.. _promotion:

Datatype promotion
==================

.. extension:: DataKinds
    :shortdesc: Enable datatype promotion.

    :since: 7.4.1

    Allow promotion of data types to kind level.

This section describes *data type promotion*, an extension to the kind
system that complements kind polymorphism. It is enabled by
:extension:`DataKinds`, and described in more detail in the paper `Giving
Haskell a Promotion <http://dreixel.net/research/pdf/ghp.pdf>`__, which
appeared at TLDI 2012.

Motivation
----------

Standard Haskell has a rich type language. Types classify terms and
serve to avoid many common programming mistakes. The kind language,
however, is relatively simple, distinguishing only regular types (kind
``Type``) and type constructors (e.g. kind ``Type -> Type -> Type``).
In particular when using advanced type
system features, such as type families (:ref:`type-families`) or GADTs
(:ref:`gadt`), this simple kind system is insufficient, and fails to
prevent simple errors. Consider the example of type-level natural
numbers, and length-indexed vectors: ::

    data Ze
    data Su n

    data Vec :: Type -> Type -> Type where
      Nil  :: Vec a Ze
      Cons :: a -> Vec a n -> Vec a (Su n)

The kind of ``Vec`` is ``Type -> Type -> Type``. This means that, e.g.,
``Vec Int Char`` is a well-kinded type, even though this is not what we
intend when defining length-indexed vectors.

With :extension:`DataKinds`, the example above can then be rewritten to: ::

    data Nat = Ze | Su Nat

    data Vec :: Type -> Nat -> Type where
      Nil  :: Vec a 'Ze
      Cons :: a -> Vec a n -> Vec a ('Su n)

With the improved kind of ``Vec``, things like ``Vec Int Char`` are now
ill-kinded, and GHC will report an error.

Overview
--------

With :extension:`DataKinds`, GHC automatically promotes every datatype
to be a kind and its (value) constructors to be type constructors. The
following types ::

    data Nat = Zero | Succ Nat

    data List a = Nil | Cons a (List a)

    data Pair a b = Pair a b

    data Sum a b = L a | R b

give rise to the following kinds and type constructors (where promoted
constructors are prefixed by a tick ``'``): ::

    Nat :: Type
    'Zero :: Nat
    'Succ :: Nat -> Nat

    List :: Type -> Type
    'Nil  :: forall k. List k
    'Cons :: forall k. k -> List k -> List k

    Pair  :: Type -> Type -> Type
    'Pair :: forall k1 k2. k1 -> k2 -> Pair k1 k2

    Sum :: Type -> Type -> Type
    'L :: k1 -> Sum k1 k2
    'R :: k2 -> Sum k1 k2

Virtually all data constructors, even those with rich kinds, can be promoted.
There are only a couple of exceptions to this rule:

-  Data family instance constructors cannot be promoted at the moment. GHC's
   type theory just isn’t up to the task of promoting data families, which
   requires full dependent types.

-  Data constructors with contexts that contain non-equality constraints cannot
   be promoted. For example: ::

     data Foo :: Type -> Type where
       MkFoo1 :: a ~ Int         => Foo a    -- promotable
       MkFoo2 :: a ~~ Int        => Foo a    -- promotable
       MkFoo3 :: Show a          => Foo a    -- not promotable

   ``MkFoo1`` and ``MkFoo2`` can be promoted, since their contexts
   only involve equality-oriented constraints. However, ``MkFoo3``'s context
   contains a non-equality constraint ``Show a``, and thus cannot be promoted.

.. _promotion-syntax:

Distinguishing between types and constructors
---------------------------------------------

In the examples above, all promoted constructors are prefixed with a single
quote mark ``'``. This mark tells GHC to look in the data constructor namespace
for a name, not the type (constructor) namespace. Consider ::

    data P = MkP    -- 1

    data Prom = P   -- 2

We can thus distinguish the type ``P`` (which has a constructor ``MkP``)
from the promoted data constructor ``'P`` (of kind ``Prom``).

As a convenience, GHC allows you to omit the quote mark when the name is
unambiguous. However, our experience has shown that the quote mark helps
to make code more readable and less error-prone. GHC thus supports
:ghc-flag:`-Wunticked-promoted-constructors` that will warn you if you
use a promoted data constructor without a preceding quote mark.

Just as in the case of Template Haskell (:ref:`th-syntax`), GHC gets
confused if you put a quote mark before a data constructor whose second
character is a quote mark. In this case, just put a space between the
promotion quote and the data constructor: ::

  data T = A'
  type S = 'A'   -- ERROR: looks like a character
  type R = ' A'  -- OK: promoted `A'`

.. _promoted-lists-and-tuples:

Promoted list and tuple types
-----------------------------

With :extension:`DataKinds`, Haskell's list and tuple types are natively
promoted to kinds, and enjoy the same convenient syntax at the type
level, albeit prefixed with a quote: ::

    data HList :: [Type] -> Type where
      HNil  :: HList '[]
      HCons :: a -> HList t -> HList (a ': t)

    data Tuple :: (Type,Type) -> Type where
      Tuple :: a -> b -> Tuple '(a,b)

    foo0 :: HList '[]
    foo0 = HNil

    foo1 :: HList '[Int]
    foo1 = HCons (3::Int) HNil

    foo2 :: HList [Int, Bool]
    foo2 = ...

For type-level lists of *two or more elements*, such as the signature of
``foo2`` above, the quote may be omitted because the meaning is unambiguous. But
for lists of one or zero elements (as in ``foo0`` and ``foo1``), the quote is
required, because the types ``[]`` and ``[Int]`` have existing meanings in
Haskell.

.. note::
    The declaration for ``HCons`` also requires :extension:`TypeOperators`
    because of infix type operator ``(':)``


.. _promotion-existentials:

Promoting existential data constructors
---------------------------------------

Note that we do promote existential data constructors that are otherwise
suitable. For example, consider the following: ::

    data Ex :: Type where
      MkEx :: forall a. a -> Ex

Both the type ``Ex`` and the data constructor ``MkEx`` get promoted,
with the polymorphic kind ``'MkEx :: forall k. k -> Ex``. Somewhat
surprisingly, you can write a type family to extract the member of a
type-level existential: ::

    type family UnEx (ex :: Ex) :: k
    type instance UnEx (MkEx x) = x

At first blush, ``UnEx`` seems poorly-kinded. The return kind ``k`` is
not mentioned in the arguments, and thus it would seem that an instance
would have to return a member of ``k`` *for any* ``k``. However, this is
not the case. The type family ``UnEx`` is a kind-indexed type family.
The return kind ``k`` is an implicit parameter to ``UnEx``. The
elaborated definitions are as follows (where implicit parameters are
denoted by braces): ::

    type family UnEx {k :: Type} (ex :: Ex) :: k
    type instance UnEx {k} (MkEx @k x) = x

Thus, the instance triggers only when the implicit parameter to ``UnEx``
matches the implicit parameter to ``MkEx``. Because ``k`` is actually a
parameter to ``UnEx``, the kind is not escaping the existential, and the
above code is valid.

See also :ghc-ticket:`7347`.


