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Map.prototype.emplace

ECMAScript proposal and reference implementation for Map.prototype.emplace.

Author: Brad Farias (GoDaddy)

Champion: Erica Pramer (GoDaddy)

Stage: 2

Motivation

Adding and updating values of a Map are tasks that developers often perform in conjunction. There are currently no Map prototype methods for either of those two things, let alone a method that does both. The workarounds involve multiple lookups and developer inconvenience while avoiding encouraging code that is surprising or is potentially error prone.

Solution: emplace

We propose the addition of a method that will add a value to a map if the map does not already have something at key, and will also update an existing value at key. It’s worthwhile having this API for the average case to cut down on lookups. It is also worthwhile for developer convenience and expression of intent.

Examples & Proposed API

The following examples would all be optimized and made simpler by emplace. The proposed API allows a developer to do one lookup and update in place:

// given counts is a Map of object => id
counts.emplace(key, {
  insert(key, map) {
    return 0;
  },
  update(existing, key, map) {
    return existing + 1;
  }
});

Normalization of values during insertion

Currently you would need to do 3 lookups:

if (!map.has(key)) {
  map.set(key, value);
}
map.get(key).doThing();

With this proposal:

map.emplace(key, {
  insert: () => value
}).doThing();

Either update or insert for a specific key

You might get new data and want to calculate some aggregate if the key exists, but just insert if it's the first value at that key.

// two lookups
old = map.get(key);
if (!old) {
  map.set(key, value);
} else {
  map.set(key, updated);
}

With this proposal:

map.emplace(key, {
  update: () => updated,
  insert: () => value
});

Just insert if missing

You might omit an update if you're handling data that doesn't change, but can still be appended.

// two lookups
if (!map1.has(key)) {
  map1.set(key, value);
}

With this proposal:

map.emplace(key, {
  insert: () => value
});

Just update if present

You might want to omit an insert if you want to perform a function on all existing values in a Map (ex. normalization).

// three lookups
if (map.has(key)) {
  old = map.get(key);
  updated = old.doThing();
  map.set(key, updated);
}

With this proposal:

if (map.has(key)) {
  map.emplace(key, {
    update: (old) => old.doThing()
  });
}

Implementations in other languages

Similar functionality exists in other languages.

Java

• computeIfPresent remaps existing entry
• computeIfAbsent insert if empty. computes the insertion value with a mapping function

C++

• emplace inserts if missing
• map[] assignment opts inserts if missing at key but also returns a value if it exists at key
• insert_or_assign inserts if missing. updates existing value by replacing with a specific new one, not by applying a function to the existing value

Rust

• and_modify Provides in-place mutable access to an occupied entry
• or_insert_with inserts if empty. insertion value comes from a mapping function

Python

• setDefault Performs a get and an insert

Elixir

• Map.update/4 Updates the item with given function if key exists, otherwise inserts given initial value

FAQ

Is the goal to simplify the API or to optimize it?

• This proposal seeks to simplify expressing intent for programmers, and should ease optimization without complex analysis. For engines without complex analysis like IOT VMs this should see wins by avoiding multiple entry lookups, at potential call stack cost.

Why not use existing JS engines that optimize by coalescing the lookup and mutation?

• This does not cover all patterns (of which there are many), things such as ordering can cause the optimization to fail.

if (!x.has(a)) x.set(a, []);
if (!y.has(b)) y.set(b, []);
x.get(a).push(1);
y.get(b).push(2);

Why error if the key doesn't exist and no insert handler is provided.

• This keeps the return type constrained to the union of insert and update without adding undefined. This alleviates a variety of static checker errors from code such as the following.

// map is a Map of object values
let x;
if (map.has(key)) {
  x = map.get(key); // can return undefined
} else {
  x = {};
  map.set(key, x);
}
// x's type is `undefined | object` 

The proposal could guarantee that the type does not include undefined:

// map is a Map of object values
let x = map.emplace(key, {
  insert: () => { return {}; }
});
// x's type is `object`

Why not have a single function that has a boolean if performing an update and the potentially existing value?

• By naming the handlers, you can increase readability and reduce overall boilerplate. Additionally, generally there are not common workflows that have code paths that cover both updating and insertion. See the following which only seeks to insert a value if none exists:

x = map.emplace(key, (updating, value) => updating ? value : []);

The proposal allows a handler to avoid the boilerplate condition and focus only on the relevant workflows:

x = map.emplace(key, {
  insert: () => []
});

Why not have a single function that inserts the value if no such key is mapped?

• By only having a single function that inserts a variety of workflows become less clear. In particular, by only having insert, a usage of a default value must be inserted with an anti-update operation already applied.

// have to set the default value to -1, not 0
const n = counts.emplace(key, () => -1);
// have to perform an additional set afterwards
counts.set(key, n + 1);

The proposal allows a handler to avoid the odd default value and avoid the extra .set. This does still require coding logic for both, but keeps the intent more readable and localized.

counts.emplace(key, {
  insert: () => 0,
  update: (v) => v + 1
});

• This can also lead to insertion of values that are incomplete/invalid regarding the intended type of the Map:

updateNameOf(key) {
  // contacts only has objects which must have a string value
  const contact = contacts.insert(key, () => ({name:null}));
  // inserted and can be assured of same reference on .get
  contact.name = getName();
  // if getName throws, contact is incomplete/invalid
  // .name remains null
}

This can be rewritten to be less error prone by moving getName above

updateNameOf(key) {
  const name = getName();
  const contact = contacts.insert(key, () => ({name:null}));
  contact.name = name;
}

This code is less error prone (unless contact.name fails assignment for example), but it still has an invalid value being inserted into the map.

This can again be rewritten to be less constraint breaking by avoiding insert() entirely.

updateNameOf(key) {
  const name = getName();
  const existing = contacts.get(key) ?? {name:null};
  contact.name = name;
  contacts.set(key);
}

This design does avoid ever inserting the invalid value into the map, but is a bit confusing to read. This proposal by combining update and insert can be a little clearer:

updateNameOf(key, name) {
  const name = getName();
  contacts.emplace(key, {
    insert: () => ({name}),
    update: (contact) => {
      contact.name = name;
      return contact;
    }
  });
}

by having both operations co-located it would feel odd to try and getName after .emplace and no invalid value is inserted into the map.

Why not have a single function that updates the value if no if the key is mapped?

• By only having a single function that updates a variety of workflows become less clear. In particular in order to guarantee that the result type is not a union with undefined it should error if the key is not mapped.

let n;
try {
  n = counts.emplace(key, (existing) => existing + 1);
} catch (e) {
  // this is a fragile detection and quite hard to determine it was
  // counts.emplace that caused an error, and not something internal to
  // the update callback
  if (e instanceof MissingEntryError) {
    counts.set(key, n = 0);
  }
}
if (n > RETRIES) {
  // ...
}

A alteration to return a boolean to see if an action is taken requires boilerplate and reduces the utility of the return value:

let updated = counts.emplace(key, (existing) => existing + 1);
if (!updated) {
  counts.set(key, 0);
}
let n = counts.get(key);
if (n > RETRIES) {
  // ...
}

The proposal allows a handler to avoid the odd default value and avoid the extra .set.

let n = counts.emplace(key, {
  insert: () => 0,
  update: (v) => v + 1
});
if (n > RETRIES) {
  // ...
}

Why not have an API that exposes an Entry to collection types?

• An Entry API is not prevented by this proposal. Explicit thought about re-entrancy was taken into consideration and was designed not to conflict with such an API. Desires for such an API should be done in a separate proposal.
• An Entry API has much stricter implications on how implementations must store the backing data for a collection due to creating persistent references.
• An Entry API is extremely complex regarding shared mutability and should be considered to be an extreme increase in scope to the goals of this proposal. See complexity such as the following about needing to think of an design an entire lifecycle and sharing scheme for multiple entry references:

let entry1 = map.mutableEntry(key);
let entry2 = map.mutableEntry(key);
entry2.remove();
entry1.insertIfMissing(0);

Why use functions instead of values for the parameters?

• You may want to apply a factory function when inserting to avoid costs of potentially heavy allocation, or the key may be determined at insertion time.

// an example of when eager allocation of the value
// is undesirable
const sharedRequests = new Map();
function request(url) {
  return sharedRequests.emplace(url, {
    insert: () => {
      return fetch(url).then(() => {
        sharedRequests.delete(url);
      });
    }
  });
}

• When updating, we will be able to perform a function on the existing value instead of just replacing the value. The action may also cause mutation or side-effects, which would want to be avoided if not updating.

const eventCounts = new Map();
obj.onevent(
  (eventName) => {
    // this API allows working with value type and primitive values
    eventCounts.emplace(eventName, {
      update: (n) => n + 1,
      insert: () => 1
    });
  }
);

This is important as primitives like BigInt, [Records, and Tuples](Records and Tuples), etc. are added to the language. This API should continue to be able to handle and work with such values as they are added.

Why are we calling this emplace?

• updateOrInsert and insertOrUpdate seem too wordy.
  • in the case that only an insert operation is provided it may do neither update nor insert.
• upsert was seen as too unique a term and the ordering was problematic as there was a desire to focus on insertion.
  • It is a combination of "update" & "insert" that is already used in other programming situations and many SQL variants use that exact term.
• emplace matches a naming precedent from C++.

What happens during re-entrancy?

• Other methods like Array methods while iterating using a higher order function do not re-iterate if mutated in a re-entrant manner. This method will modify the underlying storage cell that contains the existing value and any mutation of the map will act on new storage cells if that cell is removed from the map. This method will not perform a second lookup if the storage cell in the collection for the key is replaced with a new one.
• See issue #9 for more.

Specification

• Ecmarkup source
• HTML version

Polyfill

A polyfill is available in the core-js library. You can find it in the ECMAScript proposals section.