Given a mutable list of: ```data class Foo(val id: Int, val name: String)``` What's an ideomatic way...

# announcements

[dave08]

Given a mutable list of:

data class Foo(val id: Int, val name: String)

What's an ideomatic way of searching for an id and replacing in-place the entry with another name (without turning the list into a map)? And return the id or the found entry as a bonus..

[Big Chungus]

You can't replace the name as it's immutable. Or do you mean replacing entire element with a modified copy if it?

[dave08]

yes

[arekolek]

• do you want to replace it in-place or is it ok to remove the item and add the new one to the end of the list?
• do you want to replace only first or all occurrences?

[dave08]

Only the first, and better in-place.

[Big Chungus]

Seems like some kind of pop operator would help here then. Or if you're happy with replacing all occurences, you can use map operator

[dave08]

map? That would just recreate the whole list, whereas now I have a mutable one...

[arekolek]

And return the id or the found entry as a bonus

did you mean the old name maybe? why return id if it's going to be the same as the passed argument or null (if not found)?

[dave08]

In my case, I search on another field in the data class... I guess the contrived example is a bit too contrived 😅

[arekolek]

ok, I read "or found entry" as "of found entry", but returning the entry makes sense

[dave08]

-- found entry is also fine... I could get the id from there

I need this to create db repository stubs for unit testing... I thought it would be easier to simulate INSERT ... ON CONFLICT ... RETURNING id ... it just turned out not being such a one liner anymore...

[arekolek]

I came up with this:

fun MutableList<Foo>.replace(id: Int, name: String): Foo? {
    val index = indexOfFirst { it.id == id }
    return if (index > -1) {
        set(index, get(index).copy(name = name))
    } else {
        null
    }
}

[dave08]

Thanks! If I turn that into a more generic function, it should come in handy 🙂, I wonder if this (in some form) could be an interesting addition to the stdlib? There's already a replaceAll...

[arekolek]

Maybe 🤔

fun <T> MutableList<T>.replace(predicate: (T) -> Boolean, transformation: (T) -> T): T? {
    val index = indexOfFirst(predicate)
    return if (index > -1) {
        set(index, transformation(get(index)))
    } else {
        null
    }
}

fun MutableList<Foo>.replace(id: Int, name: String): Foo? {
    return replace({ it.id == id }) { it.copy(name = name) }
}

[Big Chungus]

I'd also make it inline

[dave08]

I did the same, but I used a receiver for the predicate... but I think I like yours better... since I think all the stdlib functions do that.

Inline to save all the lambdas, I suppose. That's also an idea!

[Big Chungus]

Even better yet, crossinline lambda args too

[arekolek]

Even better yet, crossinline lambda args too

does that change anything in the case above?

[Big Chungus]

It'll not allocate lambda args and instead just unfurl them in call sites, so you end up with code that only uses stdlib

[Big Chungus]

Basically erasing the existence of your helper fun in bytecode alltogether

[arekolek]

isn't it what inline does? bytecode seems to be exactly the same with and without crossinline

[ephemient]

don't use crossinline unless you need it, it prevents nonlocal returns

[ephemient]

you can consider using https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-list/list-iterator.html to mutate a list entry in place instead of relying on indices

[ephemient]

e.g.

Untitled.kt

[arekolek]

I think the above needs some slight modification to return the value before transformation, rather than after transformation

[ephemient]

I misread the previous code. behavior now matches

[dave08]

Thanks! I also ended up making this one (also pretty handy for stubbing repositories in tests):

fun <T> MutableList<T>.modify(predicate: (T) -> Boolean, transformation: T.() -> Unit): T? {
        val iterator = listIterator()
        while (iterator.hasNext()) {
            val value = iterator.next()
            if (predicate(value)) return value.also { value.transformation() }
        }
        return null
    }