[Hackage] #925: More flexible constraints, specified via local preferences file (was: Unused constraints result in "no available version")

[Hackage]

#925: More flexible constraints, specified via local preferences file
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  Reporter:  snoyberg            |        Owner:                    
      Type:  enhancement         |       Status:  new               
  Priority:  normal              |    Milestone:  cabal-install-0.14
 Component:  cabal-install tool  |      Version:  1.10.2.0          
  Severity:  normal              |     Keywords:                    
Difficulty:  unknown             |   Ghcversion:                    
  Platform:                      |  
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Changes (by kosmikus):

  * type:  defect => enhancement
  * milestone:  => cabal-install-0.14


Comment:

 I've changed your bug into an enhancement request. What you're describing
 is intended behaviour. A constraint passed on the command line acts the
 same as a constraint put in the build-depends of a package, i.e., it not
 only restricts the range of versions, but also causes the package to
 actually be installed.

 The closest to what you want that currently exists is --preference, and it
 should actually be sufficient in most cases. A --preference constraint
 does not prevent excluded versions from selected at all, but it will first
 try all preferred versions.

 I appreciate the attempt you make with cabal-nirvana, but I don't like
 that it works by actually changing the downloaded files. Cabal already
 supports a global preferences file that's downloaded from Hackage. It
 should probably support local files with preferences, too. That would be
 relatively easy to change.

 So there are two possible enhancements in this issue:

    * introduce a way to easily specify a large number of local
 preferences,
    * next to constraints introduced by --preference and --constraint

 The mail

 http://www.mail-archive.com/haskell-cafe@haskell.org/msg96962.html

 as well as the surrounding thread are also somewhat related.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/925#comment:2>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects