There isn't. GHC will infer kind `Type -> Type` for `MyType2`.  You can check that in `ghci` with `:kind MyType` and `:kind MyType2`.  There's also third way to write it by adding a standalone kind signature:
```
{-# LANGUAGE StandalondKindSignatures, GADTs #-}
type MyType3 :: Type -> Type
data MyType3 a where
  MkT3 :: Int -> MyType3 Int
```

A polykinded version would look like this:
```
{-# LANGUAGE PolyKinds #-}
type MyType4 :: forall a -> Type
data MyType4 a where
  MkT3 :: MyType4 Int
```
in `ghci` the kind of `MyType4` is:
```
:kind MyType4
MyType4 :: forall k. forall (a :: k) -> Type
```

Best regards,
Marcin

Sent with ProtonMail Secure Email.

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Thursday, June 10th, 2021 at 13:15, Velichko Lefterov <velichko.lefterov@gmail.com> wrote:
I tried to understand DataKinds and GADTs...
Is there a difference between:

data MyType :: Type -> Type where
  MyTypeConstructor :: Int -> MyType Int

data MyType2 a where
  MyTypeConstructor2 :: Int -> MyType2 Int