[Hackage] #649: better warning messages for why dependencies are missing / cannot be selected

[Hackage]

#649: better warning messages for why dependencies are missing / cannot be
selected
---------------------------------+------------------------------------------
  Reporter:  duncan              |        Owner:      
      Type:  enhancement         |       Status:  new 
  Priority:  normal              |    Milestone:      
 Component:  cabal-install tool  |      Version:  HEAD
  Severity:  normal              |     Keywords:      
Difficulty:  unknown             |   Ghcversion:      
  Platform:                      |  
---------------------------------+------------------------------------------

Comment(by duncan):

 Similarly, for installed constraints we should explain in error messages
 that an installed version is required and list any installed versions that
 were not suitable or were otherwise excluded.

 Consider for example:
 {{{
 build-depends: base == 4.1.*
 }}}
 Suppose someone runs `cabal install` on this when the only available
 versions of base are 3.0.3 and 4.2.0 (ie using ghc-6.12.x). Currently we
 will report:
 {{{
 cabal: cannot configure foo-1.0. It requires base ==4.1.*
 For the dependency on base ==4.1.* there are these packages: base-4.1.0.0.
 However none of them are available.
 base-4.1.0.0 was excluded because of the top level dependency base -any
 }}}
 This lists all the non-installed versions that satisfy the version
 constraint and tells us (badly) that they're no good because they're not
 installed. Instead in this situation it's probably better to say it needs
 an installed version and list the (hopefully) small number of installed
 versions, even if they do not satisfy the version constraint.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/649#comment:1>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects