6.3.4. Explicit namespaces in import/export

6.3.4.1. The type keyword in import/export lists

Since: GHC 7.6

In an import or export list, such as

module M( f, (++) ) where ...
  import N( f, (++) )
  ...

the entities f and (++) are values. However, with type operators (Type operators) it becomes possible to declare (++) as a type constructor. In that case, how would you export or import it?

The ExplicitNamespaces extension allows you to prefix the name of a type constructor in an import or export list with “type” to disambiguate this case, thus:

module M( f, type (++) ) where ...
  import N( f, type (++) )
  ...
module N( f, type (++) ) where
  data family a ++ b = L a | R b

It is also possible to use the type namespace specifier in subordinate import and export lists:

module N (C(type (#))) where
  class C a b where
    type a # b
    (#) :: a -> b -> (a # b)
module M where
  import N as T (C(type (#)))
  import N as D (C((#)))
  -- (T.#) is the associated type
  -- (D.#) is the class method

The extension ExplicitNamespaces is implied by TypeOperators and (for some reason) by TypeFamilies.

6.3.4.2. The data keyword in import/export lists

Since: GHC 9.14

In an import or export list, such as

module M( T ) where ...
  import N( T )
  ...

the entity T refers to a type constructor, even if there is also a data constructor or pattern synonym of the same name.

For a concrete example, consider the declaration data Proxy t = Proxy and the following imports:

import Data.Proxy (Proxy(Proxy))  -- imports both constructors
import Data.Proxy (Proxy(..))     -- imports both constructors
import Data.Proxy (Proxy)         -- imports the type constructor only
import Data.Proxy (type Proxy)    -- imports the type constructor only

However, how would one import only the data constructor? There are two options:

import Data.Proxy (data Proxy)     -- imports the data constructor only
import Data.Proxy (pattern Proxy)  -- imports the data constructor only

The data keyword enables the import or export a data constructor without its parent type constructor.

The pattern keyword does the same, with only a few differences:

• Required compiler versions and flags

  • pattern is provided by the PatternSynonyms extension and requires GHC ≥7.8
  • data is enabled by ExplicitNamespaces and requires GHC ≥9.14

  See Import and export of pattern synonyms.

• Restrictions on use

  • pattern is restricted to top-level imports of pattern synonyms and data constructors:

    import N (pattern P)      -- ok    (top-level)
    import N (T(pattern P))   -- error (subordinate)
    import N (pattern f)      -- error (term or field selector)
    

  • data is also permitted in subordinate import/export lists, and is applicable to term declarations (functions and constants) and field selectors:

    import N (data P)         -- ok  (top-level)
    import N (T(data P))      -- ok  (subordinate)
    import N (data f)         -- ok  (term or field selector)
    

The data keyword is preferred over pattern in import/export lists unless there is a need to support older GHC versions.

6.3.4.3. Explicit namespaces in fixity declarations and warning/deprecation pragmas

Since: GHC 9.10

When ExplicitNamespaces is enabled, it is possible to use the type and data keywords to specify the namespace of the name used in a fixity signature or a WARNING/DEPRECATED pragma. This can be useful for disambiguating between names in different namespaces that may conflict with each other.

Here is an example of using namespace specifiers to set different fixities for type-level and term-level operators:

type f $ a = f a
f $ a = f a

infixl 9 type $ -- type-level $ is left-associative with priority 9
infixr 0 data $ -- term-level $ is right-associative with priority 0

Similarly, it can be used in pragmas to deprecate only one name in a namespace:

data Solo a = MkSolo a

pattern Solo x = MkSolo x
{-# DEPRECATED data Solo "Use `MkSolo` instead" #-}

type family Head xs where
  Head (x : _) = x

pattern Head x <- (head -> x)

{-# WARNING in "x-partial" data Head "this is a partial type synonym" #-}

It is considered bad practice to use a fixity signature, WARNING pragma, or DEPRECATED pragma for a type-level name without an explicit type namespace, and doing so will become an error in a future version of GHC.

6.3.4.4. The type keyword in expressions and patterns

Since: GHC 9.10

Furthermore, ExplicitNamespaces permits the use of the type keyword in patterns and expressions:

f (type t) x = ...       -- in a pattern
r = f (type Integer) 10  -- in an expression

This is used in conjunction with RequiredTypeArguments.