Rename lookup to lookupByType, add lookupAlive

src/world.zig

@@ -38,12 +38,20 @@ pub fn World(comptime ctx: anytype) type {
             return c.ecs_progress(self.raw, delta_time);
         }
 
-        pub fn lookup(self: *Self, comptime T: type) ?Entity(ctx) {
-            const id = Lookup(ctx, T).id;
-            const result = Entity(ctx).fromRaw(self, id);
-            return if (result.isAlive()) result else null;
+        /// Returns an `Entity` for the specified `ecs_entity_t` value, or an
+        /// error if the entity is invalid or not alive in this `World`.
+        pub fn lookupAlive(self: *Self, id: c.ecs_entity_t) !Entity(ctx) {
+            return Entity(ctx).fromRaw(self, id).ensureAlive();
         }
 
+        /// Returns the component `Entity` registered for the specified
+        /// type `T`, or an error if an association has not been made.
+        pub fn lookupByType(self: *Self, comptime T: type) !Entity(ctx) {
+            return lookupAlive(self, Lookup(ctx, T).id);
+        }
+
+        // TODO: Replace these functions with an entity builder interface.
+
         pub fn entity(
             self: *Self,
             comptime add: []const type,

test/world.zig

@@ -57,8 +57,8 @@ test "World_progress_w_0" {
     try expectEql(ctx.param, null);
 
     try expectEql(ctx.e[0], e1.raw);
-    try expectEql(ctx.c[0][0], world.lookup(Position).?.raw);
-    try expectEql(ctx.c[0][1], world.lookup(Velocity).?.raw);
+    try expectEql(ctx.c[0][0], (try world.lookupByType(Position)).raw);
+    try expectEql(ctx.c[0][1], (try world.lookupByType(Velocity)).raw);
     try expectEql(ctx.s[0][0], 0);
     try expectEql(ctx.s[0][1], 0);