list of nested empty lists in Haskell

Question

Why is it possible to make such a list in Haskell:

slist = [ [], [[]], [[],[[]]] ]

As far I understand, every element has various types here (like in mathematics: Ø, {Ø} and so on). And ghci says:

> :t []
[] :: [t]
> :t [[]]
[[]] :: [[t]]

formally, I see different notes.

In other words, the first element is a simple empty list and the second one is a list of list (!) and so on.

What is wrong? Why does Haskell consider them to be the same type?

Answer 1

You are right, that in a Haskell list, all elements must be of the same type. And indeed, the type in your example is:

> :t slist
slist :: [[[[a]]]]

But the empty list [] can have any type, as long as it's of the form [b], but there are many possible bs. So there are many possible concrete types. One of them is for b to be of type [[[a]]], as in your slist.

Answer 2

Take a look at type of the first element of such a list:

> head [ [], [[]], [[],[[]]] ]
[]
it :: [[[t]]]

It's not t, but it's [[[t]]].

Why I can possibility to make in Haskell such list

Because the is nothing wrong with the type of this expression.

What is wrong? Why does Haskell consider them to be the same type?

t in the [[[[t]]]] is not a final type, it's type variable. That's why the type of the first element of this list could be a or [b] or [[c]] or [[[t]]].