# Polymorphic Kinds for Haskell

Currently thinking about adding **Polymorphic Kinds** to GHC ... this document is a WIP

This is strongly motivated by wanting to add a new KindSystem, but does not require it. See KindSystem for a discussion of the syntax of sorts (which classify kinds).

## Example: At the term level

f :: forall_kind (k :: **). forall (m :: k -> *) (a :: k) . m a -> Int f _ = 2 data T m = MkT (m Int) foo = f (Just 2) -- m = Maybe, a = Int bar = f (MkT (Just 2)) -- m = T , a = Maybe

## Example: Typeable[123..]

With polymorphic kinds, it should be possible to remove the need for
`Typeable[1,2,3,..]` classes from SYB for kinds that arn't `*`.

Because we need a way of talking about the types (and hence their kinds) in the type classes' functions, we will need a proxy data type:

data Proxy :: forall k . k -> * class Typeable (k :: **) (t :: k) where typeOf :: Proxy t -> TypeRep -- Typeable :: forall (k :: **). k -> Class -- f :: forall a. a -> Int -- forall a. f (x::a) = 3??? -- At term level we scope type variables from a separate signature -- For class decls it's unclear. One possiblity: implicitly bring k into scope, and -- infer its sort. -- When we call f, we write (f 3) not (f Int 3) -- Similarly we want to write (Typeable Int) not (Typable * Int) instance Typeable Bool where typeOf _ = mkTyCon "Prelude.Bool" [] instance Typeable Maybe where typeOf _ = mkTyConApp (mkTyCon "Prelude.Mabe") [] instance Typeable Either where typeOf _ = ... instance (Typeable (t1 :: (* -> *), Typeable (t2 :: *))) => Typeable (t1 t2) where typeOf _ = (typeOf (undefined :: Proxy t1)) `mkAppTy` (typeOf (undefined :: Proxy t2))

## Functions Quantifying over Kinds

For function signatures, we need a way of quantifying over kinds. Options:

### Option 1: Add forall_kind (or equivt.) notation

As in the example above, functions would need a new quantifier to explicitly mark when a new kind is being quantified over.

If it were to take the form `forall_kind vars .` then it shouldn't interact
with existing forms.

### Option 2: Use forall and infer kind variables from usage

f :: forall k (m :: k -> *) (a :: k) . m a -> Int f _ = 2

In the above example, it is clear* that k must quantify over kinds as it appears in the kind signatures.

*=well...

** Pros: **

- No new syntax

** Cons: **

- More complicated for both users and implementaion logic to work out what's going on

### Option 3: Completely implicit quantified kind variables

In any type signature, find all the free kind variables; bring them into scope; kind-check the type signature; fix the sorts of the kind variables. Entirely local to an explicit, user-written type signature.

f :: forall (m :: k -> *) (a :: k) . m a -> Int

**Pros:**

- In line with haskell type variables being implicitly quantified

** Cons: **

- This makes it hard to add additional constraints to the k in future (sort annotations, kind classes?)

Just occasionally, a Haskell program *needs* an explicit kind signature, because the defaulting mechanism makes the wrong choice:

data T m = MkT -- m defaults to (m::*)

Suppose you really wanted

data T (m::*->*) = MkT

Question: could the same thing happen at the next level up, so that we want explicit sort signatures? And if so, does that mean we need explicit binding sites for kind variables?

## Type Classes

Declaration options:

### Option 1: Implicit kind variables

class Bar (a :: k -> *) where -- standalone class Bar (a :: k -> *) => Baz (a :: * -> k) where -- superclass, explicit name (shared) class Bar a => Baz (a :: * -> k) where -- superclass, new k? instance Bang (a :: k -> *) => Bar (a :: k -> *) -- instance implication, explic instance Bang a => Bar (a :: k -> *) -- instance implication, explic class Foo (a :: k -> *) (b :: k -> *) where -- MPTC name shared

### Option 2: Explicit kind variables

Here reusing `forall` is probably safe, although possibly inconsistent if we go for `forall_kind` (or other) as a term level quantifier...

class forall k . Blah (a :: k -> *) where -- standalone class forall k . Baz (a :: k -> *) => Bar (a :: * -> k) where -- superclass, shared k class forall k . Baz a => Bar (a :: * -> k) where -- superclass, new k instance forall k . Bang (a :: k -> *) => Bar (a :: k -> *) -- instance implication, shared k instance forall k . Bang a => Bar (a :: k -> *) -- instance implication, new k class forall k . Foo (a :: k -> *) (b :: k -> *) where -- MPTC name shared

Again, this doesn't seem to add much now, however if we ever want to constrain the kind variables to particular sorts in the future, we will probably need binders for them.

### Option 3: Explicit kind signatures for type classes

Can't do this in general, as you need to name the variables that index the type class for use in member functions. Although for type classes with no member functions this may be a viable option. The question is what is the result of a type class?

class Blah :: forall k . (k -> *) -> CLASS where