This page gives additional implementation details for the -XPolyKinds flag. The grand design is described in the paper Giving Haskell a Promotion. Most of the work has been done and merged into GHC 7.4.1. The relevant user documentation is in [the user's guide (add link when it's up)] and on the Haskell wiki page. What still doesn't work, or doesn't work correctly, is described here.
data family T a data instance T Int = MkT data Proxy (a :: k) data S = MkS (Proxy 'MkT)
Is it ok to use the promoted data family instance constructor MkT in
the data declaration for S? No, we don't allow this. It might make
sense, but at least it would mean that we'd have to interleave
typechecking instances and data types, whereas at present we do data
types then instances.
Bug report #5682 (closed) shows a
problem in parsing promoted infix datatypes.
Future work: handle kind operators properly in the parser.
Kind synonyms (from type synonym promotion)
At the moment we are not promoting type synonyms, i.e. the following is invalid:
data Nat = Ze | Su Nattype Nat2 = Nattype family Add (m :: Nat2) (n :: Nat2) :: Nat2
We propose to change this, and make GHC promote
type synonyms to kind synonyms by default with -XDataKinds. For instance, type String = [Char]
should give rise to a kind String.
Question: are there dangerous interactions with -XLiberalTypeSynonyms? E.g. what's the kind
of *type K a = forall b. b -> a`?
By extension, we might want to have kind synonyms that do not arise from promotion: type kind K ....
And perhaps even type synonyms that never give rise to a promoted kind: type type T ....
Generalized Algebraic Data Kinds (GADKs)
Future work: this section deals with a proposal to collapse kinds and sorts into a single system
so as to allow Generalised Algebraic DataKinds (GADKs). The sort BOX should
become a kind, whose kind is again BOX. Kinds would no longer be classified by sorts;
they would be classified by kinds.
(As an aside, sets containing themselves result in an inconsistent system; see, for instance,
this example. This is not of practical
concern for Haskell.)
Collapsing kinds and sorts would allow some form of indexing on kinds. Consider the
following two types, currently not promotable in FC-pro:
data Proxy a = Proxydata Ind (n :: Nat) :: * where ...
In Proxy, a has kind forall k. k. This type is not promotable because
a does not have kind *. This is unfortunate, since a new feature (kind
polymorphism) is getting on the way of another new feature (promoting
datatypes). As for Ind, it takes an argument of kind (promoted) Nat,
which renders it non-promotable. Why is this? Well, promoted Proxy and Ind
would have sorts:
Proxy :: forall s. s -> BOXInd :: 'Nat -> BOX
But s is a sort variable, and 'Nat is the sort arising from promoting
the kind Nat (which itself arose from promoting a datatype). FC-pro has
neither sort variables nor promoted sorts. However, if there are no sorts, and
BOX is the kind of all kinds, the "sorts" ("kinds", now) of promoted Proxy
and Ind become:
Proxy :: forall k. k -> BOXInd :: Nat -> BOX
Now instead of sort variables we have kind variables, and we do not need to promote
Kind indexing alone should not require kind equality constraints; we always
require type/kind signatures for kind polymorphic stuff, so then
can be used to type check generalised algebraic kinds, avoiding the need for
coercions. While this would still require some implementation effort, it
should be "doable".