This is true.
Improper list is a list where the tail is not an empty list.

# proper list:
iex(1)> [1, 2 | []]
[1, 2]

# improper list
iex(2)> [1, 2 | 3]
[1, 2 | 3]

| pipe means or. Think about maybe_improper_list as about function that returns a type, so it has 2 args and that’s why , comma is used.

The second part describes the tail end of the entire list, not the tail of any list item. For example:

[1, 2, 3 | [] ]

In the above, the “First Part” in the type describes 1, 2, 3 and the “Second Part” describes only []. So in your example [h | t ], the type of t is the union of the “first part” and “second part” unless we know that it is the end of the list, then it is only the “second part”.

The binary “constructor” syntax, <<...>> accepts bytes, but it doesn’t produce a byte(). Try for yourself, see if <<0>> equals 0.