introduce ReadonlyTag as a Tag that can only be used to read a service from requirements (#5305)

Author: mikearnaldi

.changeset/little-singers-wash.md

@@ -0,0 +1,36 @@
+---
+"effect": minor
+---
+
+Introduce ReadonlyTag as the covariant side of a tag, enables:
+
+```ts
+import type { Context } from "effect"
+import { Effect } from "effect"
+
+export class MyRequirement extends Effect.Service<MyRequirement>()(
+  "MyRequirement",
+  { succeed: () => 42 }
+) {}
+
+export class MyUseCase extends Effect.Service<MyUseCase>()("MyUseCase", {
+  dependencies: [MyRequirement.Default],
+  effect: Effect.gen(function* () {
+    const requirement = yield* MyRequirement
+    return Effect.fn("MyUseCase.execute")(function* () {
+      return requirement()
+    })
+  })
+}) {}
+
+export function effectHandler<I, Args extends Array<any>, A, E, R>(
+  service: Context.ReadonlyTag<I, (...args: Args) => Effect.Effect<A, E, R>>
+) {
+  return Effect.fn("effectHandler")(function* (...args: Args) {
+    const execute = yield* service
+    yield* execute(...args)
+  })
+}
+
+export const program = effectHandler(MyUseCase)
+```

packages/effect/src/Context.ts

@@ -7,6 +7,7 @@
  *
  * @since 2.0.0
  */
+import type * as Effect from "./Effect.js"
 import type { Equal } from "./Equal.js"
 import type { LazyArg } from "./Function.js"
 import type { Inspectable } from "./Inspectable.js"
@@ -16,7 +17,11 @@ import type { Pipeable } from "./Pipeable.js"
 import type * as Types from "./Types.js"
 import type * as Unify from "./Unify.js"
 
-const TagTypeId: unique symbol = internal.TagTypeId
+/**
+ * @since 2.0.0
+ * @category symbol
+ */
+export const TagTypeId: unique symbol = internal.TagTypeId
 
 /**
  * @since 2.0.0
@@ -28,7 +33,7 @@ export type TagTypeId = typeof TagTypeId
  * @since 3.5.9
  * @category models
  */
-export interface Tag<in out Id, in out Value> extends Pipeable, Inspectable {
+export interface Tag<in out Id, in out Value> extends Pipeable, Inspectable, ReadonlyTag<Id, Value> {
   readonly _op: "Tag"
   readonly Service: Value
   readonly Identifier: Id
@@ -45,7 +50,27 @@ export interface Tag<in out Id, in out Value> extends Pipeable, Inspectable {
   [Unify.ignoreSymbol]?: TagUnifyIgnore
 }
 
-const ReferenceTypeId: unique symbol = internal.ReferenceTypeId
+/**
+ * @since 3.5.9
+ * @category models
+ */
+export interface ReadonlyTag<in out Id, out Value> extends Pipeable, Inspectable, Effect.Effect<Value, never, Id> {
+  readonly _op: "Tag"
+  readonly Service: Value
+  readonly Identifier: Id
+  readonly [TagTypeId]: {
+    readonly _Service: Types.Covariant<Value>
+    readonly _Identifier: Types.Invariant<Id>
+  }
+  readonly stack?: string | undefined
+  readonly key: string
+}
+
+/**
+ * @since 3.11.0
+ * @category symbol
+ */
+export const ReferenceTypeId: unique symbol = internal.ReferenceTypeId
 
 /**
  * @since 3.11.0