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Add ReducerReader for building reducers out of state and action #1969

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Add ReducerReader for building reducers out of state and action #1969

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Commits on Mar 8, 2023

  1. Add ReducerReader for building reducers out of state and action 

    We've experimented with this before and called it `_Observe`, but
weren't using it. More and more community members have built their own
versions of `_Observe` and called them `StateReader` and `ActionReader`,
etc., which evokes `ScrollViewReader` and `GeometryReader` nicely, so
let's consider shipping this tool as a first-class citizen.

A couple example uses:

```swift
var body: some ReducerProtocol<State, Action> {
  ReducerReader { state, _ in
    switch state {  // Compile-time failure if new cases are added
    case .loggedIn:
      Scope(state: /AppFeature.loggedIn, action: /AppFeature.loggedIn) {
        LoggedInFeature()
      }
    case .loggedOut:
      Scope(state: /AppFeature.loggedOut, action: /AppFeature.loggedOut) {
        LoggedOutFeature()
      }
    }
  }
}
```

```swift
var body: some ReducerProtocol<State, Action> {
  ReducerReader { state, action in
    if state.isAdmin && action.isAdmin {
      AdminFeature()
    }
  }
}
```

We'd love any feedback the community may have, especially from those
that have used this kind of reducer in their applications.

We think a single `ReducerReader` entity is the way to go vs. three
reducers, one for reading state _and_ action (`ReducerReader`), and then
one for just reading state (`StateReader`) and one for just reading
actions (`ActionReader`), since you can simply ignore the value you
don't need to read:

```swift
// Instead of:
StateReader { state in /* ... */ }
// Do:
ReducerReader { state, _ in /* ... */ }

// Instead of:
ActionReader { action in /* ... */ }
// Do:
ReducerReader { _, action in /* ... */ }
```
    @stephencelis
    stephencelis committed Mar 8, 2023
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  2. wip

    @stephencelis
    stephencelis committed Mar 8, 2023
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  3. fix

    @stephencelis
    stephencelis committed Mar 8, 2023
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