6.3.4. Explicit namespaces in import/export

ExplicitNamespaces

Implied by:

TypeOperators, TypeFamilies

Since:

7.6.1

Status:

Included in GHC2024

Enable use of explicit namespaces in module export lists, patterns, and expressions.

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

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

In addition, with PatternSynonyms you can prefix the name of a data constructor in an import or export list with the keyword pattern, to allow the import or export of a data constructor without its parent type constructor (see Import and export of pattern synonyms).

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.

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 = MkSolo

pattern Solo = MkSolo
{-# 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.