[commit: Cabal] : When finalising paired packages, cope with there being multiple choices (6d4bc5e)

[Ian Lynagh]

Repository : ssh://darcs.haskell.org//srv/darcs/packages/Cabal

On branch  : 

http://hackage.haskell.org/trac/ghc/changeset/6d4bc5e59be3cec62688a0a575e30b83875b3cef

>---------------------------------------------------------------

commit 6d4bc5e59be3cec62688a0a575e30b83875b3cef
Author: Duncan Coutts <duncan at haskell.org>
Date:   Sun Oct 5 05:38:21 2008 +0000

    When finalising paired packages, cope with there being multiple choices
    For ordinary packages we selected a single version which means
     we added an equality constraint. As a consequence we used to
    assume there was only one version left when finalising. For
    paired packages there may be two versions left if the package
    did not directly constrain the choice to just one. If there is
    more than one version remaining then we have to pick between
    them. At the moment we still pick the highest version, but
    later we can take a global preference or polciy into account.

>---------------------------------------------------------------

 .../Distribution/Client/Dependency/TopDown.hs      |   14 +++++++++-----
 1 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/cabal-install/Distribution/Client/Dependency/TopDown.hs b/cabal-install/Distribution/Client/Dependency/TopDown.hs
index cd54be2..3dd42eb 100644
--- a/cabal-install/Distribution/Client/Dependency/TopDown.hs
+++ b/cabal-install/Distribution/Client/Dependency/TopDown.hs
@@ -410,11 +410,15 @@ finaliseSelectedPackages selected constraints =
     finaliseInstalled (InstalledPackage pkg _ _) = InstallPlan.PreExisting pkg
     finaliseAvailable (SemiConfiguredPackage pkg flags deps) =
       InstallPlan.Configured (ConfiguredPackage pkg flags deps')
-      where deps' = [ packageId pkg'
-                    | dep <- deps
-                    , let pkg' = case PackageIndex.lookupDependency selected dep of
-                                   [pkg''] -> pkg''
-                                   _ -> impossible ]
+      where
+        deps' = map (packageId . pickRemaining) deps
+        pickRemaining dep =
+          case PackageIndex.lookupDependency remainingChoices dep of
+            []        -> impossible
+            [pkg']    -> pkg'
+            remaining -> maximumBy bestByPref remaining
+        bestByPref = comparing (\p -> (isPreferred p, packageVersion p))
+        isPreferred _ = True
 
 -- | Improve an existing installation plan by, where possible, swapping
 -- packages we plan to install with ones that are already installed.