Add isResource type predicate (#2)

* Add isResource type predicate

* v0.1.1

deno.json

@@ -1,10 +1,10 @@
 {
   "name": "@kure/schema",
-  "version": "0.1.0",
+  "version": "0.1.1",
   "exports": {
     ".": "./mod.ts"
   },
   "imports": {
-    "@kure/spec": "jsr:@kure/spec@^0.1.0"
+    "@kure/spec": "jsr:@kure/spec@^0.1.2"
   }
-}
+}

impl.ts

@@ -29,3 +29,60 @@ export function factory<R extends Resource>(
 
   return build;
 }
+
+/**
+ * Create a [type predicate][] for the given {@link ResourceType resource type}.
+ *
+ * [type predicate]: https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates
+ *
+ * @example
+ * ```ts
+ * import { Pod, Service } from "jsr:@kure/api/core";
+ * import { isResource } from "jsr:@kure/schema";
+ *
+ * export const isPod = isResource(Pod);
+ * export const isService = isResource(Service);
+ * ```
+ */
+export function isResource<R extends Resource>(
+  type: ResourceType<R>
+): (target: unknown) => target is R;
+/**
+ * [Check][type predicate] if a value is an instance of a
+ * {@link ResourceType resource type}.
+ *
+ * [type predicate]: https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates
+ *
+ * @example
+ * ```ts
+ * import { Deployment } from "jsr:@kure/api/apps";
+ * import { isResource, type Resource } from "jsr:@kure/schema";
+ *
+ * function updateScale(replicas: number, resources: Resource[]) {
+ *   for (const resource of resources) {
+ *     if (isResource(Deployment, resource)) {
+ *       // TypeScript now knows `resource` is a Deployment
+ *       resource.spec.replicas = replicas;
+ *     }
+ *   }
+ * }
+ * ```
+ */
+export function isResource<R extends Resource>(
+  type: ResourceType<R>,
+  target: unknown
+): target is R;
+export function isResource<R extends Resource>(
+  type: ResourceType<R>,
+  target?: unknown
+) {
+  if (arguments.length < 2)
+    return (target: unknown) => isResource(type, target);
+
+  return (
+    target != null &&
+    typeof target === "object" &&
+    (target as Resource).apiVersion === type.apiVersion &&
+    (target as Resource).kind === type.kind
+  );
+}

mod.ts

@@ -1,6 +1,6 @@
 export type { Received, ReceivedObjectMeta } from "./disposition.ts";
 export * from "./identity.ts";
-export { factory } from "./impl.ts";
+export { factory, isResource } from "./impl.ts";
 export * from "./meta.ts";
 export * from "./resource.ts";