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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`

and`Proxy`

have polymorphic kinds:Proxy :: forall k. k -> * 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

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 7.6**, provided

- You did not mention
`Typeable1`

etc explicitly - You used
`deriving( Typeable )`

to write instances.

**In GHC 7.10:**

- Remove
`Data.OldTypeable`