Evolving Immutable objects

Write declarative denormalization of your Immutable.js object that run as fast (almost) as traditional denormalizations done through imperative updates.

Theory

Immutable.js uses hash-tries to represent immutable data. This allows for fast creation of new objects from old objects. If you create a map const map1 = Map({a: 11, b: 12, c: 13, d: 14}) and const map2 = map1.set('e', 15), the map map2 will share most of the memory with the map map1. A neat side-effect of this is the ease of diffing immutable objects that share common edit history.

Read more about the hash-trie diffing here: immutable-js/immutable-js#953

If a denormalized object must be computed from an immutable object it has to be done anew every time the source object is replaced. However with a cheap diff, a set of changes can be extracted from two object and then applied to the denormalized object.

Example

Imagine you have the likesById object:

const likes = immutable.Set([
  {id: 'lA', postId: 'pA', userId: 'uA'},
  {id: 'lB', postId: 'pB', userId: 'uB'},
  {id: 'lC', postId: 'pC', userId: 'uC'},
  {id: 'lD', postId: 'pD', userId: 'uD'},
  {id: 'lE', postId: 'pD', userId: 'uC'},
])
const likesById = Map(likes.map(like => ([like.id, like])))

If you want to create an aggregate of likes by post id you could write something like that:

const simpleGetLikesByPostId = (likesById) => {
  const likesByPostId = immutable.Map().asMutable()
  likesById.forEach((like, likeId) => {
    likesByPostId.update(like.postId, (likes) => (likes || immutable.Map()).set(likeId, like))
  })
  return likesByPostId.asImmutable()
}

The resulting datastructure is computed anew each time simpleGetLikesByPostId is called. Even if the likesById datastructure didn't change (or changed only slightly) the same expensive grouping computation is performed.

Instead the getLikesByPostId can be rewritten to using differential memization:

const diffMemGetLikesByPostId = (() => {
  let previousLikesById = immutable.Map()
  let previousLikesByPostId = immutable.Map()

  return (likesById) => {
    const difference = likesById.diffFrom(previousLikesById)

    let newLikesByPostId = previousLikesByPostId.asMutable()

    difference.added.forEach((like, likeId) => {
      newLikesByPostId.update(like.postId, (likes) => (likes || immutable.Map()).set(likeId, like))
    })
    difference.removed.forEach((like, likeId) => {
      newLikesByPostId.update(like.postId, (likes) => likes.remove(likeId))
      if(newLikesByPostId.get(like.postId).isEmpty()) {
        newLikesByPostId.remove(like.postId)
      }
    })
    difference.updated.forEach(({ prev: prevLike, next: nextLike }, likeId) => {
      if(prevLike.postId !== nextLike.postId) {
        newLikesByPostId.update(prevLike.postId, (likes) => likes.remove(likeId))
        if(newLikesByPostId.get(prevLike.postId).isEmpty()) {
          newLikesByPostId.remove(prevLike.postId)
        }
        newLikesByPostId.update(nextLike.postId, (likes) => (likes || immutable.Map()).set(likeId, nextLike))
      } else {
        newLikesByPostId.update(prevLike.postId, (likes) => likes.set(likeId, nextLike))
      }
    })

    previousLikesByPostId = newLikesByPostId.asImmutable()
    previousLikesById = likesById
    return previousLikesByPostId
  }
})()

Now each time diffMemGetLikesByPostId is called, the data used in the previous computation is reused and only the changes in the likesById argument are applied to the newly computed result. Writing all your aggregations like that can be cumbersome, and that is what evolving-immutable is for:

const getLikesByPostId = semiPureFunction({
  createMemory: () => ({
    groupLikesByPostId: group(like => like.postId)
  }),
  executeFunction: ({ groupLikesByPostId }, likesById) => {
    return groupLikesByPostId(likesById)
  }
})

Chains

The chains are here to simplify the composition of multiple operations. Chains can not be created during an execution of another chain to prevent the loss of memoization. The can however be attached to each other in various ways during the setup. A chain can map over all elements of the dataset using a second chain, before filtering the transformed elements with the third chain:

const chain = EvImm.startChain()
  .addMapStep(
    EvImm.startChain()
      .addFilterStep(value => value.param > 3)
      .addGroupStep(value => value.id)
      .endChain()
  )
  .addFilterStep(
    (groups) => !groups.isEmpty()
  )

errendir/evolving-immutable

Evolving Immutable objects

Theory

Example

Chains