the comment is wrong, what Gleam lacks is interfaces.

Which feels super strange, but doesn't seem to really be a problem, e.g. imagine a language where you'd write

    fun sum_all_numbers(Iterable<T> it) { it.fold(...) } # an interface
    sum_all_numbers(a_list) # a list implements iterable

    fun sum_all_numbers(Iterator<T> it) { it.fold(...) } # a concrete type
    sum_all_numbers(a_list.iterator) # get the caller to build the right object