In some cases this lack can cause repeated code and work in later applying fold etc to the Grouping

# announcements

[Nir]

In some cases this lack can cause repeated code and work in later applying fold etc to the Grouping

[Nir]

Here's an example that motivated this question, there isn't much extra "work" really but a bit of extra code

[Nir]

fun List<Food>.toAllergenPossibilities() = asSequence()
    .flatMap { food -> food.allergens.map { it to food.ingredients } }
    .groupingBy { (allergen, _) -> allergen }
    .foldTo(mutableMapOf(),
        initialValueSelector = { _, (_, ingredients) -> ingredients.toMutableSet() }) { _, set, (_, ingredients) ->
        set.apply { retainAll(ingredients) }
    }

[Nir]

So, in this example, in foldTo, we need to destructure ingredients out of the value twice, even though allergen is already our key

[Nir]

Instead we could write it like this:

fun List<Food>.toAllergenPossibilities() = asSequence()
    .flatMap { food -> food.allergens.map { it to food.ingredients } }
    .groupingBy { (allergen, _) -> allergen }
    .mapValues { it.second }
    .foldTo(mutableMapOf(),
        initialValueSelector = { _, ingredients -> ingredients.toMutableSet() }) { _, set, ingredients ->
        set.apply { retainAll(ingredients) }
    }

[Nir]

Anyway can implement this myself I suppose, maybe the standard library would be interested in it

[Nir]

unfortunately this is pretty hard to implement without a lot of overhead, given how the Grouping interface is specified and the extension functions are written

[Nir]

I created an issue and summarized the problem here: https://youtrack.jetbrains.com/issue/KT-44503

[Nir]

If anyone knows why Grouping has this iterator + keyOf interface, instead of simply providing an iterator<Pair<T, K>>, I'd be curious to know

[ilya.gorbunov]

Iterator<Pair<T, K>> requires to allocate a pair for every element, this would significantly increase memory traffic of groupingBy operations.

[Nir]

Fair enough, that makes sense

[Nir]

Then, is the lack of valueOf also a conscious decision for perf reasons or was it simply not considered?