# redux-sac

## Slice and compose redux-type reducers.

### `subReducer(key, reducer, additionalKey, ...)`

Creates a wrapper reducer that call `reducer`
on the substate specified by `key`.
You may use dot notation.
Rest of the state is left untouched.

```js
	const r = subReducer("persons", personReducer);

    r({persons: ["John", "Jill"], cars: ["Honda"]}, cityon);
    // => {
    //   persons: personReducer(["John", "Jill"], action), 
    //   cars: ["Honda"]
    // }
```

Respects redux convention that if no change was made,
the identical object should be returned. So in previous case,
if `personReducer` would return the identical array,
`r` would return the state object it was passed in.

If persons were deeper in hierarchy, it could have been created as
`const r = subReducer("files.persons", personReducer);` for example.

You may pass additional keys (also with possible dot-notation)
as addition arguments. In that case, additional parts of state
will be fetched and passed to a sub-reducer:

```js
	const r = subReducer("persons", personReducer, "assets.cars");

    r({persons: ["John", "Jill"], assets: {cars: ["Honda"]}}, action);
    // => {
    //   persons: personReducer(["John", "Jill"], action, ["Honda"]), 
    //   assets: {cars: ["Honda"]}
    // }
```

This technique is mentioned in Redux docs, in "Beyond combineReducers" page.

### `composeReducers(reducer1, reducer2, ...)`

Creates a wrapper reducer that calls passed reducers
one after another, passing intermediate states.
and returning the result of the last one.

Useful to "concatenate" a few `subReducer`s. like:

```js
	composeReducers(
	    subReducer("files.persons", personReducer, "assets.swag"),
	    subReducer("files.clients", clientReducer, "news"),
	    subReducer("assets", assetReducer),
	    baseReducer
	)
```

## Redux helpers

### `cowValueModel(key, ...)`

Creates an object with set of functions allowing
to set or get specified key from any object.
Get when one arg, set when two args.
Specify keys by passing a list of keys to `cowValueModel`.
You can use dot notation.

Setting return a copy with specified (sub-)property changed;
in case no change actually happens, returns the original object.

```js
    const name = cowValueModel("name");
    
    name({name: "Tom"});
    // => "Tom"
    name({name: "Tom"}, "Jerry");
    // => {name: "Jerry"}
    
    const city = cowValueModel("address.city");
    const city2 = cowValueModel("address", "city");
    const object = {address: {city: "New York"}};
    
    city(object);
    // => "New York"
    city2(object);
    // => "New York"
    city(object, "London");
    // => {address: {city: "London"}}
    city2(object, "London");
    // => {address: {city: "London"}}
    object;
    // => {address: {city: "New York"}}
    city(object, "New York") === object;
    // => true
    city2(object, "New York") === object;
    // => true
```

If you put a number in a list of keys to use,
an object will be treated as an array (unlike the default string case,
where it is treated as an object), so copy wil be created
using `[...obj]`, not using `{...obj}`.

That way you can create eg. `const c = cowValueObject("person", 34, "name")`
to access `obj.person[34].name` with `c(obj)` / `c(obj, val)`.