From c6cd6327525886e38f4f34e716555f295a864f0e Mon Sep 17 00:00:00 2001
From: aleclarson <alec.stanford.larson@gmail.com>
Date: Sat, 2 Feb 2019 11:33:47 -0500
Subject: [PATCH] types: async produce

---
 __tests__/produce.ts | 47 +++++++++++++++++++++++++++++++++++++++-----
 src/immer.d.ts       | 16 +++++++--------
 2 files changed, 49 insertions(+), 14 deletions(-)

diff --git a/__tests__/produce.ts b/__tests__/produce.ts
index 41cdfcd3..4b31cdb3 100644
--- a/__tests__/produce.ts
+++ b/__tests__/produce.ts
@@ -196,15 +196,52 @@ it("does not enforce immutability at the type level", () => {
     exactType(result, {} as any[])
 })
 
-it("can produce nothing", () => {
-    let result = produce({}, _ => nothing)
+it("can produce an undefined value", () => {
+    let base = {} as {readonly a: number}
+
+    // Return only nothing.
+    let result = produce(base, _ => nothing)
     exactType(result, undefined)
+
+    // Return maybe nothing.
+    let result2 = produce(base, draft => {
+        if (draft.a > 0) return nothing
+    })
+    exactType(result2, {} as typeof base | undefined)
+})
+
+it("can return the draft itself", () => {
+    let base = {} as {readonly a: number}
+    let result = produce(base, draft => draft)
+
+    // Currently, the `readonly` modifier is lost.
+    exactType(result, {} as {a: number} | undefined)
+})
+
+it("can return a promise", () => {
+    let base = {} as {readonly a: number}
+
+    // Return a promise only.
+    exactType(
+        produce(base, draft => {
+            return Promise.resolve(draft.a > 0 ? null : undefined)
+        }),
+        {} as Promise<{readonly a: number} | null>
+    )
+
+    // Return a promise or undefined.
+    exactType(
+        produce(base, draft => {
+            if (draft.a > 0) return Promise.resolve()
+        }),
+        {} as (Promise<{readonly a: number}> | {readonly a: number})
+    )
 })
 
 it("works with `void` hack", () => {
-    let obj = {} as {readonly a: number}
-    let res = produce(obj, s => void s.a++)
-    exactType(res, obj)
+    let base = {} as {readonly a: number}
+    let copy = produce(base, s => void s.a++)
+    exactType(copy, base)
 })
 
 it("works with generic parameters", () => {
diff --git a/src/immer.d.ts b/src/immer.d.ts
index c7baa150..c5bee92d 100644
--- a/src/immer.d.ts
+++ b/src/immer.d.ts
@@ -43,17 +43,15 @@ export interface Patch {
 
 export type PatchListener = (patches: Patch[], inversePatches: Patch[]) => void
 
-type IsVoidLike<T> = T extends void | undefined ? 1 : 0
-
 /** Converts `nothing` into `undefined` */
-type FromNothing<T> = Nothing extends T ? Exclude<T, Nothing> | undefined : T
+type FromNothing<T> = T extends Nothing ? undefined : T
 
 /** The inferred return type of `produce` */
-export type Produced<T, Return> = IsVoidLike<Return> extends 0
-    ? FromNothing<Return>
-    : IsVoidLike<Return> extends 1
-    ? T
-    : T | FromNothing<Exclude<Return, void>>
+export type Produced<Base, Return> = Return extends void
+    ? Base
+    : Return extends Promise<infer Result>
+    ? Promise<Result extends void ? Base : FromNothing<Result>>
+    : FromNothing<Return>
 
 type ImmutableTuple<T extends ReadonlyArray<any>> = {
     readonly [P in keyof T]: Immutable<T[P]>
@@ -168,7 +166,7 @@ export function createDraft<T>(base: T): Draft<T>
  * Pass a function as the 2nd argument to generate Immer patches based on the
  * changes that were made.
  */
-export function finishDraft<T>(base: Draft<T>, listener?: PatchListener): T
+export function finishDraft<T>(draft: T, listener?: PatchListener): Immutable<T>
 
 /** Get the underlying object that is represented by the given draft */
 export function original<T>(value: T): T | void