Jolan Rensen

if we do it by default, then we would get duplicate columns, because the key columns are often in the groups as well

But then, what qualifies as "new"? - `groupBy { expr { myCol } }`, yes - but `groupBy { myCol + 1 }`? - or `groupBy { myCol named "other"...

There's also the case where a user creates a new `expr` column with a duplicate name that should still be kept, so my suggestion is the following: Create a `concatWithKeys()`...

Alternatively, what's arguably a lot simpler, we could just explode the groups column. Like: ```kt internal fun GroupBy.concatWithKeys(): DataFrame = toDataFrame().explode { groups } ``` This will generate extra key...

They, unfortunately, won't work on it until after K2 is stabilized.

Actually, I think this is simply a result of Kotlin's type inference. This is the function in question: ```kotlin public inline fun DataFrame.add( column: ColumnAccessor, infer: Infer = Infer.Nulls, noinline...

https://youtrack.jetbrains.com/issue/KT-59207/Trailing-lambda-operator-invoke-resolution

Random Sunday night thought that might just work: make `expression` of type `AddExpression` and make `S : R`. Then `R` is defined by `column` and `S` must follow `R`. Edit:...

Seems like a lot of different functions break if I revert it back. Edit: oh, only like 3 But it was removed for a reason. My understanding of `out`/`in` (as...

Unfortunately the same issue applies for the KProperties API: ```kotlin public inline fun DataFrame.add( property: KProperty, infer: Infer = Infer.Nulls, noinline expression: AddExpression ): DataFrame = (this + mapToColumn(property, infer,...