Is it possible to type a non-empty list?

[daniel-nagy]

Hi,

I've been experimenting with Gleam recently and I was wondering if it was possible to type a non-empty list in Gleam? e.g.

pub fn of(items: NonEmptyList(a)) -> Node(a) {
  // ...
}

of([]) // error
of(["👍"]) // ok

Without being able to type a non-empty list the signature would need to be

pub fn of(items: List(a)) -> Option(Node(a)) {
  // ...
}

but then the constructor may return None which makes it more cumbersome to work with.

[daniel-nagy]

The following works but feels wrong

let assert Some(list) = linked_list.of([1, 2])

[daniel-nagy]

I added an Empty state to my Node type and that removed the need for a non-empty list. i.e.

pub type Node(a) {
  Node(data: a, next: Node(a))
  Empty
}

A non-empty list was not necessary in this case. I'm not sure if it would be useful in other cases or if it is just a skill issue 😅.

P.S. Yes, I'm aware the build-in list type is a singly-linked list. I'm doing this as a learning exercise.

[lpil]

Hello! There's a non-empty list in this package: https://github.com/giacomocavalieri/non_empty_list

[lpil]

One thing to keep in mind is that in Gleam the Option type is never used for success and failure, only things that are nullable. https://hexdocs.pm/gleam_stdlib/gleam/option.html#Option