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Today's Topics:
1. Re: DataKinds and GADTs question? (c...@coot.me)
----------------------------------------------------------------------
Message: 1
Date: Sat, 12 Jun 2021 11:13:01 +0000
From: c...@coot.me
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] DataKinds and GADTs question?
Message-ID:
<WCVdiFMzZCPwzMoRiCQFWKAvs1YldwXZ1yay-shqB3irFEDOTqYfPF6izJ_WvruhtUxypjj5uD4zq-d-iZNGrAJ1HACVrCweFZiWJD88g0c=@coot.me>
Content-Type: text/plain; charset="utf-8"
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.lefte...@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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