View source for GHC/QualifiedModuleExport

[[Category:GHC|Qualified Module Exports]]

== Motivation ==

There are a number of libraries in the Haskell ecosystem which encourage qualified imports to avoid name clashes with other modules. For example:

<haskell>
   -- Import the ByteString and Text types un-qualified.
  import Data.ByteString (ByteString)
  import Data.Map.Strict (Map)
  import Data.Text (Text)

  -- Import ByteString and Text modules qualified to avoid name collisions.
  import qualified Data.ByteString.Char8 as BS
  import qualified Data.Map.Strict as Map
  import qualified Data.Text as T

</haskell>

In larger projects the above lines will need to be added to every file that uses <hask>ByteString</hask>, <hask>Map</hask> and <hask>Text</hask>.

The problem for the types can easily be fixed by creating a Prelude module which does the following:

<haskell>
  module MyPrelude
      ( Text
      , Map
      , ByteString
      ) where
  import Data.ByteString (ByteString)
  import Data.Map.Strict (Map)
  import Data.Text (Text)

</haskell>

and then importing MyPrelude to access the types. Unfortunately there is currently no way to add qualified module exports to <hask>MyPrelude</hask> and have those qualified names available to modules that import <hask>MyPrelude</hask>.

== Background ==
The following is a list of previous documents and discussions about Haskell's module system:

* Iavor S. Diatchki et al's formal specification of the Haskell 98 module system: http://purely-functional.net/yav/publications/modules98.pdf
* Module system in the Haskell 98 report: http://www.haskell.org/onlinereport/modules.html (code at: https://github.com/yav/module-98 )
* Module system in the Haskell 2010 report: http://www.haskell.org/onlinereport/haskell2010/haskellch5.html
* A thread from the haskell-cafe mailing list: http://www.haskell.org/pipermail/haskell-cafe/2005-March/thread.html
* A related proposal for nested modules: http://osdir.com/ml/glasgow-haskell-users@haskell.org/2012-01/msg00171.html

== The Proposal ==

This proposal extends the export mechanism so that the following is possible:

<haskell>
  module MyPrelude
      ( Text
      , Map
      , ByteString
      , qualified module BS
      , qualified module Map
      , qualified module T
      ) where
  import Data.ByteString (ByteString)
  import Data.Map.Strict (Map)
  import Data.Text (Text)

  import qualified Data.ByteString.Char8 as BS
  import qualified Data.Map.Strict as Map
  import qualified Data.Text as T

</haskell>

Files which originally had 6 lines of imports can now replace those 6 lines with a single import of <hask>MyPrelude</hask>.