feature(toPayload): use generics

[jotatoledo]

[x] Feature request

What is the current behavior?

Currently, using .map(toPayload) will return an Observable<any> as result.

In most cases, developers will use toPayload to extract the information of an already filtered action (with ofType operator), so they know exactly what the action looks like and what is the type of the contained payload. But still, the type of it gets lost, what can result in typos or other problems.

Expected behavior:

Calling .map(toPayload) should be type aware; with this I mean that the type of the payload should be relevant for the resulting Observable stream,

This could be easily archieved by passing a type value to the function, so that the returned value matches the passed type. For example, the definition of the function could be changed to the following:

export function toPayload<T = any>(action: Action):  T {
  return (action as any).payload;
}

And then the function should be used as follows:

.map(toPayload<SomeInterface>)

Could this be possible?

Thanks in advance.

[jotatoledo]

Just noticed that this was already addressed by #161