Quacks like a list, but it's not a list

Elixir lists allow you to group lists of elements. Even though we see them in iex in this way:

list = [1, 2]

It almost looks like an array in other languages, but it is not. It’s a linked list.

Each list item only contains its value and a pointer to the next element in the list. And the last element is an empty list.

So the previous list, in fact under the hoods, is built like this:

list = [1 | [2 | []]]

It makes sense to make it look like an array, or else this is a bit unreadable.

One thing that can happen is that we can mess up and don’t construct the list correctly.

iex(1)> list = [1 | 2]
[1 | 2]

In this case, the last element is not an empty list, so this ends up being an improper list.

It looks like a duck…

iex(2)> is_list(list)
true

It quacks like a duck…

iex(3)> [head|tail] = list
[1 | 2]
iex(4)> head
1
iex(5)> tail
2

But it’s not a duck.

iex()> length(list)
** (ArgumentError) errors were found at the given arguments:
  * 1st argument: not a list

    :erlang.length([1 | 2])

One day this will happen for sure.