6.3.4. Explicit namespaces in import/export

ExplicitNamespaces

Implied by:TypeOperators, TypeFamilies Since:7.6.1

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.