6.4.10. Datatype promotion

DataKinds
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 DataKinds, and described in more detail in the paper Giving Haskell a Promotion, which appeared at TLDI 2012.

6.4.10.1. 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 (Type families) or GADTs (Generalised Algebraic Data Types (GADTs)), 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 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.

6.4.10.2. Overview

With 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.

6.4.10.3. 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 -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 (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'`

6.4.10.5. 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 #7347.