atpath/index.js

const copyArrayWith = (key, value) => obj => {
    const result = obj == null ? [] : [...obj];
    result[key] = value;
    return result;
};

const copyMapWith = (key, value) => obj => {
    const result = obj == null ? new Map() : new Map(obj);
    result.set(key, value);
    return result;
};

const copyObjectWith = (key, value) => obj => {
    const result = obj == null ? {} : {...obj};
    result[key] = value;
    return result;
};

const isMapKey = key => typeof key === 'object' && key.isKeyInMap === true;

export const keyInMap = key => ({isKeyInMap: true, key});

export const kim = keyInMap;

const copyWith = (key, value) =>
    typeof key === 'number' ? copyArrayWith(key, value) :
        isMapKey(key) ? copyMapWith(key.key, value) :
            copyObjectWith(key, value);

function parseKeysInto (keyDescriptions, keys) {
    keyDescriptions.forEach(each => {
        if (typeof each === 'number' || isMapKey(each)) keys.push(each);
        else if (Array.isArray(each)) fillKeys(each);
        else keys.push(...each.toString().split('.'));
    });
    return keys;
}

const getKey = (x, key) => x == null ? undefined : isMapKey(key) ? x.get(key.key) : x[key];

const getKeyWithCheck = (x, key) =>
    x == null ? null :
        isMapKey(key) ? x.has(key.key) ? {value: x.get(key.key)} : null :
            x.hasOwnProperty(key) ? {value: x[key]} : null;

const chain = (list, stop, step) => (function worker (index) {
    return index >= list.length ? stop : step(list[index], worker(index + 1));
})(0);

const copyOnModification = (obj, key, value, modifierFn) => {
    const modified = modifierFn(value);
    return value === modified ? obj : copyWith(key, modified)(obj);
};

const atPathBeginningWith = prefix => (...keyDescriptions) => {
    const keys = parseKeysInto(keyDescriptions, [...prefix]);

    return Object.assign(atPathBeginningWith(keys), {
        get: obj => keys.reduce(getKey, obj),

        put: val => chain(keys, () => val, (key, next) => x =>
            copyOnModification(x, key, getKey(x, key), next)
        ),

        map: fn => chain(keys, fn, (key, next) => x => {
            const valueWithCheck = getKeyWithCheck(x, key);
            if (valueWithCheck == null) return x;
            return copyOnModification(x, key, valueWithCheck.value, next);
        })
    });
};

export const atPath = atPathBeginningWith([]);

export const cowWorkshop = (keys, fn = x => x) => (obj, {result = obj, resultKeys = keys, diff} = {}) => {
    keys.forEach((key, index) => {
        const value = fn(deget(key)(obj));
        if (typeof value === "undefined") return;
        const modifier = decow(resultKeys[index])(value);
        const oldResult = result;
        result = modifier(oldResult);
        if (result !== oldResult) {
            diff = modifier(diff);
        }
    });
    return {result, diff};
};