Doubt Error Tree in Haskell

Question

My teacher, in a matter of Haskell asked to set tree in the following way:

Being that defining this way gives this error:

And in many corners of the internet, the tree in Haskell they define using Data as the image below:

Then I wonder if the teacher is correct in declaring tree this way, because the way he asked us to define the error in the compiler.

Answer 1

You are correct in your suspicion. The keyword type should not have been used in this case. The correct is data.

Briefly:

In Haskell, we have three basic ways to type:

• Using data for new types.
• Using type to declare synonyms of types, i.e., alternative names for existing types.
• Using newtype to define new types equivalent to existing types.

I recommend you read more about type reporting, as the type system is a key part of Haskell. You can start with Wikibook in English, where I got that reference.