function copyWith (obj, key, value) {
    const result = typeof key === 'number' ? [...obj] : {...obj};
    result[key] = value;
    return result;
}

export const cowValueModel = (key, keys = key.split('.')) => {
    function setField (x = {}, index, val) {
        if (index >= keys.length) return val;
        const key = keys[index],
            value = x[key],
            modified = setField(value, index + 1, val);
        return value === modified ? x : copyWith(x, key, modified);
    }

    const GET_SENTINEL = {};

    return (obj, val = GET_SENTINEL) =>
        val === GET_SENTINEL ?
            keys.reduce((x = {}, key) => x[key], obj) :
            setField(obj, 0, val);
};