From 5e4c40bad83d5367463be478aca1a9ffca63f3c5 Mon Sep 17 00:00:00 2001
From: Oliver Heard <oliver@reason.ink>
Date: Thu, 22 Feb 2018 16:28:06 +0000
Subject: [PATCH] Improves type safety in immer.d.ts and fixes 97

 - Fixed two-arg produce() curries being returned as one-arg curries

 - Added a one-arg produce() curry signature

 - Current state may now be Readonly<T> via new type MaybeReadonly<T>

 - produce() and curries now return Readonly<T>

 - Added Draft<T> that ensures the draft is always writable (fixes 97)

 - Reversed the order of produce() signatures so that the most
   specific is used first

 - Added non-doc style comments to curry-producing produce() variants

 - Added a note to the varadic signature of produce() about Typescript
   issue 5453
---
 src/immer.d.ts | 45 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 32 insertions(+), 13 deletions(-)

diff --git a/src/immer.d.ts b/src/immer.d.ts
index 17aec72f..3c0d4950 100644
--- a/src/immer.d.ts
+++ b/src/immer.d.ts
@@ -1,3 +1,6 @@
+export type MaybeReadonly<T> = T|Readonly<T>
+export type Draft<T> = { [P in keyof T]: T[P] }
+
 /**
  * Immer takes a state, and runs a function against it.
  * That function can freely mutate the state, as it will create copies-on-write.
@@ -11,22 +14,38 @@
  * @returns The next state: a new state, or the current state if nothing was modified
  */
 export default function<S = any>(
-    currentState: S,
-    recipe: (this: S, draftState: S) => void
-): S
+    currentState: MaybeReadonly<S>,
+    recipe: (this: Readonly<S>, draftState: Draft<S>) => void
+): Readonly<S>
+
 // curried invocations
-export default function<S = any, A = any, B = any, C = any>(
-    recipe: (this: S, draftState: S, a: A, b: B, c: C) => void
-): (currentState: S, a: A, b: B, c: C) => S
-export default function<S = any, A = any, B = any>(
-    recipe: (this: S, draftState: S, a: A, b: B) => void
-): (currentState: S, a: A, b: B) => S
+
+// 0 additional arguments
+export default function<S = any>(
+    recipe: (this: Readonly<S>, draftState: Draft<S>) => void
+): (currentState: MaybeReadonly<S>) => Readonly<S>
+
+// 1 additional argument of type A
 export default function<S = any, A = any>(
-    recipe: (this: S, draftState: S, a: A) => void
-): (currentState: S) => S
+    recipe: (this: Readonly<S>, draftState: Draft<S>, a: A) => void
+): (currentState: MaybeReadonly<S>, a: A) => Readonly<S>
+
+// 2 additional arguments of types A and B
+export default function<S = any, A = any, B = any>(
+    recipe: (this: Readonly<S>, draftState: Draft<S>, a: A, b: B) => void
+): (currentState: MaybeReadonly<S>, a: A, b: B) => Readonly<S>
+
+// 3 additional arguments of types A, B and C
+export default function<S = any, A = any, B = any, C = any>(
+    recipe: (this: Readonly<S>, draftState: Draft<S>, a: A, b: B, c: C) => void
+): (currentState: MaybeReadonly<S>, a: A, b: B, c: C) => Readonly<S>
+
+// any number of additional arguments, but with loss of type safety
+// this may be alleviated if "variadic kinds" makes it into Typescript:
+// https://github.com/Microsoft/TypeScript/issues/5453
 export default function<S = any>(
-    recipe: (this: S, draftState: S, ...extraArgs: any[]) => void
-): (currentState: S, ...extraArgs: any[]) => S
+    recipe: (this: Readonly<S>, draftState: Draft<S>, ...extraArgs: any[]) => void
+): (currentState: MaybeReadonly<S>, ...extraArgs: any[]) => Readonly<S>
 
 /**
  * Automatically freezes any state trees generated by immer.