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... 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-ga..., 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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