resolve #111: allow passing initial state to curried producer function

__tests__/curry.js

@@ -10,9 +10,6 @@ function runTests(name, useProxies) {
 
         it("should check arguments", () => {
             expect(() => produce()).toThrow(/produce expects 1 or 2 arguments/)
-            expect(() => produce(() => {}, {})).toThrow(
-                /the second argument to produce should be a function/
-            )
             expect(() => produce(new Buffer(""), () => {})).toThrow(
                 /the first argument to an immer producer should be a primitive, plain object or array/
             )
@@ -21,7 +18,10 @@ function runTests(name, useProxies) {
             )
             expect(() => produce({}, {})).toThrow(/should be a function/)
             expect(() => produce({})).toThrow(
-                /first argument should be a function/
+                /if first argument is not a function, the second argument to produce should be a function/
+            )
+            expect(() => produce(() => {}, () => {})).toThrow(
+                /if first argument is a function .* the second argument to produce cannot be a function/
             )
         })
 
@@ -51,5 +51,17 @@ function runTests(name, useProxies) {
             expect(reducer(undefined, 3)).toEqual({hello: "world"})
             expect(reducer({}, 3)).toEqual({index: 3})
         })
+
+        it("should support passing an initial state as second argument", () => {
+            const reducer = produce(
+                (item, index) => {
+                    item.index = index
+                },
+                {hello: "world"}
+            )
+
+            expect(reducer(undefined, 3)).toEqual({hello: "world", index: 3})
+            expect(reducer({}, 3)).toEqual({index: 3})
+        })
     })
 }

readme.md

@@ -36,7 +36,7 @@ The Immer package exposes a default function that does all the work.
 
 `produce(currentState, producer: (draftState) => void): nextState`
 
-There is also a curried overload that is explained [below](#currying)
+There is also a curried overload that is explained [below](#currying).
 
 ## Example
 
@@ -168,8 +168,7 @@ onBirthDayClick2 = () => {
 
 ## Currying
 
-`produce` can be called with one or two arguments.
-The one argument version is intended to be used for currying. This means that you get a pre-bound producer that only needs a state to produce the value from.
+Passing a function as the first argument to `produce` is intended to be used for currying. This means that you get a pre-bound producer that only needs a state to produce the value from.
 The producer function gets passed in the draft, and any further arguments that were passed to the curried function.
 
 For example:
@@ -202,7 +201,22 @@ const byId = produce((draft, action) => {
 ```
 
 Note that `state` is now factored out (the created reducer will accept a state, and invoke the bound producer with it).
-One think to keep in mind; you cannot use this construction to initialize an uninitialized state. E.g. `draft = {}` doesn't do anything useful.
+
+If you want to initialize an uninitialized state using this construction, you can do so by passing the initial state as second argument to `produce`:
+
+```javascript
+import produce from 'immer'
+
+const byId = produce((draft, action) => {
+  switch (action.type) {
+    case RECEIVE_PRODUCTS:
+      action.products.forEach(product => {
+        draft[product.id] = product
+      })
+    })
+  }
+}, { 1: { id: 1, name: "product-1" } })
+```
 
 ## Auto freezing
 
@@ -216,9 +230,9 @@ One think to keep in mind; you cannot use this construction to initialize an uni
 ## Returning data from producers
 
 It is not needed to return anything from a producer, as Immer will return the (finalized) version of the `draft` anyway.
-However, it allowed to just `return draft`.
+However, it is allowed to just `return draft`.
 
-It is also allowed to return abritrarily other data from the producer function. But _only_ if you didn't modify the draft.
+It is also allowed to return arbitrarily other data from the producer function. But _only_ if you didn't modify the draft.
 This can be useful to produce an entirely new state. Some examples:
 
 ```javascript

src/immer.d.ts

@@ -1,13 +1,14 @@
 /**
  * Immer takes a state, and runs a function against it.
  * That function can freely mutate the state, as it will create copies-on-write.
- * This means that the original state will stay unchanged, and once the function finishes, the modified state is returned
+ * This means that the original state will stay unchanged, and once the function finishes, the modified state is returned.
  *
- * If only one argument is passed, this is interpreted as the recipe, and will create a curried function that will execute the recipe
- * any time it is called with a base state
+ * If the first argument is a function, this is interpreted as the recipe, and will create a curried function that will execute the recipe
+ * any time it is called with the current state.
  *
  * @param currentState - the state to start with
- * @param thunk - function that receives a proxy of the current state as first argument and which can be freely modified
+ * @param recipe - function that receives a proxy of the current state as first argument and which can be freely modified
+ * @param initialState - if a curried function is created and this argument was given, it will be used as fallback if the curried function is called with a state of undefined
  * @returns The next state: a new state, or the current state if nothing was modified
  */
 export default function<S = any>(
@@ -16,16 +17,20 @@ export default function<S = any>(
 ): S
 // curried invocations
 export default function<S = any>(
-    recipe: (this: S, draftState: S, ...extraArgs: any[]) => void
+    recipe: (this: S, draftState: S, ...extraArgs: any[]) => void,
+    initialState?: S
 ): (currentState: S, ...extraArgs: any[]) => S
 export default function<S = any, A = any>(
-    recipe: (this: S, draftState: S, a: A) => void
+    recipe: (this: S, draftState: S, a: A) => void,
+    initialState?: S
 ): (currentState: S, a: A) => S
 export default function<S = any, A = any, B = any>(
-    recipe: (this: S, draftState: S, a: A, b: B) => void
+    recipe: (this: S, draftState: S, a: A, b: B) => void,
+    initialState?: S
 ): (currentState: S, a: A, b: B) => S
 export default function<S = any, A = any, B = any, C = any>(
-    recipe: (this: S, draftState: S, a: A, b: B, c: C) => void
+    recipe: (this: S, draftState: S, a: A, b: B, c: C) => void,
+    initialState?: S
 ): (currentState: S, a: A, b: B, c: C) => S
 
 /**

src/immer.js

@@ -15,24 +15,35 @@ import {produceEs5} from "./es5"
  * @returns {any} a new state, or the base state if nothing was modified
  */
 export default function produce(baseState, producer) {
+    // prettier-ignore
+    if (arguments.length !== 1 && arguments.length !== 2) throw new Error("produce expects 1 or 2 arguments, got " + arguments.length)
+
     // curried invocation
-    if (arguments.length === 1) {
-        const producer = baseState
+    if (typeof baseState === "function") {
         // prettier-ignore
-        if (typeof producer !== "function") throw new Error("if produce is called with 1 argument, the first argument should be a function")
+        if (typeof producer === "function") throw new Error("if first argument is a function (curried invocation), the second argument to produce cannot be a function")
+
+        const initialState = producer
+        const recipe = baseState
+
         return function() {
             const args = arguments
-            return produce(args[0], draft => {
+
+            const currentState =
+                args[0] === undefined && initialState !== undefined
+                    ? initialState
+                    : args[0]
+
+            return produce(currentState, draft => {
                 args[0] = draft // blegh!
-                return producer.apply(draft, args)
+                return recipe.apply(draft, args)
             })
         }
     }
 
     // prettier-ignore
     {
-        if (arguments.length !== 2)  throw new Error("produce expects 1 or 2 arguments, got " + arguments.length)
-        if (typeof producer !== "function") throw new Error("the second argument to produce should be a function")
+        if (typeof producer !== "function") throw new Error("if first argument is not a function, the second argument to produce should be a function")
     }
 
     // if state is a primitive, don't bother proxying at all and just return whatever the producer returns on that value

src/immer.js.flow

@@ -3,13 +3,14 @@
 /**
  * Immer takes a state, and runs a function against it.
  * That function can freely mutate the state, as it will create copies-on-write.
- * This means that the original state will stay unchanged, and once the function finishes, the modified state is returned
+ * This means that the original state will stay unchanged, and once the function finishes, the modified state is returned.
  *
- * If only one argument is passed, this is interpreted as the recipe, and will create a curried function that will execute the recipe
- * any time it is called with a base state
+ * If the first argument is a function, this is interpreted as the recipe, and will create a curried function that will execute the recipe
+ * any time it is called with the current state.
  *
  * @param currentState - the state to start with
- * @param thunk - function that receives a proxy of the current state as first argument and which can be freely modified
+ * @param recipe - function that receives a proxy of the current state as first argument and which can be freely modified
+ * @param initialState - if a curried function is created and this argument was given, it will be used as fallback if the curried function is called with a state of undefined
  * @returns The next state: a new state, or the current state if nothing was modified
  */
 declare export default function produce<S>(
@@ -18,16 +19,20 @@ declare export default function produce<S>(
 ): S
 // curried invocations
 declare export default function produce<S, A, B, C>(
-    recipe: (draftState: S, a: A, b: B, c: C) => void
+    recipe: (draftState: S, a: A, b: B, c: C) => void,
+    initialState?: S
 ): (currentState: S, a: A, b: B, c: C) => S
 declare export default function produce<S, A, B>(
-    recipe: (draftState: S, a: A, b: B) => void
+    recipe: (draftState: S, a: A, b: B) => void,
+    initialState?: S
 ): (currentState: S, a: A, b: B) => S
 declare export default function produce<S, A>(
-    recipe: (draftState: S, a: A) => void
+    recipe: (draftState: S, a: A) => void,
+    initialState?: S
 ): (currentState: S) => S
 declare export default function produce<S>(
-    recipe: (draftState: S, ...extraArgs: any[]) => void
+    recipe: (draftState: S, ...extraArgs: any[]) => void,
+    initialState?: S
 ): (currentState: S, ...extraArgs: any[]) => S
 
 /**