Tuple.prototype.groupBy

[acutmore]

Array.prototype.groupBy getting stage 3 made me think, I wonder what Tuple.prototype.groupBy would do 🤔

1. return an object of arrays
2. return a record of tuples
3. return an object of tuples

[nicolo-ribaudo]

The lists should be tuples, because they are "sublists" of the original tuple: tup.groupBy(computeKey).myKey is tup.filter(v => computeKey(v) === "myKey").

I don't have an opinion regarding the external wrapper, but if we choose to produce an object it will be easy to convert it to a record because it only contains primitives: Record(tup.groupBy(computeKey)).

[nicolo-ribaudo]

Well, since Tuple#groupByToMap will return a map probably Tuple#groupBy should return an object.

cc @jridgewell, if you have any opinion about this.

[ljharb]

X.prototype.groupBy should return an object of X's, just like it does on Array, which means the values must be Tuples.

That said, it would be perfectly reasonable for it to return a Record of Tuples in this case, instead of an object of Tuples.

[jridgewell]

X.prototype.groupBy should return an object of X's, just like it does on Array, which means the values must be Tuples.

I don't think must, but I think this is the right choice. Whether it's a record or an object, doesn't really matter to me.

[acutmore]

Thinking some more: Returning an object has the benefit of supporting grouping by symbol.

[ljharb]

Why couldn’t you group by a symbol in a Record?

[nicolo-ribaudo]

Records don't allow symbol keys (because they cannot be sorted).

[ljharb]

Oof, right. In that case it should probably return an object.

[rickbutton]

alternatively, Record.prototype.groupBy could return a record and just throw when you try to return a symbol from the callback, although kinda gross

[jridgewell]

It was suggested in tc39/proposal-array-grouping#30 (comment) that we look at Map.groupBy(array, cb), and now that we're dealing with possible Record return values, that's is looking more promising. Instead of having a groupByMap that returns a Map instance, the constructor that the static groupBy is called on decides the output type.

So we could have:

• Array.prototype.groupBy(cb), which returns an object
• Tuple.prototype.groupBy(cb), which returns an object
• Map.groupBy(arrayLike, cb), which returns a Map
• Record.groupBy(arrayLike, cb), which returns a Record

[ljharb]

I do like that.

[rricard]

We intend to have an opened draft PR but will only consider a merge once https://github.com/tc39/proposal-array-grouping goes to stage 4

[rricard]

PR is now open and will be merged conditionally if there is a progression to Stage 4 on proposal-array-grouping