docs: add LayerMap example (#27388)

packages/core/src/plugin/layer-map.example.ts

@@ -0,0 +1,94 @@
+export * as LayerMapExample from "./layer-map.example"
+
+import { Context, Effect, Layer, LayerMap } from "effect"
+import { Npm } from "../npm"
+
+/**
+ * Tutorial: split global services from context-specific services.
+ *
+ * Use this pattern when part of the app should be constructed once at the app edge,
+ * while another part should be cached per request/project/workspace key.
+ *
+ * In this example:
+ * - Npm.Service is the global service. It is not keyed by request context and should
+ *   be provided once by the application runtime.
+ * - ConfigService is context-specific. It is built from a RequestContext key and is
+ *   cached by LayerMap for that key.
+ * - ConfigServiceMap.layer owns the cache. Provide it once globally, then each
+ *   request can provide ConfigServiceMap.get(context) to select the right instance.
+ *
+ * Lifetime model:
+ * - ConfigServiceMap.layer has the app/global lifetime and depends on Npm.Service.
+ * - ConfigServiceMap.get(context) has the request/context lifetime and provides
+ *   ConfigService for exactly that context key.
+ * - The cached ConfigService entry stays alive while something is using it. Once idle,
+ *   it remains cached for idleTimeToLive, then its scope is finalized.
+ * - invalidate(context) removes the cache entry for future lookups. Active users keep
+ *   running on the old instance; the next lookup can create a fresh instance.
+ *
+ * Key model:
+ * - Keys can be strings, structs, classes, arrays, etc.
+ * - Prefer primitive or immutable keys. Effect uses Hash / Equal semantics for cache
+ *   lookup, so mutating an object after it has been used as a key is a bug.
+ */
+
+export type RequestContext = {
+  readonly directory: string
+  readonly workspace: string
+}
+
+export class RequestContextRef extends Context.Service<RequestContextRef, RequestContext>()(
+  "@opencode/example/RequestContextRef",
+) {}
+
+export interface ConfigServiceShape {
+  readonly directory: string
+  readonly workspace: string
+  readonly nextUse: () => Effect.Effect<number>
+  readonly which: Npm.Interface["which"]
+}
+
+export class ConfigService extends Context.Service<ConfigService, ConfigServiceShape>()(
+  "@opencode/example/ConfigService",
+) {}
+
+const configServiceLayer = Layer.effect(
+  ConfigService,
+  Effect.gen(function* () {
+    const context = yield* RequestContextRef
+    const npm = yield* Npm.Service
+
+    let useCount = 0
+
+    return ConfigService.of({
+      directory: context.directory,
+      workspace: context.workspace,
+      nextUse: () => Effect.succeed(++useCount),
+      which: npm.which,
+    })
+  }),
+)
+
+export class ConfigServiceMap extends LayerMap.Service<ConfigServiceMap>()("@opencode/example/ConfigServiceMap", {
+  lookup: (context: RequestContext) =>
+    configServiceLayer.pipe(Layer.provide(Layer.succeed(RequestContextRef, RequestContextRef.of(context)))),
+  idleTimeToLive: "5 minutes",
+}) {}
+
+export const appLayer = ConfigServiceMap.layer
+
+export const readConfig = Effect.fn("LayerMapExample.readConfig")(function* () {
+  const config = yield* ConfigService
+
+  return {
+    directory: config.directory,
+    workspace: config.workspace,
+    useCount: yield* config.nextUse(),
+  }
+})
+
+export const handleRequest = Effect.fn("LayerMapExample.handleRequest")(function* (context: RequestContext) {
+  return yield* readConfig().pipe(Effect.provide(ConfigServiceMap.get(context)))
+})
+
+export const invalidateContext = (context: RequestContext) => ConfigServiceMap.invalidate(context)