Reorder update signature to State, Action, Environment (#14)

This PR changes the order of update function arguments:

- Changes update function signature from `update(State, Environment, Action)` to `update(State, Action, Environment)`
- Changes Store from `Store<State, Environment, Action>` to `Store<State, Action, Environment>`

Why?

- This order of arguments is used by two other Elm-like libraries: [Swift Composable Architecture](https://github.com/pointfreeco/swift-composable-architecture) and [Redux-like state container](https://swiftwithmajid.com/2019/09/18/redux-like-state-container-in-swiftui/).
- Context arguments like environment often seem to be passed last in Swift. E.g. [`UIViewRepresentable.Context`](https://developer.apple.com/documentation/swiftui/uiviewrepresentable/updateuiview(_:context:)) is passed last to methods in SwiftUI.
- State and Action are always used, but Environment might be nil, or an empty struct. You don't always need environment. It is optional.
- The order reads better in the signature, since state and action are always paired together in update function and `Update` struct.

README.md

@@ -33,8 +33,8 @@ struct AppState: Equatable {
     /// State update function
     static func update(
         state: AppState,
-        environment: AppEnvironment,
-        action: AppAction
+        action: AppAction,
+        environment: AppEnvironment
     ) -> Update<AppState, AppAction> {
         switch action {
         case .increment:
@@ -77,7 +77,7 @@ A `Store` is a source of truth for application state. It's an [ObservableObject]
 Store exposes a single [`@Published`](https://developer.apple.com/documentation/combine/published) property, `state`, which represents your application state. `state` is read-only, and cannot be updated directly. Instead, like Elm or Redux, all `state` changes happen through a single `update` function, with the signature:
 
 ```
-(State, Environment, Action) -> Update<State, Action>
+(State, Action, Environment) -> Update<State, Action>
 ```
 
 The `Update` returned is a small struct that contains a new state, plus any optional effects and animations associated with the state transition (more about that in a bit).
@@ -171,8 +171,8 @@ You can subscribe to an effects publisher by returning it as part of an Update:
 ```swift
 func update(
     state: State,
-    environment: Environment,
-    action: Action
+    action: Action,
+    environment: Environment
 ) -> Update<State, Action> {
     switch action {
     // ...
@@ -196,8 +196,8 @@ Use `Update.animation` to set an explicit [Animation](https://developer.apple.co
 ```swift
 func update(
     state: State,
-    environment: Environment,
-    action: Action
+    action: Action,
+    environment: Environment
 ) -> Update<State, Action> {
     switch action {
     // ...

Sources/ObservableStore/ObservableStore.swift

@@ -102,7 +102,7 @@ where State: Equatable {
 /// See https://guide.elm-lang.org/architecture/
 /// and https://guide.elm-lang.org/webapps/structure.html
 /// for more about this approach.
-public final class Store<State, Environment, Action>: ObservableObject
+public final class Store<State, Action, Environment>: ObservableObject
 where State: Equatable {
     /// Stores cancellables by ID
     private(set) var cancellables: [UUID: AnyCancellable] = [:]
@@ -112,8 +112,8 @@ where State: Equatable {
     /// Update function for state
     public var update: (
         State,
-        Environment,
-        Action
+        Action,
+        Environment
     ) -> Update<State, Action>
     /// Environment, which typically holds references to outside information,
     /// such as API methods.
@@ -134,8 +134,8 @@ where State: Equatable {
     public init(
         update: @escaping (
             State,
-            Environment,
-            Action
+            Action,
+            Environment
         ) -> Update<State, Action>,
         state: State,
         environment: Environment
@@ -232,7 +232,7 @@ where State: Equatable {
     /// `.receive(on: DispatchQueue.main)`).
     public func send(_ action: Action) {
         // Generate next state and effect
-        let next = update(self.state, self.environment, action)
+        let next = update(self.state, action, self.environment)
         // Set `state` if changed.
         //
         // Mutating state (a `@Published` property) will fire `objectWillChange`

Tests/ObservableStoreTests/ObservableStoreTests.swift

@@ -29,8 +29,8 @@ final class ObservableStoreTests: XCTestCase {
         /// State update function
         static func update(
             state: Self,
-            environment: Environment,
-            action: Action
+            action: Action,
+            environment: Environment
         ) -> Update<Self, Action> {
             switch action {
             case .increment:
@@ -214,8 +214,8 @@ final class ObservableStoreTests: XCTestCase {
         /// Update function for Fx tests (below)
         static func update(
             state: Self,
-            environment: Environment,
-            action: Action
+            action: Action,
+            environment: Environment
         ) -> Update<Self, Action> {
             switch action {
             case .setTitle(let title):