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# 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

Currently we do not handle kind variables in the source language. So the following is invalid, for instance:

type family Apply (f :: k1 -> k2) (a :: k1)

Naturally we want to allow this. The syntax we propose is the one above, as described in the paper. (At least until ExplicitTypeApplication gets implemented.)

**Future work:** allow kind variable annotation.
Since the core language has all the support for kind variables, this shouldn't be too hard.

# Kind defaulting in type families

At the moment, when you define a type family without `-XPolyKinds` like this:

type family F a

it gets kind `* -> *`. There are no constraints on the kind of `a`, so we
default it to `*`. We also default the return kind of `F` to `*`.
The same happens for data families, and also for plain datatypes with phantom
types.

When you turn `-XPolyKinds` on, however, we currently give `F` the kind
`forall (k :: BOX). k -> *`. This is unsatisfactory for two reasons:

- The behaviour of kind generalisation changes when we turn
`-XPolyKinds`on, even though it doesn't really have to. We could still default to`*`unless you give a kind signature. So if you want`F`to be kind polymorphic, you should write`type family F (a :: k)`. This, of course, requires supporting explicit kind variables.

- Unlike the parameters, however, the return kind of
`F`is defaulted to`*`. This seems rather arbitrary. We should either generalise both arguments and return kind, or default both. In case we choose to default, the more general kind can be obtained by giving a signature:type family F (a :: k1) :: k2

**Future work:** do more consistent kind defaulting.

# #5682 (proper handling of infix promoted constructors)

Bug report #5682 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 Nat type Nat2 = Nat type family Add (m :: Nat2) (n :: Nat2) :: Nat2

**Future work:** promote type synonyms to kind synonyms.

# Better support for kinds in Template Haskell

Currently there is no support for promoted datatypes, or the kind `Constraint`, in Template Haskell.

**Future work:** address #5612, designing and implementing a way for Template Haskell to reify the new kinds.

# 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 = Proxy data 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 -> BOX Ind :: '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 -> BOX Ind :: Nat -> BOX

Now instead of sort variables we have kind variables, and we do not need to promote
`Nat` again.

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".