[Haskell-beginners] DataKinds and GADTs question?

[coot at coot.me]

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

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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

On Thursday, June 10th, 2021 at 13:15, Velichko Lefterov <velichko.lefterov at 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
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