|Version 20 (modified by 5 years ago) (diff),|
Kind polymorphism and datatype promotion
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.
Explicit kind variables
Kind defaulting in type families
#5682 (proper handling of infix promoted constructors)
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 Nat type Nat2 = Nat type family Add (m :: Nat2) (n :: Nat2) :: Nat2
Future work: promote type synonyms to kind synonyms.
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
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 = Proxy data Ind (n :: Nat) :: * where ...
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)
which renders it non-promotable. Why is this? Well, promoted
would have sorts:
Proxy :: forall s. s -> BOX Ind :: 'Nat -> BOX
s is a sort variable, and
'Nat is the sort arising from promoting
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 :: forall k. k -> BOX Ind :: 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 wobbly types 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".