I have a `var list: List<Item>`, what would ...

# getting-started

[Marcin Wisniowski]

I have a var list: List<Item>, what would be an idiomatic way of replacing one item of this list? It’s not a MutableList, so I need a way to generate a new copy of this list, with a single element replaced.

[Marcin Wisniowski]

I could do something like list = list.take(indexOfItem) + newItem + list.drop(indexOfItem + 1) but it feels like it could be simpler.

[Adam S]

Does order matter? Should the new item have the same index as the old item?

[Marcin Wisniowski]

It does.

[Marcin Wisniowski]

Hmm, maybe

items = items.map { if (it == oldItem) newItem else it }

[Adam S]

you beat me to it, I was going to suggest exactly that :)

[Adam S]

you might want to have a require(oldItem in items) check

[Adam S]

if you wanted to make this list more reusable you could create a value class that has a private mutable list that implements List<T> via delegation

[Marcin Wisniowski]

It’s a var items by remember { mutableStateOf(listOf(originalContents...)) }
that I need mutate in response to click handlers in Compose. Normally the handlers would go up higher but in this case it’s a Compose @Preview.

[Joffrey]

As a side note, you could also use persistent collections if you use this pattern often:
https://github.com/Kotlin/kotlinx.collections.immutable

They offer the mutate extension which simplifies this kind of operation

[Joffrey]

I think Compose already uses this library behind the scenes so it shouldn't bring more dependencies to your project since it seems you're already using Compose

[CLOVIS]

In case you're not aware, you can also use val items = remember { mutableStateListOf(originalContents) } then use the regular mutable replace . Compose will still recompose correctly.

[Marcin Wisniowski]

What’s the replace method? I’m not seeing it. There is replaceAll which looks like it would solve it, but sadly it requires API level 24 on Android.

[ephemient]

I don't think Compose uses kotlinx.collections.immutable directly, but the compiler recognizes it as immutable

[ephemient]

val newList = buildList(list.size) {
    addAll(list)
    set(indexOfItem, newItem)
}

[CLOVIS]

@Marcin Wisniowski Sorry, it's called set , not replace

[Marcin Wisniowski]

That does work quite nicely, thanks.