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




_______________________________________________
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.
_______________________________________________
Haskell-Cafe mailing list
To (un)subscribe, modify options or view archives go to:
http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
Only members subscribed via the mailman list are allowed to post.