I'm thinking about making a contribution to simpli...

# arrow

[James Baker]

I'm thinking about making a contribution to simplify converting a NonEmptyList to a NonEmptySet.

Currently, I think the way to do this is: nonEmptyList.toNonEmptySetOrNull()!!. This isn't ideal because the orNull case can never happen leading to unnecessary error handling (in this case with the !!).

Is this something people would be interested and if so which of these is preferred?

Option 1: Have a toSet() function in the class body, so all toSet() calls return a a NonEmptySet (I like this option because the Set will always be "`NonEmpty`", this would change the behaviour of existing code but it wouldn't be a breaking change):

// Sudocode implementation
class NonEmptyList<out A> {
  fun toSet(): NonEmptySet<A> = NonEmptySet(head, tail.toSet())
}

// Usage
nonEmptyList.toSet()

Option 2: Have an extension function toNonEmptySet() (more explicit and maybe a safer change than Option 1):

// Implementation
fun <A> NonEmptyList<A>.toNonEmptySet(): NonEmptySet<A> = NonEmptySet(head, tail.toSet())

// Usage
nonEmptyList.toNonEmptySet()

[James Baker]

Bonus question: Should there be an equivalent for going from NonEmptySet to NonEmptyList?

[Alejandro Serrano Mena]

I think the best option here would be to have a NonEmptyCollection interface (maybe just with a first() returning T), make both types implement that interface, and then make toNonEmptySet and toNonEmptyList work on those

[sindrenm]

If the idea is only converting from a NonEmptyList, then toNonEmptySet sounds good to me. If you also want to add an extension on any potentially-including-nulls collection, then that'd be something like toNonEmptySetOrNull, IMO).

[sindrenm]

I don't think toSet is great, just because we already know Kotlin's sets to potentially be empty. Expressive is better, in this case, IMO.

[Alejandro Serrano Mena]

this PR introduces NonEmptyCollection and the functions you mentioned https://github.com/arrow-kt/arrow/pull/3068

[James Baker]

@Alejandro Serrano Mena Thank you for raising that. It looks good to me 👍