feat(switchReduce): Added type safety way to reduce state

[gionkunz]

I've been trying to work out a preferred solution for me when dealing with reducers. I wanted to have type safety so that I don't need to guess what the payload of an action is. This is the result which I currently use in my project and I think it would be valuable for many other developers. Also I think switch is the worst language construct that ever existed (right after goto) and I believe we should try to reduce the amount of developers who need to use it. It's error prone, unsafe and a complete syntax mess. This is an example of how to use switchReduce in conjunction with TypedAction<T> which I've also added to the README.md. What do you think?

import { ActionReducer, TypedAction, switchReduce } from '@ngrx/store';

export class AddAction implements TypedAction<number> {
  readonly type = 'ADD';
  constructor(public readonly payload: number) {}
}
export class SubtractAction implements TypedAction<number> {
  readonly type = 'SUBTRACT';
  constructor(public readonly payload: number) {}
}
export class ResetAction implements TypedAction<any> {
  readonly type = 'RESET';
  readonly payload: any;
  constructor() {}
}

export const counterReducer: ActionReducer<number> =
  (state: number = 0, action: TypedAction<any>) =>
    switchReduce(state, action)
      .byClass(AddAction, (num: number) => {
        return state + num;
      })
      .byClass(SubtractAction, (num: number) => {
        return state - num;
      })
      .byClass(ResetAction, () => {
        return 0;
      })
      .reduce();

[gionkunz]

I just saw this is somewhat an extension to #382

[jotatoledo]

Currently on my reducer functions I can always see the type of the payload. The only thing that isnt type safe on my current project is the return of the toPayload function when I call map with it as argument on any observable.

Does this solves the describe problem with toPayload by any luck? That would be awesome

[gionkunz]

I guess the operator could be extended in such a way, ill take a look