[Hackage] #390: add cabal unpack command

[Hackage]

#390: add cabal unpack command
---------------------------------+------------------------------------------
  Reporter:  duncan              |        Owner:                 
      Type:  enhancement         |       Status:  new            
  Priority:  normal              |    Milestone:                 
 Component:  cabal-install tool  |      Version:                 
  Severity:  normal              |   Resolution:                 
  Keywords:                      |   Difficulty:  easy (<4 hours)
Ghcversion:  6.8.3               |     Platform:                 
---------------------------------+------------------------------------------
Comment (by duncan):

 Replying to [comment:3 Saizan]:
 > I've needed this command a lot lately, so i've written this patch.

 Yay! The patch looks good too.

 > There are 2 TODOs there since i wasn't sure what was best from an UI
 pov:
 >
 > When no precise version is specified it defaults to the latest that
 match,

 That's sensible.

 > should we also take the packagePreferences field in consideration here?

 Hmm. Not sure actually.

 > Currently it ignores exceptions (however if something goes wrong the
 functions involved still print error messages), summarizing failures at
 the end like in 'cabal install' is probably what we want.

 I'd suggest just failing on the first error. So I'd just remove the `try`.
 Since it does not do anything with dependencies the only time we can have
 multiple errors is if the user specifies multiple packages to unpack. If
 the first fails then they'd probably want to deal with that rather than
 having all the others unpack too (as possibly fail for the same reason).

 What happens if the user specifies no packages? Getting a helpful error
 would be nice.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/390#comment:4>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects