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kotlinlang

How can i convert a

`List&lt;Either&lt;A,B&gt;&gt;`

            *TO*

`Either&lt;List&lt;A&gt;,List&lt;B&gt;&gt;`
            OR
`ValidaterNel&lt;A,List&lt;B&gt;&gt;`

basically it iterates over either and collect both errors and success,
• *`.sequence` shortCircuits* which isnt wanted here*,*
• *`.traverse {}`* is there but I thought there would a extension function for above as its a common useCase I guess, *is there an extension for above* *?*

have builtSomething like this:

```
fun &lt;A, B&gt; Iterable&lt;Either&lt;A, B&gt;&gt;.toValidatedNel(): ValidatedNel&lt;A, List&lt;B&gt;&gt; {
    return traverse(semigroup = Semigroup.nonEmptyList()) { it.toValidatedNel() }
}```


This is your best bet, there is nothing out of the box for this in Arrow atm. We are unifying `Either` and `Validated` in Arrow 2.x.x, so this would be covered by that.

still on going but work is happening in this branch <https://github.com/arrow-kt/arrow/pull/2778>

have come up with below:

_Suggestions are welcome._

Hey <@U015KQK93FW>, I don’t think your observation is correct.

It starts with an `acc = Valid(emptyList())` and then it runs `f` with the first element of the `Iterable`. If `val res = f(a)` returns `Invalid` it will check the state of `acc` and combine the results when needed.

So for example `listOf(1).traverse { Invalid(it) }` it will start with `acc = Valid(emptyList&lt;B&gt;())` and then run `val res = Invalid(1)`.

Since `res` is `Invalid` and `acc = Valid` it will return the `Invalid` case.

Here are a property based test that verify this, <https://github.com/arrow-kt/arrow/blob/96c774a08b1c86f369f075b4b8dd81a8904121d1/arrow-libs/core/arrow-core/src/commonTest/kotlin/arrow/core/IterableTest.kt#L193|https://github.com/arrow-kt/arrow/blob/96c774a08b1c86f369f075b4b8dd81a8904121d1/ar[…]ore/arrow-core/src/commonTest/kotlin/arrow/core/IterableTest.kt>