Add type-level utils to asserting layer types (#5920)

Author: mattiamanzati

.changeset/dry-cycles-shake.md

@@ -0,0 +1,5 @@
+---
+"effect": minor
+---
+
+Add type-level utils to asserting layer types

packages/effect/dtslint/Layer.tst.ts

@@ -1,4 +1,4 @@
-import { Layer, Schedule } from "effect"
+import { Context, Layer, Schedule } from "effect"
 import { describe, expect, it } from "tstyche"
 
 interface In1 {}
@@ -19,6 +19,8 @@ interface Out3 {}
 
 declare const layer3: Layer.Layer<Out3, Err3, In3>
 
+class TestService1 extends Context.Tag("TestService1")<TestService1, {}>() {}
+
 describe("Layer", () => {
   it("merge", () => {
     expect(Layer.merge).type.not.toBeCallableWith()
@@ -40,4 +42,28 @@ describe("Layer", () => {
     expect(Layer.retry(layer1, Schedule.recurs(1))).type.toBe<Layer.Layer<Out1, Err1, In1>>()
     expect(layer1.pipe(Layer.retry(Schedule.recurs(1)))).type.toBe<Layer.Layer<Out1, Err1, In1>>()
   })
+
+  it("ensureSuccessType", () => {
+    expect(layer1.pipe(Layer.ensureSuccessType<Out1>())).type.toBe<Layer.Layer<Out1, Err1, In1>>()
+  })
+
+  it("ensureErrorType", () => {
+    const withoutError = Layer.succeed(TestService1, {})
+    expect(withoutError.pipe(Layer.ensureErrorType<never>())).type.toBe<Layer.Layer<TestService1, never, never>>()
+
+    const withError = layer1
+    expect(withError.pipe(Layer.ensureErrorType<Err1>())).type.toBe<Layer.Layer<Out1, Err1, In1>>()
+  })
+
+  it("ensureRequirementsType", () => {
+    const withoutRequirements = Layer.succeed(TestService1, {})
+    expect(withoutRequirements.pipe(Layer.ensureRequirementsType<never>())).type.toBe<
+      Layer.Layer<TestService1, never, never>
+    >()
+
+    const withRequirement = layer1
+    expect(withRequirement.pipe(Layer.ensureRequirementsType<In1>())).type.toBe<
+      Layer.Layer<Out1, Err1, In1>
+    >()
+  })
 })

packages/effect/src/Layer.ts

@@ -1226,3 +1226,55 @@ export const updateService = dual<
     layer,
     map(context(), (c) => Context.add(c, tag, f(Context.unsafeGet(c, tag))))
   ))
+
+// -----------------------------------------------------------------------------
+// Type constraints
+// -----------------------------------------------------------------------------
+
+/**
+ * A no-op type constraint that enforces the success channel of a Layer conforms to
+ * the specified success type `ROut`.
+ *
+ * @example
+ * import { Layer } from "effect"
+ *
+ * // Ensure that the layer produces the expected services.
+ * const program = Layer.succeed(MyService, new MyServiceImpl()).pipe(Layer.ensureSuccessType<MyService>())
+ *
+ * @since 3.20.0
+ * @category Type constraints
+ */
+export const ensureSuccessType =
+  <ROut>() => <ROut2 extends ROut, E, RIn>(layer: Layer<ROut2, E, RIn>): Layer<ROut2, E, RIn> => layer
+
+/**
+ * A no-op type constraint that enforces the error channel of a Layer conforms to
+ * the specified error type `E`.
+ *
+ * @example
+ * import { Layer } from "effect"
+ *
+ * // Ensure that the layer does not expose any unhandled errors.
+ * const program = Layer.succeed(MyService, new MyServiceImpl()).pipe(Layer.ensureErrorType<never>())
+ *
+ * @since 3.20.0
+ * @category Type constraints
+ */
+export const ensureErrorType = <E>() => <ROut, E2 extends E, RIn>(layer: Layer<ROut, E2, RIn>): Layer<ROut, E2, RIn> =>
+  layer
+
+/**
+ * A no-op type constraint that enforces the requirements channel of a Layer conforms to
+ * the specified requirements type `RIn`.
+ *
+ * @example
+ * import { Layer } from "effect"
+ *
+ * // Ensure that the layer does not have any requirements.
+ * const program = Layer.succeed(MyService, new MyServiceImpl()).pipe(Layer.ensureRequirementsType<never>())
+ *
+ * @since 3.20.0
+ * @category Type constraints
+ */
+export const ensureRequirementsType =
+  <RIn>() => <ROut, E, RIn2 extends RIn>(layer: Layer<ROut, E, RIn2>): Layer<ROut, E, RIn2> => layer