Is there any built-in way to ensure lists are empty, which gives a nel when it fails? Sort-of like `...

# arrow

[Emil Kantis]

Is there any built-in way to ensure lists are empty, which gives a nel when it fails? Sort-of like

ensureEmpty(list) { nel -> ProblemWithNel(nel) }

[simon.vergauwen]

No, but I've had more of these questions and I think we should consider facilitating them.

Adding useful functions to the DSL can benefit multiple types, and DSLs at once.

[simon.vergauwen]

Some transformations, especially NonEmpty are hard since after the isNotEmpty or isEmpty check you don't know anything.

[Emil Kantis]

Want me to raise an issue? Or will you create one? 🙂 I guess you know more about what the desired API should look like. Personally I added this:

@RaiseDSL
public inline fun <Error, B : Any> Raise<Error>.ensureEmpty(
   value: Collection<B>,
   raise: (NonEmptyList<B>) -> Error,
) {
   value.toNonEmptyListOrNull()?.let { nel ->
      raise(raise(nel))
   }
}

[simon.vergauwen]

That's perfect for a impl/solution ☺️ Yes, please create a ticket. Not sure if we'll add this one but I think it's good to have an open discussion. I know @timrijckaert once proposed the opposite. ensureNotEmpty for Iterable -> NonEmptyList.

[simon.vergauwen]

There were a couple more, but I cannot remember them.

[Emil Kantis]

https://github.com/arrow-kt/arrow/issues/3393