Better typings for reducers

[kosmogradsky]

Reducer in Redux is defined like so:

export type Reducer<S = any, A extends Action = AnyAction> = (
  state: S | undefined,
  action: A
) => S

I think it would be better to define LoopReducer in a similar way so the action type is taken from the parameter and the parameter has the same default as Reducer

[bdwain]

but reducers are called with all actions? Why would you ever provide a different type there?

[kosmogradsky]

You would want this to provide more exact type for better type inference in the reducer switch case. For example, in your app you can have:

type Action = {
  type: 'ACTION_ONE',
  payload: number
} | {
  type: 'ACTION_TWO',
  payload: string
} | {
  type: 'ACTION_THREE',
  payload: {
    property1: number,
    property2: string
  }
}

If you provide Action type to Reducer as a type parameter you'll get better type inference for payloads.

[bdwain]

But it still needs to accept other action types for the default case, right? How does that work

…

On Tue, Feb 26, 2019 at 7:50 AM Evgeny Kosmogradsky-Ivanov < ***@***.***> wrote: You would want this to provide more exact type for better type inference in the reducer switch case. For example, in your app you can have: type Action = { type: 'ACTION_ONE', payload: number } | { type: 'ACTION_TWO', payload: string } | { type: 'ACTION_THREE', payload: { property1: number, property2: string } } If you provide Action type to Reducer as a type parameter you get better type inference for payloads. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#210 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ADzDDp3CxIUykZYBa575770SX9HDPmzwks5vRTuPgaJpZM4a_v7m> .

[kosmogradsky]

You would still have a default case which returns the state unchanged. If all the members of Action union type are handled with respectful cases then the default case will have the action typed as never.

[bdwain]

sorry for the delay. I was on vacation the last few weeks. Sounds good!