Add an --only-dependencies flag to "install" (see ticket #697)

This flag installs only the dependencies of the packages that were
explicitly mentioned on the command line. This is useful for using
cabal-install in development environments, where the developer needs
the dependencies to build the package in development, but does not yet
want to install the package itself.

cabal-install/Distribution/Client/Install.hs

@@ -135,7 +135,6 @@ import Distribution.Simple.BuildPaths ( exeExtension )
 
 --TODO:
 -- * add --upgrade-deps flag
--- * add --only-deps flag
 -- * eliminate upgrade, replaced by --upgrade-deps and world target
 -- * assign flags to packages individually
 --   * complain about flags that do not apply to any package given as target
@@ -175,14 +174,16 @@ install, upgrade
 install verbosity packageDB repos comp conf
   globalFlags configFlags configExFlags installFlags targets =
 
-    installWithPlanner verbosity context planner targets
+    installWithPlanner verbosity context (planner onlyDeps) targets
 
   where
     context :: InstallContext
     context = (packageDB, repos, comp, conf,
                globalFlags, configFlags, configExFlags, installFlags)
 
-    planner :: Planner
+    onlyDeps = fromFlag (installOnlyDeps installFlags)
+
+    planner :: Bool -> Planner
     planner
       | null targets = planLocalPackage verbosity
                          comp configFlags configExFlags
@@ -261,8 +262,9 @@ planLocalPackage :: Verbosity
                  -> Compiler
                  -> ConfigFlags
                  -> ConfigExFlags
+                 -> Bool
                  -> Planner
-planLocalPackage verbosity comp configFlags configExFlags installed
+planLocalPackage verbosity comp configFlags configExFlags onlyDeps installed
   (AvailablePackageDb available availablePrefs) = do
   pkg <- readPackageDescription verbosity =<< defaultPackageDesc verbosity
   let -- The trick is, we add the local package to the available index and
@@ -286,9 +288,16 @@ planLocalPackage verbosity comp configFlags configExFlags installed
       preferences = mergePackagePrefs PreferLatestForSelected
                                       availablePrefs configExFlags
 
-  return $ resolveDependenciesWithProgress buildPlatform (compilerId comp)
+      plan = resolveDependenciesWithProgress buildPlatform
+             (compilerId comp)
              installed' available' preferences constraints targets
 
+      isLocalPackage = (== localPackageName) . packageName
+      localPackageName = packageName pkg
+
+  -- If we only want dependencies, then remove the local package from
+  -- the install plan after we have built it.
+  return $ if onlyDeps then removeFromPlan isLocalPackage plan else plan
 
 -- | Make an 'InstallPlan' for the given dependencies.
 --
@@ -299,10 +308,11 @@ planRepoPackages :: PackagesPreferenceDefault
                  -> ConfigExFlags
                  -> InstallFlags
                  -> [UnresolvedDependency]
+                 -> Bool
                  -> Planner
 planRepoPackages defaultPref comp
   globalFlags configFlags configExFlags installFlags
-  deps installed (AvailablePackageDb available availablePrefs) = do
+  deps onlyDeps installed (AvailablePackageDb available availablePrefs) = do
 
   deps' <- addWorldPackages deps
        >>= IndexUtils.disambiguateDependencies available
@@ -316,13 +326,18 @@ planRepoPackages defaultPref comp
                  ++ [ PackageVersionConstraint name ver
                     | Dependency name ver <- configConstraints configFlags ]
       preferences = mergePackagePrefs defaultPref availablePrefs configExFlags
-  return $ resolveDependenciesWithProgress buildPlatform (compilerId comp)
+      plan = resolveDependenciesWithProgress buildPlatform (compilerId comp)
              installed' available preferences constraints targets
+  return $ if onlyDeps then removeFromPlan (isGivenDep deps) plan else plan
   where
     hideGivenDeps pkgs index =
-      foldr PackageIndex.deletePackageName index
+      foldr PackageIndex.deletePackageName index (givenDepNames pkgs)
+
+    givenDepNames pkgs =
         [ name | UnresolvedDependency (Dependency name _) _ <- pkgs ]
 
+    isGivenDep pkgs = (`elem` givenDepNames pkgs) . packageName
+
     addWorldPackages :: [UnresolvedDependency] -> IO [UnresolvedDependency]
     addWorldPackages targets = case partition World.isWorldTarget targets of
       ([], _)               -> return targets
@@ -337,6 +352,19 @@ planRepoPackages defaultPref comp
         worldFile = fromFlag $ globalWorldFile globalFlags
 
 
+-- | Adapt InstallPlan.removePackages to work on Progress so that it
+-- can be integrated with the planners
+--
+removeFromPlan :: (InstallPlan.PlanPackage -> Bool)
+               -> Progress step String InstallPlan
+               -> Progress step String InstallPlan
+removeFromPlan shouldRemove =
+    foldProgress Step Fail $ \plan ->
+    case InstallPlan.removePackages shouldRemove plan of
+      Right plan'   -> Done plan'
+      Left problems ->
+          Fail $ unlines $ map InstallPlan.showPlanProblem problems
+
 mergePackagePrefs :: PackagesPreferenceDefault
                   -> Map.Map PackageName VersionRange
                   -> ConfigExFlags

cabal-install/Distribution/Client/InstallPlan.hs

@@ -22,6 +22,7 @@ module Distribution.Client.InstallPlan (
   ready,
   completed,
   failed,
+  removePackages,
 
   -- ** Query functions
   planPlatform,
@@ -181,6 +182,20 @@ new platform compiler index =
 toList :: InstallPlan -> [PlanPackage]
 toList = PackageIndex.allPackages . planIndex
 
+-- | Remove packages from the install plan. This will result in an
+-- error if there are remaining packages that depend on any matching
+-- package. This is primarily useful for obtaining an install plan for
+-- the dependencies of a package or set of packages without actually
+-- installing the package itself, as when doing development.
+--
+removePackages :: (PlanPackage -> Bool) -> InstallPlan
+               -> Either [PlanProblem] InstallPlan
+removePackages shouldRemove plan =
+    new (planPlatform plan) (planCompiler plan) newIdx
+    where
+      newIdx =
+          PackageIndex.fromList $ filter (not . shouldRemove) $ toList plan
+
 -- | The packages that are ready to be installed. That is they are in the
 -- configured state and have all their dependencies installed already.
 -- The plan is complete if the result is @[]@.

cabal-install/Distribution/Client/Setup.hs

@@ -517,6 +517,7 @@ data InstallFlags = InstallFlags {
     installReinstall    :: Flag Bool,
     installUpgradeDeps  :: Flag Bool,
     installOnly         :: Flag Bool,
+    installOnlyDeps     :: Flag Bool,
     installRootCmd      :: Flag String,
     installSummaryFile  :: [PathTemplate],
     installLogFile      :: Flag PathTemplate,
@@ -533,6 +534,7 @@ defaultInstallFlags = InstallFlags {
     installReinstall    = Flag False,
     installUpgradeDeps  = Flag False,
     installOnly         = Flag False,
+    installOnlyDeps     = Flag False,
     installRootCmd      = mempty,
     installSummaryFile  = mempty,
     installLogFile      = mempty,
@@ -634,6 +636,12 @@ installOptions showOrParseArgs =
           "Do not record the packages in the world file."
           installOneShot (\v flags -> flags { installOneShot = v })
           trueArg
+
+      , option [] ["only-dependencies"]
+          "Install only the dependencies necessary to build the listed packages"
+          installOnlyDeps (\v flags -> flags { installOnlyDeps = v })
+          trueArg
+
       ] ++ case showOrParseArgs of      -- TODO: remove when "cabal install" avoids
           ParseArgs ->
             option [] ["only"]
@@ -651,6 +659,7 @@ instance Monoid InstallFlags where
     installReinstall    = mempty,
     installUpgradeDeps  = mempty,
     installOnly         = mempty,
+    installOnlyDeps     = mempty,
     installRootCmd      = mempty,
     installSummaryFile  = mempty,
     installLogFile      = mempty,
@@ -665,6 +674,7 @@ instance Monoid InstallFlags where
     installReinstall    = combine installReinstall,
     installUpgradeDeps  = combine installUpgradeDeps,
     installOnly         = combine installOnly,
+    installOnlyDeps     = combine installOnlyDeps,
     installRootCmd      = combine installRootCmd,
     installSummaryFile  = combine installSummaryFile,
     installLogFile      = combine installLogFile,