Nonempty lists with clean syntax in Reason (requires Bucklescript 6.*)

Nonempty.re

type empty;

type nonempty;

type t(_, _) =
  | []: t('a, empty)
  | ::('a, t('a, _)): t('a, nonempty);

let rec fromList: ('a, list('a)) => t('a, nonempty) =
  x =>
    fun
    | [] => [x]
    | [head, ...rest] => [x, ...fromList(head, rest)];

let rec toList: type a. ('b, t('b, a)) => list('b) =
  x =>
    fun
    | [] => []
    | [x] => [x]
    | [head, ...rest] => [x, ...toList(head, rest)];

let rec map: type a. (t('b, a), 'b => 'c) => t('c, a) =
  fun
  | [] => (_ => [])
  | [head, ...tail] => (f => [f(head), ...map(tail, f)]);

let head: t('a, nonempty) => 'a =
  fun
  | [head, ..._] => head;

[1]->head->Js.log; // Compiles and works as expected, notice that an int is returned, not an option(int)
// []->head->Js.log; // Doesn't even compile!