[Hackage] #474: cabal-install needlessly reinstalls an existing package

[Hackage]

#474: cabal-install needlessly reinstalls an existing package
---------------------------------+------------------------------------------
  Reporter:  judah               |        Owner:  kosmikus          
      Type:  defect              |       Status:  new               
  Priority:  high                |    Milestone:  cabal-install-0.16
 Component:  cabal-install tool  |      Version:  1.6.0.1           
  Severity:  normal              |     Keywords:  solver            
Difficulty:  unknown             |   Ghcversion:                    
  Platform:                      |  
---------------------------------+------------------------------------------
Changes (by kosmikus):

  * priority:  normal => high
  * milestone:  cabal-install-0.14 => cabal-install-0.16


Comment:

 Ok, I can reproduce a similar situation and it seems it's still handled as
 described in this bug. Using an imaginary version `process-1.0.1.6` with
 `ghc-7.0.4`, I get:
 {{{
 In order, the following would be installed:
 Cabal-1.10.2.0 (reinstall) changes: process-1.0.1.5 -> 1.0.1.6
 deepseq-1.3.0.0 (new package)
 text-0.11.1.13 (new package)
 transformers-0.2.2.0 (new package)
 mtl-2.0.1.0 (new package)
 parsec-3.1.2 (new package)
 network-2.3.0.11 (new package)
 HTTP-4000.2.2 (new package)
 zlib-0.5.3.3 (new package)
 cabal-install-0.10.2 -bytestring-in-base (new package)
 Warning: The following packages are likely to be broken by the reinstalls:
 ghc-7.0.4
 bin-package-db-0.0.0.0
 Use --force-reinstalls if you want to install anyway.
 }}}

 Using the modular solver does not help, as it's using the same policy to
 pick the latest installed package by default, and doing this does not lead
 to an error, only to a reinstallation -- although that reinstallation
 breaks the `ghc` and `bin-package-db` packages.

 We can fix this by using the `--avoid-reinstalls` flag. However, I don't
 want to make that flag the default, because it may lead to confusing
 install plans, as we can actually observe here:
 {{{
 In order, the following would be installed (use -v for more details):
 Cabal-1.10.1.0
 deepseq-1.3.0.0
 text-0.11.1.13
 transformers-0.2.2.0
 mtl-2.0.1.0
 parsec-3.1.2
 network-2.3.0.11
 HTTP-4000.2.2
 zlib-0.5.3.3
 cabal-install-0.10.2
 }}}
 Now, we get an older version of `Cabal`, which is also not what we really
 want.

 We get the "correct" solution by passing `--constraint="process ==
 1.0.1.5":
 {{{

 Resolving dependencies...
 In order, the following would be installed (use -v for more details):
 deepseq-1.3.0.0
 text-0.11.1.13
 transformers-0.2.2.0
 mtl-2.0.1.0
 parsec-3.1.2
 network-2.3.0.11
 HTTP-4000.2.2
 zlib-0.5.3.3
 cabal-install-0.10.2
 }}}

 One option to fix problems like this one would be to actually try to find
 several install plans for a given problem and try to pick the "best".
 Picking an older uninstalled version should have a higher penalty as
 picking an older installed version.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/474#comment:10>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects