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Design of Redux-go v2

dannypsnl profile image 林子篆 Originally published at dannypsnl.github.io on ・4 min read

Redux is a single flow state manager. I porting it from JS to Go at last year.

But there had one thing make me can’t familiar with it, that is the type of state!

In Redux, we have store combined with many reducers. Then we dispatch action into the store to updating our state. That means our state could be anything.

In JS, we have a reducer like:

const counter = (state = 0, action) => {
    switch action.type {
    case "INC":
        return state + action.payload
    case "DEC":
        return state - action.payload
    default:
        return state
    }
}
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It looks good because we don’t have type limit at here. In Redux-go v1, we have:

func counter(state interface{}, action action.Action) interface{} {
    if state == nil {
        return 0
    }
    switch action.Type {
    case "INC":
        return state.(int) + action.Args["payload"].(int)
    case "DEC":
        return state.(int) - action.Args["payload"].(int)
    default:
        return state
    }
}
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Look at those assertions, of course, it’s safe because you should know which type are you using. But just so ugly.

So I decide to change this. In v2, we have:

func counter(state int, payload int) int {
    return state + payload
}
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Wait, what!!!?

So I have to explain the magic behind it.

First is how to get the type of state that user wanted. The answer is reflect package.

But how? Let’s dig in v2/store function: New.

func New(reducers ...interface{}) *Store
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As you see, we have to accept any type been a reducer at parameters part.

Then let’s see type: Store(only core part)

type Store struct {
    reducers []reflect.Value
    state map[uintptr]reflect.Value
}
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Yp, we store the reflect result that type is reflect.Value.

But why? Because if we store interface{}, we have to call reflect.ValueOf every time we want to call it! That will become too slow.

And state will have an explanation later.

So in the New body.

func New(reducers ...interface{}) *Store {
    // malloc a new store and point to it
    newStore := &Store{
        reducers: make([]reflect.Value, 0),
        state: make(map[uintptr]reflect.Value),
    }
    // range all reducers, of course
    for _, reducer := range reducers {
        r := reflect.ValueOf(reducer)
        checkReducer(r)
        // Stop for while
    }
}
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Ok, what is checkReducer? Let’s take a look now!

func checkReducer(r reflect.Value) {
    // Ex. nil
    if r.Kind() == reflect.Invalid {
        panic("It's an invalid value")
    }

    // reducer :: (state, action) -> state

    // Missing state or action
    // Ex. func counter(s int) int
    if r.Type().NumIn() != 2 {
        panic("reducer should have state & action two parameter, not thing more")
    }
    // Return mutiple result, Redux won't know how to do with this
    // Ex. func counter(s int, p int) (int, error)
    if r.Type().NumOut() != 1 {
        panic("reducer should return state only")
    }
    // Return's type is not input type, Redux don't know how would you like to handle this
    // Ex. func counter(s int, p int) string
    if r.Type().In(0) != r.Type().Out(0) {
        panic("reducer should own state with the same type at any time, if you want have variant value, please using interface")
    }
}
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Now back to New

// ...
for _, reducer := range reducers {
    // ...
    checkReducer(r)
    newStore.reducers = append(newStore.reducers, r)

    newStore.state[r.Pointer()] = r.Call(
        []reflect.Value{
            reflect.Zero(r.Type().In(0)),
            reflect.Zero(r.Type().In(1)),
        },
    )[0]
}
return newStore
// ...
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So that’s how state work, using an address of reducer mapping its state.

reflect.Value.Call this method allow you to invoke a reflect.Value from a function.

It’s parameter types required by signature. It always returns several reflect.Value, but because of we just very sure we only return one thing, so we can just extract index 0.

Then is state, why I choose to use a pointer but not function name this time?

Thinking about this:

// pkg a
func Reducer(s int, p int) int
// pkg b
func Reducer(s int, p int) int
// pkg main
func main() {
    store := store.New(a.Reducer, b.Reducer)
}
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Which one should we pick? Of course, we can try to left package name make it can be identified.

But next is really hard:

func main() {
    counter := func(s int, p int) int { return s + p }
    store := store.New(counter)
}
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If you think the counter name is counter, that is totally wrong, its name is func1.

So, I decide using function itself to get mapping state. That is new API: StateOf

func (s *Store) StateOf(reducer interface{}) interface{} {
    place := reflect.Valueof(reducer).Pointer()
    return s.state[place].Interface()
}
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The point is reflect.Value.Interface, this method returns the value it owns.

The reason we return interface{} at here is because, we have no way to convert to user wanted to type, and user always knows what them get actually, just for convenience we let user can use any type for their state, so they don’t need to do state.(int) these assertions.

Now, you just work like this:

func main() {
    counter := func(s int, payload int) int {
        return s + payload
    }
    store := store.New(counter)
    store.Dispatch(10)
    store.Dispatch(100)
    store.Dispatch(-30)
    fmt.Printf("%d\n", store.StateOf(counter)) // expected: 80
}
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These are the biggest breakthrough for v2, thanks for reading.

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