Thanks Richard, indeed that proposal would not help.  The snippet I posted is a valid GADT syntax.  When it is compiled with `DataKinds`, `PolyKinds` and `KindSignatures` ghc is is quite explicit why it refuses it: Expected a type, but `x` has a kind `a`, which is exactly what you said.

However, I think it would be quite reasonable to accept it. In general ghc would only need to generate code for the case `a ~ Type`, all other use cases at term level must be refused. It might be enough to add a type inference rule which injects `a ~ Type` for any such term.

Another missing puzzle is that there's no way to specify that one only wants the promoted types / kinds without the term level part. This could be done by specifying an explicit kind signature:
```
type K0 :: a -> K a
data K a where
  K0 :: forall a (x :: a) -> K a
```
Now this is refused with `The standalone kind signature for 'K0' lacks an accompynying binding`.

It's not the first time I stumbled upon this.  The latest incarnation is a gist in which I worked out how to encode pipelining using a type level queue / list in a session type framework which we use at work for developing protocols:
https://gist.github.com/coot/b568ebc7bac2e4e31cb54bf3939419d8#file-pipelined-hs-L94

Richard, does it sound reasonable to you? If so, what would be the right process to propose / implement such a feature? I don't think it would require a new extension, rather modify how `DataKind` works in presence of `PolyKinds` and `StandaloneKindSignatures`.

Maybe this is in scope of the dependent type Haskell workstream that you're doing?

Best regards,
Marcin

‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Monday, May 3rd, 2021 at 16:11, Richard Eisenberg <rae@richarde.dev> wrote:

Perhaps Marcin is looking for https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0402-gadt-syntax.rst, but that proposal would not accept the code in the original post, for the reasons Kai describes -- you just cannot have (x :: a) on one side of an arrow.

Richard

On May 3, 2021, at 9:10 AM, Kai-Oliver Prott <kai.prott@hotmail.de> wrote:

Hi,

I think I don't quite understand the question.

The code you propose seems wrong to me for a different reason:
The type constructor (->) for function values has Kind: * -> * -> *

In fact, GHC will print this error when compiling the snippet with the corresponding extensions.

Promoting a data type is done differently.

Can you elaborate on what exactly it is you are proposing?

Sincerely
Kai Prott

On 03.05.21 14:04, coot@coot.me wrote:
Hello,

Currently with `DataKind` extension, Haskell allows to promote terms / types to types / kinds.  Currently, one cannot write:

```
data K a where
  K0 :: forall (x :: a). x -> K a
```

Because `K0` is both a term and a type constructor, and as a term and one cannot represent `x` of kind `a`.  Is there a proposal or an issue to allow such declaration and error at use sites of `K0` as a term, rather than at declaration site?

Best regards,
Marcin Szamotulski




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