type Foo = {id: number};

const fooReducer = (state: Foo, action) => {
    switch (action) {
        case "RENAME":
            return state.id === action.id
                ? { ...state, name: action.name }
                : state;
        default: return state;
    }
}

const foosReducer = (state: Foo[] = [], action) => {
    switch (action) {
        case "ADD_FOO": return [...state, action.foo];
        case "RENAME": return state.map(foo => fooReducer(foo, action));
        default: return state;
    }
}

const alternativeFooReducer = (state: Foo, action) => {
    switch (action) {
        case "RENAME": return { ...state, name: action.name };
        default: return state;
    }
}

const alternativeFoosReducer = (state: Foo[] = [], action) => {
    switch (action) {
        case "ADD_FOO": return [...state, action.foo];
        case "RENAME": {
            const fooToRename = state.find(f => f.id === action.id);
            const index = state.indexOf(fooToRename);
            return [
                ...state.slice(0, index),
                alternativeFooReducer(fooToRename, action),
                ...state.slice(index + 1)
            ]
        }
        default: return state;
    }
}