return a proxy Layer from LayerMap service (#4898)

Author: tim-smart

.changeset/true-eagles-peel.md

@@ -0,0 +1,52 @@
+---
+"effect": minor
+---
+
+return a proxy Layer from LayerMap service
+
+The new usage is:
+
+```ts
+import { NodeRuntime } from "@effect/platform-node"
+import { Context, Effect, FiberRef, Layer, LayerMap } from "effect"
+
+class Greeter extends Context.Tag("Greeter")<
+  Greeter,
+  {
+    greet: Effect.Effect<string>
+  }
+>() {}
+
+// create a service that wraps a LayerMap
+class GreeterMap extends LayerMap.Service<GreeterMap>()("GreeterMap", {
+  // define the lookup function for the layer map
+  //
+  // The returned Layer will be used to provide the Greeter service for the
+  // given name.
+  lookup: (name: string) =>
+    Layer.succeed(Greeter, {
+      greet: Effect.succeed(`Hello, ${name}!`)
+    }),
+
+  // If a layer is not used for a certain amount of time, it can be removed
+  idleTimeToLive: "5 seconds",
+
+  // Supply the dependencies for the layers in the LayerMap
+  dependencies: []
+}) {}
+
+// usage
+const program: Effect.Effect<void, never, GreeterMap> = Effect.gen(
+  function* () {
+    // access and use the Greeter service
+    const greeter = yield* Greeter
+    yield* Effect.log(yield* greeter.greet)
+  }
+).pipe(
+  // use the GreeterMap service to provide a variant of the Greeter service
+  Effect.provide(GreeterMap.get("John"))
+)
+
+// run the program
+program.pipe(Effect.provide(GreeterMap.Default), NodeRuntime.runMain)
+```