Should `NonEmptyList`s be assumed immutable? Or is...

# arrow

[Youssef Shoaib [MOD]]

Should `NonEmptyList`s be assumed immutable? Or is the only assumption that the list is never empty? I ask because the `wrap`ing APIs say that you need to ensure the list stays nonempty, but it doesn't clearly say whether you can mutate the underlying list or not

[Alejandro Serrano.Mena]

It should only assume it's not empty (and stays not empty). Wrapping a mutable list should be perfectly OK

[Ulrich Schuster]

For consistency, wouldn't that be a NonEmptyMutableList? A Kotlin List is immutable, so I would expect a NonEmptyList to be the same (immutability is the default).

[Youssef Shoaib [MOD]]

Nope! A Kotlin List is not immutable, it's read only. Case in point: val list: List<Int> = mutableListOf()

[Ulrich Schuster]

Sorry for being sloppy – that's what I meant.

In my opinion, The NonEmpty wrapper should adhere to the Kotlin naming convention for collections.

[Youssef Shoaib [MOD]]

It does, though.
All that we're saying is that the following is completely valid:

val mutable = mutableListOf(1)
val nonEmpty = mutable.wrapAsNonEmptyListOrThrow()
println(nonEmpty) // [1]
mutable.add(2)
println(nonEmpty) // [1, 2]

(note that wrapAsNonEmptyListOrThrow has an opt-in because you must ensure that the list you're wrapping never becomes empty)

[Ulrich Schuster]

As a simple user of the arrow libraries, that looks disturbing. You put a guarantee into the type that the type cannot deliver.

[Youssef Shoaib [MOD]]

I didn't, the user who calls wrapAsNonEmptyListOrThrow and opts-in (to the aptly named PotentiallyUnsafeNonEmptyOperation) does.
These functions are provided firstly for performance, but it's also perfectly legitimate to wrap a mutable list, as long as you, the person who explicitly opts-in, decides that your list will never become empty in the future

[Youssef Shoaib [MOD]]

FWIW, I'm working on a set of safe (so no opt-in) APIs that'll allow for NonEmptyMutableList to be a thing, and truly ensuring that the list cannot ever get empty

[Ulrich Schuster]

hmmm, as a user of the non-mutable `List`` I would not like to be bothered by an opt-in that doesn't apply to me.

[Youssef Shoaib [MOD]]

If none of your code uses that opt-in, you're fine. The regular toNonEmptyList() functions do a defensive copy.

[Youssef Shoaib [MOD]]

Let me make it very clear, for future readers as well:
If you never opt-in to PotentiallyUnsafeNonEmptyOperation, all your NonEmptyList-related code will never encounter empty lists (modulo bugs, of course).
If you opt-in to PotentiallyUnsafeNonEmptyOperation, it's your responsibility to make sure that the list passed to wrapAsNonEmptyXXX never goes empty. If you're a library, be dang sure that it doesn't, or your users will be very unhappy!
Within Arrow itself, we use that method sparingly, and (AFAIK) never to wrap a mutable list (I just remembered one exception, but it's in experimental Accumulate code, and it only calls add and addAll on the mutable list, so it's safe)
In fact, using the wrapAsNonEmptyXXX methods with a guaranteed-immutable list as the receiver is completely safe (of course, the orThrow variants will throw in such cases, which is to be expected).

[Youssef Shoaib [MOD]]

FWIW, here's the PR that adds buildNonEmptyList/Set