|Version 9 (modified by dreixel, 6 months ago)|
Defining kinds without an associated datatype
When using -XDataKinds GHC automatically promotes every datatype to a kind, and its constructors to types. This forces us to declare a datatype for every kind. However, sometimes we are not interested in the datatype at all, only on the kind. Consider the following data kind that defines a small universe for generic programming:
data Universe star = Sum Universe Universe | Prod Universe Universe | K star
This universe comes with an associated interpretation:
data Interpretation :: Universe * -> * where L :: Interpretation a -> Interpretation (Sum a b) R :: Interpretation b -> Interpretation (Sum a b) Prod :: Interpretation a -> Interpretation b -> Interpretation (Prod a b) K :: a -> Interpretation (K a)
In this case, having to declare a datatype for Universe has two disadvantages:
- We lose constructor name space, because the datatype constructor names will be taken, even though we will never use them. So Prod and K cannot be used as constructors of Interpretation as above, because those are also constructors of Universe.
- We cannot use kinds (such as *) while defining a datatype, so we are forced to make Universe a parametrised datatype, and later always instantiate this parameter to * (like in the kind of Interpretation).
Proposal: allow defining kinds directly, as in the following example:
data kind Universe = Sum Universe Universe | Prod Universe Universe | K *
By using data kind, we tell GHC that we are only interested in the Universe kind, and not the datatype. Consequently, Sum, Prod, and K will be types only, and not constructors. Note however that this would imply being able to parse kinds (*, at the very least) on the right-hand side of data kind declarations. To avoid this, we propose instead using a kind Type (or Star), defined in GHC.Exts, that acts as a synonym of *.
This ticket to track this request is #6024.
Defining datatypes without an associated kind
By extension, we might want to define a datatype that will never be promoted, even with -XDataKinds. For that we propose the syntax data type D ....