComponents.Texture;
 namespace gaemstone.Client.Systems;
 
 [Module]
-[DependsOn(typeof(RenderingComponents))]
-[DependsOn(typeof(ResourceComponents))]
-[DependsOn(typeof(Windowing))]
+[DependsOn<gaemstone.Client.Components.RenderingComponents>]
+[DependsOn<gaemstone.Client.Components.ResourceComponents>]
+[DependsOn<gaemstone.Client.Systems.Windowing>]
 public class TextureManager
 	: IModuleInitializer
 {

src/gaemstone.Client/Systems/Windowing.cs

@@ -26,7 +26,7 @@ public class Windowing
 		public GameWindow(IWindow handle) => Handle = handle;
 	}
 
-	[System(typeof(SystemPhase.PreFrame))]
+	[System<SystemPhase.PreFrame>]
 	public static void ProcessWindow(GameWindow window, Canvas canvas)
 		=> canvas.Size = window.Handle.Size;
 }

src/gaemstone.Client/gaemstone.Client.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
+    <LangVersion>preview</LangVersion>
     <TargetFramework>net6.0</TargetFramework>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>

src/gaemstone/ECS/Attributes.cs

@@ -0,0 +1,107 @@
+using System;
+using static gaemstone.Flecs.Core;
+
+namespace gaemstone.ECS;
+
+/// <summary>
+/// When present on an attribute attached to a type that's part of a module
+/// being registered automatically through <see cref="ModuleManager.Register"/>,
+/// an entity is automatically created and <see cref="LookupExtensions.CreateLookup"/>
+/// called on it, meaning it can be looked up using <see cref="Universe.Lookup(Type)"/>.
+/// </summary>
+public interface ICreateEntityAttribute {  }
+
+/// <summary>
+/// By default, entities registered automatically have their symbol
+/// (a globally unique identifier) set equal to their name. If they
+/// are also marked with this attribute, the symbol won't be set.
+/// </summary>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
+public class PrivateAttribute : Attribute {  }
+
+/// <summary>
+/// Register the proxied type instead of the one marked with this attribute.
+/// This can be used to make types not registered in a module available,
+/// including types registered in other assemblies.
+/// </summary>
+public class ProxyAttribute<T> : ProxyAttribute
+	{ public ProxyAttribute() : base(typeof(T)) {  } }
+
+
+/// <summary>
+/// Marked entity automatically has the specified entity added to it when
+/// automatically registered. Equivalent to <see cref="EntityBase.Add{T}"/>.
+/// </summary>
+public class AddAttribute<TEntity> : AddEntityAttribute
+	{ public AddAttribute() : base(typeof(TEntity)) {  } }
+
+/// <summary>
+/// Marked entity automatically has the specified relationship pair added to it when
+/// automatically registered, Equivalent to <see cref="EntityBase.Add{TRelation, TTarget}"/>.
+/// </summary>
+public class AddAttribute<TRelation, TTarget> : AddRelationAttribute
+	{ public AddAttribute() : base(typeof(TRelation), typeof(TTarget)) {  } }
+
+/// <seealso cref="Tag"/>
+[AttributeUsage(AttributeTargets.Struct)]
+public class TagAttribute : AddAttribute<Tag>, ICreateEntityAttribute {  }
+
+/// <summary>
+/// Marked entity represents a relationship type, meaning it may be used as
+/// the "relation" in a pair. However, this attribute is purely informational.
+/// </summary>
+/// <remarks>
+///   The relationship may have component data associated with
+///   it when added to an entity under these circumstances:
+///   <list type="bullet">
+///     <item>If marked as a <see cref="TagAttribute"/>, does not carry data.</item>
+///     <item>If marked as a <see cref="ComponentAttribute"/>, carries the relation's data.</item>
+///     <item>If marked with neither, will carry the target's data, if it's a component.</item>
+///   </list>
+/// </remarks>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
+public class RelationAttribute : Attribute, ICreateEntityAttribute {  }
+
+
+/// <seealso cref="IsA"/>
+public class IsAAttribute<TTarget> : AddAttribute<IsA, TTarget> {  }
+
+/// <seealso cref="ChildOf"/>
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
+public class ChildOfAttribute<TTarget> : AddAttribute<ChildOf, TTarget> {  }
+
+/// <seealso cref="DependsOn"/>
+public class DependsOnAttribute<TTarget> : AddAttribute<DependsOn, TTarget> {  }
+
+
+/// <seealso cref="Exclusive"/>
+public class ExclusiveAttribute : AddAttribute<Exclusive> {  }
+
+/// <seealso cref="With"/>
+public class WithAttribute<TTarget> : AddAttribute<With, TTarget> {  }
+
+
+// Base attributes for other attributes.
+
+[AttributeUsage(AttributeTargets.Struct)]
+public class ProxyAttribute : Attribute
+{
+	public Type Type { get; }
+	internal ProxyAttribute(Type type) => Type = type;
+}
+
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = true)]
+public class AddEntityAttribute : Attribute
+{
+	public Type Entity { get; }
+	internal AddEntityAttribute(Type entity) => Entity = entity;
+}
+
+[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = true)]
+public class AddRelationAttribute : Attribute
+{
+	public Type Relation { get; }
+	public Type Target { get; }
+	internal AddRelationAttribute(Type relation, Type target)
+		{ Relation = relation; Target = target; }
+}

src/gaemstone/ECS/Entity.cs

@@ -4,15 +4,15 @@ using static flecs_hub.flecs;
 namespace gaemstone.ECS;
 
 [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
-public class EntityAttribute : Attribute
-	{ public string? Name { get; set; } }
-
-[AttributeUsage(AttributeTargets.Struct)]
-public class TagAttribute : EntityAttribute {  }
-
-/// <summary> Unused, purely informational. </summary>
-[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
-public class RelationAttribute : Attribute {  }
+public class EntityAttribute : Attribute, ICreateEntityAttribute
+	{ public string? Name { get; init; } }
+
+/// <summary>
+/// A singleton is a single instance of a tag or component that can be retrieved
+/// without explicitly specifying an entity in a query, where it is equivalent
+/// to <see cref="SourceAttribute{}"/> with itself as the generic type parameter.
+/// </summary>
+public class SingletonAttribute : EntityAttribute {  }
 
 public readonly struct Entity
 	: IEquatable<Entity>

src/gaemstone/ECS/Game.cs

@@ -6,13 +6,9 @@ namespace gaemstone.ECS;
 /// Entity for storing global game state and configuration.
 /// Parameters can use <see cref="GameAttribute"/> to source this entity.
 /// </summary>
-[Entity, Tag]
+[Entity]
 public struct Game {  }
 
-/// <summary> Short for <c>[Source(typeof(Game))]</c>. </summary>
+/// <summary> Equivalent to <see cref="SourceAttribute{Game}"/>. </summary>
 [AttributeUsage(AttributeTargets.Parameter)]
-public class GameAttribute : SourceAttribute
-{
-	public GameAttribute()
-		: base(typeof(Game)) {  }
-}
+public class GameAttribute : SourceAttribute<Game> {  }

src/gaemstone/ECS/Identifier.cs

@@ -11,9 +11,10 @@ public readonly struct Identifier
 	public bool IsPair     => ecs_id_is_pair(Value);
 	public bool IsWildcard => ecs_id_is_wildcard(Value);
 
+	public IdentifierFlags Flags => (IdentifierFlags)(Value & ECS_ID_FLAGS_MASK);
+
 	public Entity RelationUnsafe => new(new() { Data = (Value & ECS_COMPONENT_MASK) >> 32 });
 	public Entity TargetUnsafe   => new(new() { Data =  Value & ECS_ENTITY_MASK });
-	public IdentifierFlags Flags => (IdentifierFlags)(Value & ECS_ID_FLAGS_MASK);
 
 	public Identifier(ecs_id_t value) => Value = value;
 

src/gaemstone/ECS/Module.cs

@@ -16,15 +16,6 @@ public class ModuleAttribute : Attribute
 	}
 }
 
-[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
-public class DependsOnAttribute : Attribute
-{
-	public Type Type { get; }
-
-	public DependsOnAttribute(Type type)
-		{ Type = type; }
-}
-
 public interface IModuleInitializer
 {
 	void Initialize(EntityRef module);

src/gaemstone/ECS/Observer.cs

@@ -11,10 +11,11 @@ namespace gaemstone.ECS;
 public class ObserverAttribute : Attribute
 {
 	public Type Event { get; }
-	public string? Expression { get; set; }
-
-	public ObserverAttribute(Type @event) => Event = @event;
+	public string? Expression { get; init; }
+	internal ObserverAttribute(Type @event) => Event = @event; // Use generic type instead.
 }
+public class ObserverAttribute<TEvent> : ObserverAttribute
+	{ public ObserverAttribute() : base(typeof(TEvent)) {  } }
 
 public static class ObserverExtensions
 {
@@ -49,12 +50,12 @@ public static class ObserverExtensions
 				typeof(Action<Iterator>), instance, method);
 		} else {
 			var gen = IterActionGenerator.GetOrBuild(universe, method);
-			filter = (attr.Expression != null) ? new(attr.Expression) : new(gen.Terms.ToArray());
+			filter  = (attr.Expression != null) ? new(attr.Expression) : new(gen.Terms.ToArray());
 			iterAction = iter => gen.RunWithTryCatch(instance, iter);
 		}
 
 		filter.Name = method.Name;
-		var @event = universe.LookupOrThrow(attr.Event);
+		var @event  = universe.LookupOrThrow(attr.Event);
 		return universe.RegisterObserver(filter, @event, iterAction);
 	}
 }

src/gaemstone/ECS/System.cs

@@ -17,8 +17,10 @@ public class SystemAttribute : Attribute
 	public string? Expression { get; set; }
 
 	public SystemAttribute() : this(typeof(SystemPhase.OnUpdate)) {  }
-	public SystemAttribute(Type phase) => Phase = phase;
+	internal SystemAttribute(Type phase) => Phase = phase; // Use generic type instead.
 }
+public class SystemAttribute<TPhase> : SystemAttribute
+	{ public SystemAttribute() : base(typeof(TPhase)) {  } }
 
 public static class SystemExtensions
 {
@@ -70,12 +72,12 @@ public static class SystemExtensions
 			iterAction = (Action<Iterator>)Delegate.CreateDelegate(typeof(Action<Iterator>), instance, method);
 		} else {
 			var gen = IterActionGenerator.GetOrBuild(universe, method);
-			query = (attr?.Expression != null) ? new(attr.Expression) : new(gen.Terms.ToArray());
+			query   = (attr?.Expression != null) ? new(attr.Expression) : new(gen.Terms.ToArray());
 			iterAction = iter => gen.RunWithTryCatch(instance, iter);
 		}
 
 		query.Name = method.Name;
-		var phase = universe.LookupOrThrow(attr?.Phase ?? typeof(SystemPhase.OnUpdate));
+		var phase  = universe.LookupOrThrow(attr?.Phase ?? typeof(SystemPhase.OnUpdate));
 		return universe.RegisterSystem(query, phase, iterAction);
 	}
 

src/gaemstone/ECS/TermAttributes.cs

@@ -3,13 +3,15 @@ using System;
 namespace gaemstone.ECS;
 
 // TODO: Make it possible to use [Source] on systems.
-
 [AttributeUsage(AttributeTargets.Parameter)]
 public class SourceAttribute : Attribute
 {
 	public Type Type { get; }
 	public SourceAttribute(Type type) => Type = type;
 }
+public class SourceAttribute<TEntity> : SourceAttribute
+	{ public SourceAttribute() : base(typeof(TEntity)) {  } }
+
 
 // Parameters types marked with [Tag] are equivalent to [Has].
 [AttributeUsage(AttributeTargets.Parameter)]
@@ -23,6 +25,9 @@ public class InAttribute : Attribute {  }
 [AttributeUsage(AttributeTargets.Parameter)]
 public class OutAttribute : Attribute {  }
 
+// [AttributeUsage(AttributeTargets.Parameter)]
+// public class OrAttribute : Attribute {  }
+
 [AttributeUsage(AttributeTargets.Parameter)]
 public class NotAttribute : Attribute {  }
 

src/gaemstone/ECS/Universe+Modules.cs

@@ -49,13 +49,14 @@ public class ModuleManager
 				$"Existing module {type} with name '{path}' not found");
 
 			// This implementation is pretty naive. It simply gets all nested
-			// types which are tagged with [Entity] attribute or a subtype
-			// thereof and creates a lookup mapping. No sanity checking.
+			// types which are tagged with an ICreateEntityAttribute base
+			// attribute and creates a lookup mapping. No sanity checking.
 
-			foreach (var nested in type.GetNestedTypes())
-				if (nested.Get<EntityAttribute>() is EntityAttribute nestedAttr)
-					Universe.LookupOrThrow(entity, nestedAttr.Name ?? nested.Name)
-						.CreateLookup(nested);
+			foreach (var nested in type.GetNestedTypes()) {
+				if (!nested.GetCustomAttributes(true).OfType<ICreateEntityAttribute>().Any()) continue;
+				var name = nested.Get<EntityAttribute>()?.Name ?? nested.Name;
+				Universe.LookupOrThrow(entity, name).CreateLookup(nested);
+			}
 
 			return entity;
 
@@ -141,9 +142,10 @@ internal class ModuleInfo
 
 		var module = Universe.New(path).Add<Module>();
 
-		// Add module dependencies from [DependsOn] attributes.
-		foreach (var dependsAttr in Type.GetMultiple<DependsOnAttribute>()) {
-			var dependsPath = ModuleManager.GetModulePath(dependsAttr.Type);
+		// Add module dependencies from [DependsOn<>] attributes.
+		foreach (var dependsAttr in Type.GetMultiple<AddRelationAttribute>().Where(attr =>
+				attr.GetType().GetGenericTypeDefinition() == typeof(DependsOnAttribute<>))) {
+			var dependsPath = ModuleManager.GetModulePath(dependsAttr.Target);
 			var dependency = Universe.Lookup(dependsPath) ??
 				Universe.New(dependsPath).Add<Module>().Disable().Build();
 
@@ -169,35 +171,39 @@ internal class ModuleInfo
 	private void RegisterNestedTypes()
 	{
 		foreach (var type in Type.GetNestedTypes()) {
-			if (type.Get<EntityAttribute>() is not EntityAttribute attr) continue;
+			if (!type.GetCustomAttributes(true).OfType<ICreateEntityAttribute>().Any()) continue;
+
+			// If proxied type is specified, use it instead of the marked type.
+			// Attributes are still read from the original type.
+			var proxyType = type.Get<ProxyAttribute>()?.Type ?? type;
 
-			if (attr.Name != null) {
-				try { EntityPath.ValidateName(attr.Name); }
-				catch (Exception ex) { throw new Exception(
-					$"{type} has invalid entity name '{attr.Name}: {ex.Message}'", ex); }
+			if (!type.Has<ComponentAttribute>() && (!proxyType.IsValueType || proxyType.GetFields().Length > 0)) {
+				var typeHint = (proxyType != type) ? $"{proxyType.Name} (proxied by {type})" : type.ToString();
+				throw new Exception($"Type {typeHint} must be an empty, used-defined struct.");
 			}
 
-			var name   = attr.Name ?? type.Name;
-			var entity = Entity.NewChild(name).Symbol(name);
-			switch (attr) {
+			var name = type.Get<EntityAttribute>()?.Name ?? proxyType.Name;
+			try { EntityPath.ValidateName(name); }
+			catch (Exception ex) { throw new Exception(
+				$"{type} has invalid entity name '{name}: {ex.Message}'", ex); }
 
-				case TagAttribute:
-					if (!type.IsValueType || type.GetFields().Length > 0) throw new Exception(
-						$"Tag {type} must be an empty, used-defined struct.");
-					entity.Add<Tag>().Build().CreateLookup(type);
-					break;
+			var builder = Entity.NewChild(name);
+			if (!type.Has<PrivateAttribute>()) builder.Symbol(name);
 
-				case ComponentAttribute:
-					entity.Build().CreateComponent(type);
-					break;
+			foreach (var attr in type.GetMultiple<AddEntityAttribute>())
+				builder.Add(Universe.LookupOrThrow(attr.Entity));
+			foreach (var attr in type.GetMultiple<AddRelationAttribute>())
+				builder.Add(Universe.LookupOrThrow(attr.Relation), Universe.LookupOrThrow(attr.Target));
 
-				default:
-					if (!type.IsValueType || type.GetFields().Length > 0) throw new Exception(
-						$"Entity {type} must be an empty, used-defined struct.");
-					entity.Build().CreateLookup(type);
-					break;
+			var entity = builder.Build();
 
-			}
+			if (type.Has<SingletonAttribute>())
+				entity.Add(entity);
+
+			if (type.Has<ComponentAttribute>())
+				entity.CreateComponent(proxyType);
+			else
+				entity.CreateLookup(proxyType);
 		}
 	}
 

src/gaemstone/ECS/Universe.cs

@@ -26,8 +26,7 @@ public unsafe partial class Universe
 
 		ChildOf = LookupOrThrow<Flecs.Core.ChildOf>();
 
-		// Create "Game" singleton entity, which
-		// stores global state, configuration, ...
+		// Create "Game" entity to store global state.
 		New("Game").Symbol("Game").Build()
 			.CreateLookup<Game>().Add<Game>();
 	}

src/gaemstone/Flecs/SystemPhase.cs

@@ -1,24 +1,30 @@
 using gaemstone.ECS;
+using static gaemstone.Flecs.Pipeline;
 
 namespace gaemstone.Flecs;
 
 [Module("flecs", "pipeline")]
 public static class SystemPhase
 {
-	[Entity] public struct PreFrame {  }
+	[Entity, Add<Phase>]
+	public struct PreFrame {  }
 
 	/// <summary>
 	/// This phase contains all the systems that load data into your ECS.
 	/// This would be a good place to load keyboard and mouse inputs.
 	/// </summary>
-	[Entity] public struct OnLoad {  }
+	[Entity, Add<Phase>]
+	[DependsOn<PreFrame>]
+	public struct OnLoad {  }
 
 	/// <summary>
 	/// Often the imported data needs to be processed. Maybe you want to associate
 	/// your keypresses with high level actions rather than comparing explicitly
 	/// in your game code if the user pressed the 'K' key.
 	/// </summary>
-	[Entity] public struct PostLoad {  }
+	[Entity, Add<Phase>]
+	[DependsOn<OnLoad>]
+	public struct PostLoad {  }
 
 	/// <summary>
 	/// Now that the input is loaded and processed, it's time to get ready to
@@ -27,13 +33,17 @@ public static class SystemPhase
 	/// This can be a good place to prepare the frame, maybe clean up some
 	/// things from the previous frame, etcetera.
 	/// </summary>
-	[Entity] public struct PreUpdate {  }
+	[Entity, Add<Phase>]
+	[DependsOn<PostLoad>]
+	public struct PreUpdate {  }
 
 	/// <summary>
 	/// This is usually where the magic happens! This is where you put all of
 	/// your gameplay systems. By default systems are added to this phase.
 	/// </summary>
-	[Entity] public struct OnUpdate {  }
+	[Entity, Add<Phase>]
+	[DependsOn<PreUpdate>]
+	public struct OnUpdate {  }
 
 	/// <summary>
 	/// This phase was introduced to deal with validating the state of the game
@@ -42,7 +52,9 @@ public static class SystemPhase
 	/// This phase is for righting that wrong. A typical feature to implement
 	/// in this phase would be collision detection.
 	/// </summary>
-	[Entity] public struct OnValidate {  }
+	[Entity, Add<Phase>]
+	[DependsOn<OnUpdate>]
+	public struct OnValidate {  }
 
 	/// <summary>
 	/// When your game logic has been updated, and your validation pass has ran,
@@ -50,7 +62,9 @@ public static class SystemPhase
 	/// detection system detected collisions in the <c>OnValidate</c> phase,
 	/// you may want to move the entities so that they no longer overlap.
 	/// </summary>
-	[Entity] public struct PostUpdate {  }
+	[Entity, Add<Phase>]
+	[DependsOn<OnValidate>]
+	public struct PostUpdate {  }
 
 	/// <summary>
 	/// Now that all of the frame data is computed, validated and corrected for,
@@ -59,13 +73,19 @@ public static class SystemPhase
 	/// A good example would be a system that calculates transform matrices from
 	/// a scene graph.
 	/// </summary>
-	[Entity] public struct PreStore {  }
+	[Entity, Add<Phase>]
+	[DependsOn<PostUpdate>]
+	public struct PreStore {  }
 
 	/// <summary>
 	/// This is where it all comes together. Your frame is ready to be
 	/// rendered, and that is exactly what you would do in this phase.
 	/// </summary>
-	[Entity] public struct OnStore {  }
+	[Entity, Add<Phase>]
+	[DependsOn<PreStore>]
+	public struct OnStore {  }
 
-	[Entity] public struct PostFrame {  }
+	[Entity, Add<Phase>]
+	[DependsOn<OnStore>]
+	public struct PostFrame {  }
 }

src/gaemstone/Utility/CStringExtensions.cs

@@ -13,9 +13,10 @@ public unsafe static class CStringExtensions
 	public static unsafe byte[]? FlecsToBytes(this CString str)
 	{
 		if (str.IsNull) return null;
-		var length  = 0;
 		var pointer = (byte*)(nint)str;
-		while (true) if (pointer[length++] == 0) break;
+		// Find length of the string by locating the NUL character.
+		var length = 0; while (true) if (pointer[length++] == 0) break;
+		// Create span over the region, NUL included, copy it into an array.
 		return new Span<byte>(pointer, length).ToArray();
 	}
 

src/gaemstone/Utility/IL/IterActionGenerator.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.Linq;
 using System.Reflection;
 using System.Reflection.Emit;
@@ -9,6 +10,8 @@ using gaemstone.ECS;
 
 namespace gaemstone.Utility.IL;
 
+// TODO: Implement "or" operator.
+// TODO: Support tuple syntax to match relationship pairs.
 public unsafe class IterActionGenerator
 {
 	private static readonly ConstructorInfo _entityRefCtor = typeof(EntityRef).GetConstructors().Single();
@@ -35,7 +38,7 @@ public unsafe class IterActionGenerator
 
 	public Universe Universe { get; }
 	public MethodInfo Method { get; }
-	public ParamInfo[] Parameters { get; }
+	public IReadOnlyList<ParamInfo> Parameters { get; }
 
 	public IReadOnlyList<Term> Terms { get; }
 	public Action<object?, Iterator> GeneratedAction { get; }
@@ -44,12 +47,12 @@ public unsafe class IterActionGenerator
 	public void RunWithTryCatch(object? instance, Iterator iter)
 	{
 		try { GeneratedAction(instance, iter); } catch {
-			Console.WriteLine("Exception occured while running:");
-			Console.WriteLine("    " + Method);
-			Console.WriteLine();
-			Console.WriteLine("Method's IL code:");
-			Console.WriteLine(ReadableString);
-			Console.WriteLine();
+			Console.Error.WriteLine("Exception occured while running:");
+			Console.Error.WriteLine("    " + Method);
+			Console.Error.WriteLine();
+			Console.Error.WriteLine("Method's IL code:");
+			Console.Error.WriteLine(ReadableString);
+			Console.Error.WriteLine();
 			throw;
 		}
 	}
@@ -59,165 +62,172 @@ public unsafe class IterActionGenerator
 		Universe = universe;
 		Method   = method;
 
-		Parameters = method.GetParameters().Select(ParamInfo.Build).ToArray();
-		// if (!Parameters.Any(c => c.IsRequired && (c.Kind != ParamKind.Unique)))
-		// 	throw new ArgumentException($"At least one parameter in {method} is required");
-
-		var name  = "<>Query_" + string.Join("_", Parameters.Select(p => p.UnderlyingType.Name));
+		var name = "<>Query_" + string.Join("_", method.Name);
 		var genMethod = new DynamicMethod(name, null, new[] { typeof(object), typeof(Iterator) });
 		var IL = new ILGeneratorWrapper(genMethod);
 
 		var instanceArg = IL.Argument<object?>(0);
 		var iteratorArg = IL.Argument<Iterator>(1);
 
-		// If parameters only contains global unique paremeters (such as
-		// Universe or TimeSpan), or is empty, just run the system, since
-		// without terms it won't match any entities anyway.
-		if (Parameters.All(p => p.Kind == ParamKind.GlobalUnique)) {
-			if (!Method.IsStatic) IL.Load(instanceArg);
-			foreach (var p in Parameters) {
-				IL.Comment($"Global unique parameter {p.ParameterType.GetFriendlyName()}");
-				_globalUniqueParameters[p.ParameterType](IL, iteratorArg);
-			}
-			IL.Call(Method);
-			IL.Return();
-
-			Terms = Array.Empty<Term>();
-			GeneratedAction = genMethod.CreateDelegate<Action<object?, Iterator>>();
-			ReadableString  = IL.ToReadableString();
-
-			return;
-		}
-
-		var terms = new List<Term>();
-		var fieldLocals = new ILocal[Parameters.Length];
-		var tempLocals  = new ILocal[Parameters.Length];
+		var fieldIndex = 1;
+		var parameters = new List<(ParamInfo Info, Term? Term, ILocal? FieldLocal, ILocal? TempLocal)>();
+		foreach (var info in method.GetParameters()) {
+			var p = ParamInfo.Build(info);
 
-		for (var i = 0; i < Parameters.Length; i++) {
-			var p = Parameters[i];
-			if (p.Kind <= ParamKind.Unique) continue;
+			// If the parameter is unique, we don't create a term for it.
+			if (p.Kind <= ParamKind.Unique)
+				{ parameters.Add((p, null, null, null)); continue; }
 
-			// Add an entry to the terms to look for this type.
-			terms.Add(new(universe.LookupOrThrow(p.UnderlyingType)) {
+			// Create a term to add to the query.
+			var term = new Term(universe.LookupOrThrow(p.UnderlyingType)) {
 				Source = (p.Source != null) ? (TermID)Universe.LookupOrThrow(p.Source) : null,
 				InOut = p.Kind switch {
-					ParamKind.In => TermInOutKind.In,
+					ParamKind.In  => TermInOutKind.In,
 					ParamKind.Out => TermInOutKind.Out,
 					ParamKind.Not or ParamKind.Not => TermInOutKind.None,
 					_ => default,
 				},
 				Oper = p.Kind switch {
 					ParamKind.Not => TermOperKind.Not,
+					ParamKind.Or  => TermOperKind.Or,
 					_ when !p.IsRequired => TermOperKind.Optional,
 					_ => default,
 				},
-			});
-
-			// Create a Span<T> local and initialize it to iterator.Field<T>(i).
-			var spanType = typeof(Span<>).MakeGenericType(p.FieldType);
-			fieldLocals[i] = IL.Local(spanType, $"field_{i}");
-			if (p.Kind is ParamKind.Has or ParamKind.Not) {
-				// If a "has" or "not" parameter is a struct, we require a temporary local that
-				// we can later load onto the stack when loading the arguments for the action.
-				if (p.ParameterType.IsValueType) {
-					IL.Comment($"temp_{i} = default({p.ParameterType});");
-					tempLocals[i] = IL.Local(p.ParameterType);
-					IL.LoadAddr(tempLocals[i]);
-					IL.Init(tempLocals[i].LocalType);
-				}
-			} else if (p.IsRequired) {
-				IL.Comment($"field_{i} = iterator.Field<{p.FieldType.Name}>({terms.Count})");
-				IL.Load(iteratorArg);
-				IL.LoadConst(terms.Count);
-				IL.Call(_iteratorFieldMethod.MakeGenericMethod(p.FieldType));
-				IL.Store(fieldLocals[i]);
-			} else {
-				IL.Comment($"field_{i} = iterator.MaybeField<{p.FieldType.Name}>({terms.Count})");
-				IL.Load(iteratorArg);
-				IL.LoadConst(terms.Count);
-				IL.Call(_iteratorMaybeFieldMethod.MakeGenericMethod(p.FieldType));
-				IL.Store(fieldLocals[i]);
+			};
+
+			var spanType   = typeof(Span<>).MakeGenericType(p.FieldType);
+			var fieldLocal = IL.Local(spanType, $"{info.Name}Field");
+			var tempLocal  = (ILocal?)null;
+
+			switch (p.Kind) {
+				case ParamKind.Has or ParamKind.Not:
+					if (!p.ParameterType.IsValueType) break;
+					// If a "has" or "not" parameter is a struct, we require a temporary local that
+					// we can later load onto the stack when loading the arguments for the action.
+					IL.Comment($"{info.Name}Temp = default({p.ParameterType});");
+					tempLocal = IL.Local(p.ParameterType);
+					IL.LoadAddr(tempLocal);
+					IL.Init(tempLocal.LocalType);
+					break;
+
+				case ParamKind.Nullable:
+					IL.Comment($"{info.Name}Field = iterator.MaybeField<{p.FieldType.Name}>({fieldIndex})");
+					IL.Load(iteratorArg);
+					IL.LoadConst(fieldIndex);
+					IL.Call(_iteratorMaybeFieldMethod.MakeGenericMethod(p.FieldType));
+					IL.Store(fieldLocal);
+
+					IL.Comment($"{info.Name}Temp = default({p.ParameterType});");
+					tempLocal = IL.Local(p.ParameterType);
+					IL.LoadAddr(tempLocal);
+					IL.Init(tempLocal.LocalType);
+					break;
+
+				default:
+					IL.Comment($"{info.Name}Field = iterator.Field<{p.FieldType.Name}>({fieldIndex})");
+					IL.Load(iteratorArg);
+					IL.LoadConst(fieldIndex);
+					IL.Call(_iteratorFieldMethod.MakeGenericMethod(p.FieldType));
+					IL.Store(fieldLocal);
+					break;
 			}
 
-			if (p.Kind == ParamKind.Nullable) {
-				IL.Comment($"temp_{i} = default({p.ParameterType});");
-				tempLocals[i] = IL.Local(p.ParameterType);
-				IL.LoadAddr(tempLocals[i]);
-				IL.Init(tempLocals[i].LocalType);
-			}
+			parameters.Add((p, term, fieldLocal, tempLocal));
+			fieldIndex++;
 		}
 
 		// If there's any reference type parameters, we need to define a GCHandle local.
-		var hasReferenceType = Parameters
-			.Where(p => p.Kind > ParamKind.Unique)
-			.Any(p => !p.UnderlyingType.IsValueType);
+		var hasReferenceType = parameters
+			.Where(p => p.Info.Kind > ParamKind.Unique)
+			.Any(p => !p.Info.UnderlyingType.IsValueType);
 		var handleLocal = hasReferenceType ? IL.Local<GCHandle>() : null;
 
-		using (IL.For(() => IL.Load(iteratorArg, _iteratorCountProp), out var currentLocal)) {
-			if (!Method.IsStatic) IL.Load(instanceArg);
-			for (var i = 0; i < Parameters.Length; i++) {
-				var p = Parameters[i];
-				if (p.Kind == ParamKind.GlobalUnique) {
-					IL.Comment($"Global unique parameter {p.ParameterType.GetFriendlyName()}");
-					_globalUniqueParameters[p.ParameterType](IL, iteratorArg);
-				} else if (p.Kind == ParamKind.Unique) {
-					IL.Comment($"Unique parameter {p.ParameterType.GetFriendlyName()}");
-					_uniqueParameters[p.ParameterType](IL, iteratorArg, currentLocal);
-				} else if (p.Kind is ParamKind.Has or ParamKind.Not) {
-					if (p.ParameterType.IsValueType)
-						IL.LoadObj(tempLocals[i]!);
+		IDisposable? forLoopBlock = null;
+		ILocal<int>? forCurrentLocal = null;
+		// If all parameters are fixed, iterator count will be 0, but since
+		// the query matched, we want to run the callback at least once.
+		if (parameters.Any(p => !p.Info.IsFixed))
+			forLoopBlock = IL.For(() => IL.Load(iteratorArg, _iteratorCountProp), out forCurrentLocal);
+
+		if (!Method.IsStatic)
+			IL.Load(instanceArg);
+
+		foreach (var (info, term, fieldLocal, tempLocal) in parameters) {
+			switch (info.Kind) {
+
+				case ParamKind.GlobalUnique:
+					IL.Comment($"Global unique parameter {info.ParameterType.GetFriendlyName()}");
+					_globalUniqueParameters[info.ParameterType](IL, iteratorArg);
+					break;
+
+				case ParamKind.Unique:
+					IL.Comment($"Unique parameter {info.ParameterType.GetFriendlyName()}");
+					_uniqueParameters[info.ParameterType](IL, iteratorArg, forCurrentLocal!);
+					break;
+
+				case ParamKind.Has or ParamKind.Not:
+					if (info.ParameterType.IsValueType)
+						IL.LoadObj(tempLocal!);
 					else IL.LoadNull();
-				} else {
-					var spanType = typeof(Span<>).MakeGenericType(p.FieldType);
+					break;
+
+				default:
+					var spanType = typeof(Span<>).MakeGenericType(info.FieldType);
 					var spanItemMethod   = spanType.GetProperty("Item")!.GetMethod!;
 					var spanLengthMethod = spanType.GetProperty("Length")!.GetMethod!;
 
-					IL.Comment($"Parameter {p.ParameterType.GetFriendlyName()}");
-					if (p.IsByRef) {
-						IL.LoadAddr(fieldLocals[i]!);
-						IL.Load(currentLocal);
+					IL.Comment($"Parameter {info.ParameterType.GetFriendlyName()}");
+					if (info.IsByRef) {
+						IL.LoadAddr(fieldLocal!);
+						if (info.IsFixed) IL.LoadConst(0);
+						else IL.Load(forCurrentLocal!);
 						IL.Call(spanItemMethod);
-					} else if (p.IsRequired) {
-						IL.LoadAddr(fieldLocals[i]!);
-						IL.Load(currentLocal);
+					} else if (info.IsRequired) {
+						IL.LoadAddr(fieldLocal!);
+						if (info.IsFixed) IL.LoadConst(0);
+						else IL.Load(forCurrentLocal!);
 						IL.Call(spanItemMethod);
-						IL.LoadObj(p.FieldType);
+						IL.LoadObj(info.FieldType);
 					} else {
 						var elseLabel = IL.DefineLabel();
 						var doneLabel = IL.DefineLabel();
-						IL.LoadAddr(fieldLocals[i]!);
+						IL.LoadAddr(fieldLocal!);
 						IL.Call(spanLengthMethod);
 						IL.GotoIfFalse(elseLabel);
-							IL.LoadAddr(fieldLocals[i]!);
-							IL.Load(currentLocal);
+							IL.LoadAddr(fieldLocal!);
+							if (info.IsFixed) IL.LoadConst(0);
+							else IL.Load(forCurrentLocal!);
 							IL.Call(spanItemMethod);
-							IL.LoadObj(p.FieldType);
-							if (p.Kind == ParamKind.Nullable)
-								IL.New(p.ParameterType);
+							IL.LoadObj(info.FieldType);
+							if (info.Kind == ParamKind.Nullable)
+								IL.New(info.ParameterType);
 						IL.Goto(doneLabel);
 						IL.MarkLabel(elseLabel);
-							if (p.Kind == ParamKind.Nullable)
-								IL.LoadObj(tempLocals[i]!);
+							if (info.Kind == ParamKind.Nullable)
+								IL.LoadObj(tempLocal!);
 							else IL.LoadNull();
 						IL.MarkLabel(doneLabel);
 					}
 
-					if (!p.UnderlyingType.IsValueType) {
-						IL.Comment($"Convert nint to {p.UnderlyingType.GetFriendlyName()}");
+					if (!info.UnderlyingType.IsValueType) {
+						IL.Comment($"Convert nint to {info.UnderlyingType.GetFriendlyName()}");
 						IL.Call(_handleFromIntPtrMethod);
 						IL.Store(handleLocal!);
 						IL.LoadAddr(handleLocal!);
 						IL.Call(_handleTargetProp.GetMethod!);
-						IL.Cast(p.UnderlyingType);
+						IL.Cast(info.UnderlyingType);
 					}
-				}
+					break;
 			}
-			IL.Call(Method);
 		}
+		IL.Call(Method);
+
+		forLoopBlock?.Dispose();
 
 		IL.Return();
 
-		Terms = terms.AsReadOnly();
+		Parameters = parameters.Select(p => p.Info).ToImmutableList();
+		Terms      = parameters.Where(p => p.Term != null).Select(p => p.Term!).ToImmutableList();
 		GeneratedAction = genMethod.CreateDelegate<Action<object?, Iterator>>();
 		ReadableString  = IL.ToReadableString();
 	}
@@ -228,7 +238,6 @@ public unsafe class IterActionGenerator
 	public class ParamInfo
 	{
 		public ParameterInfo Info { get; }
-		public int Index { get; }
 
 		public ParamKind Kind { get; }
 		public Type ParameterType { get; }
@@ -239,36 +248,33 @@ public unsafe class IterActionGenerator
 
 		public bool IsRequired => (Kind <  ParamKind.Nullable);
 		public bool IsByRef    => (Kind >= ParamKind.In) && (Kind <= ParamKind.Ref);
+		public bool IsFixed    => (Kind == ParamKind.GlobalUnique) || (Source != null);
 
-		private ParamInfo(
-			ParameterInfo info, int index, ParamKind kind,
-			Type paramType, Type underlyingType)
+		private ParamInfo(ParameterInfo info, ParamKind kind,
+		                  Type paramType, Type underlyingType)
 		{
-			Info  = info;
-			Index = index;
-
+			Info = info;
 			Kind = kind;
 			ParameterType  = paramType;
 			UnderlyingType = underlyingType;
 			// Reference types have a backing type of nint - they're pointers.
 			FieldType = underlyingType.IsValueType ? underlyingType : typeof(nint);
 
-			// FIXME: Reimplement singletons somehow.
-			// if (UnderlyingType.Has<EntityAttribute>()) Source = underlyingType;
+			if (UnderlyingType.Has<SingletonAttribute>()) Source = underlyingType;
 			if (Info.Get<SourceAttribute>() is SourceAttribute attr) Source = attr.Type;
 			// TODO: Needs support for the new attributes.
 		}
 
-		public static ParamInfo Build(ParameterInfo info, int index)
+		public static ParamInfo Build(ParameterInfo info)
 		{
 			if (info.IsOptional)              throw new ArgumentException("Optional parameters are not supported\nParameter: " + info);
 			if (info.ParameterType.IsArray)   throw new ArgumentException("Arrays are not supported\nParameter: " + info);
 			if (info.ParameterType.IsPointer) throw new ArgumentException("Pointers are not supported\nParameter: " + info);
 
 			if (_globalUniqueParameters.ContainsKey(info.ParameterType))
-				return new(info, index, ParamKind.GlobalUnique, info.ParameterType, info.ParameterType);
+				return new(info, ParamKind.GlobalUnique, info.ParameterType, info.ParameterType);
 			if (_uniqueParameters.ContainsKey(info.ParameterType))
-				return new(info, index, ParamKind.Unique, info.ParameterType, info.ParameterType);
+				return new(info, ParamKind.Unique, info.ParameterType, info.ParameterType);
 
 			var isByRef    = info.ParameterType.IsByRef;
 			var isNullable = info.IsNullable();
@@ -276,13 +282,13 @@ public unsafe class IterActionGenerator
 			if (info.Has<NotAttribute>()) {
 				if (isByRef || isNullable) throw new ArgumentException(
 					"Parameter with NotAttribute must not be ByRef or nullable\nParameter: " + info);
-				return new(info, index, ParamKind.Not, info.ParameterType, info.ParameterType);
+				return new(info, ParamKind.Not, info.ParameterType, info.ParameterType);
 			}
 
 			if (info.Has<HasAttribute>() || info.ParameterType.Has<TagAttribute>()) {
 				if (isByRef || isNullable) throw new ArgumentException(
 					"Parameter with HasAttribute / TagAttribute must not be ByRef or nullable\nParameter: " + info);
-				return new(info, index, ParamKind.Has, info.ParameterType, info.ParameterType);
+				return new(info, ParamKind.Has, info.ParameterType, info.ParameterType);
 			}
 
 			var kind = ParamKind.Normal;
@@ -308,15 +314,23 @@ public unsafe class IterActionGenerator
 			if (underlyingType.IsPrimitive) throw new ArgumentException(
 				"Primitives are not supported\nParameter: " + info);
 
-			return new(info, index, kind, info.ParameterType, underlyingType);
+			return new(info, kind, info.ParameterType, underlyingType);
 		}
 	}
 
 	public enum ParamKind
 	{
-		/// <summary> Parameter is not part of terms, handled uniquely, such as Universe. </summary>
+		/// <summary>
+		/// Not part of the resulting query's terms.
+		/// Same value across a single invocation of a callback.
+		/// For example <see cref="ECS.Universe"/> or <see cref="TimeSpan"/>.
+		/// </summary>
 		GlobalUnique,
-		/// <summary> Parameter is unique per matched entity, such as EntityRef. </summary>
+		/// <summary>
+		/// Not part of the resulting query's terms.
+		/// Unique value for each iterated entity.
+		/// For example <see cref="EntityRef"/>.
+		/// </summary>
 		Unique,
 		/// <summary> Passed by value. </summary>
 		Normal,
@@ -332,9 +346,15 @@ public unsafe class IterActionGenerator
 		/// Automatically applied for types with <see cref="TagAttribute"/>.
 		/// </summary>
 		Has,
-		/// <summary> Struct passed as <c>Nullable&lt;T&gt;</c>. </summary>
+		/// <summary> Struct or class passed as <see cref="T?"/>. </summary>
 		Nullable,
 		/// <summary>
+		/// Matches any terms in a chain of "or" terms.
+		/// Applied with <see cref="OrAttribute"/>.
+		/// Implies <see cref="Nullable"/>.
+		/// </summary>
+		Or,
+		/// <summary>
 		/// Only checks for absence.
 		/// Applied with <see cref="NotAttribute"/>.
 		/// </summary>

src/gaemstone/gaemstone.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
+    <LangVersion>preview</LangVersion>
     <TargetFramework>net6.0</TargetFramework>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>