[GHC] #8753: Import constructor but not the data type

8 Feb 2014

      #8753: Import constructor but not the data type
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       Reporter:  andreas.abel      |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 How do I import a constructor but not its data type?  The constructor
 might have the name of a data type which I do not want to bring into
 scope.

 Use case:  Library (A) defines a parametrized data type (D a) which I want
 to use in my code (B) in instantiated form (D Int) but with the same name
 (D).  I want to pattern match against inhabitants of type D, so I need the
 constructor(s) in scope.

 Here is how it could look like if import lists let me specify whether I
 want to import a type or a constructor with name "D".

 module A where

 data D a = D a

 module B where

 import A (constructor D)
 import qualified A
 D = A.D Int

 f :: D -> Int
 f (D x) = x

 Haskell has different name spaces for type and constructors, but does not
 let the user talk about these name spaces where it matters, namely in
 import/export lists.  At least I found no documentation instructing me how
 to do this.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8753
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