Version 9 (modified by dreixel, 2 years ago) (diff) |
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# Kind-polymorphic `Typeable`

The page describes an improved implementation of the `Typeable` class, using polymorphic kinds. Technically it is straightforward, but it represents a non-backward-compatible change to a widely used library, so we need to make a plan for the transition.

Relevant tickets we could thereby fix: #5391, #5863.

Open question: what are the corresponding changes to `Data.Data`? See #4896,

## The current `Typeable` class

The current `Typeable` class is:

class Typeable (a :: *) where typeOf :: a -> TypeRep

Because it is mono-kinded we also have

class Typeable1 (f :: *->*) where typeOf1 :: f a -> TypeRep

and so on up to `Typeable7`. It's a mess, and we cannot make `Typeable` at all for
type constructors with higher kinds like

Foo :: (* -> *) -> *

See #5391

## The new `Typeable` class

Having polymorphic kinds lets us say this:

data Proxy t = Proxy class Typeable t where typeRep :: proxy t -> TypeRep

Notice that

`Typeable`has a polymorphic kind:Typeable :: forall k. k -> Constraint

- The method is called
`typeRep`rather than`typeOf`

- One reason for the name change is that the argument is not a value of the type
`t`, but a value of type`(proxy t)`. We have to do this because`t`may have any kind, so we can't saytypeOf :: t -> TypeRep

- You can instantiate
`proxy`to whatever you want; one common choice is the poly-kinded`Proxy`datatype:Proxy:: forall k. k -> *

Now we can give give kind-specific instances:

instance Typeable Int where typeRep _ = ... instance Typeable [] where typeRep _ = ... instance (Typeable a, Typeable b) => Typeable (a b) where typeRep _ = ...

A use of `deriving( Typeable )` for a type constructor `T` would always generate

instance Typable T where typeRep _ = ....

i.e. an instance of `T` itself, not applied to anything.

### Aside

Iavor suggested:

class Typeable (a :: k) where typeRep :: TTypeRep a newtype TTypeRep a = TR TypeRep

Is this perhaps better?

## A change-over plan

**In GHC 7.8:**

- Rename
`Data.Typeable`to`Data.OldTypeable`and deprecate the whole module.

- Define a new library
`Data.Typeable`with the new definitions in them.

- Include in
`Data.Typeable`old methods for backward compatibility, but deprecate them:typeOf :: forall a. Typeable a => a -> TypeRep typeOf _ = typeRep (Proxy :: Proxy a) typeOf1 :: forall t (a :: *). Typeable t => t a -> TypeRep typeOf1 _ = typeRep (Proxy :: Proxy t)

- Make
`deriving( Typeable )`work with whatever`Typeable`class is in scope. So what it does will be determined by whether you say`import Data.Typeable`or`import Data.OldTypeable`.

**I think that means that old programs will continue to work in GHC 7.8**, provided

- You did not mention
`Typeable1`etc explicitly - You used
`deriving( Typeable )`to write instances.

**In GHC 7.10:**

- Remove
`Data.OldTypeable`