Clean up dependency selection logic ever so slightly

No need for `ResolvedDependency` to duplicate field across variants, or
`selectDependency` to ferry argument into return value

Cabal/Distribution/Simple/Configure.hs

@@ -980,29 +980,27 @@ configureDependencies
     -> IO [(PackageName, ComponentName, InstalledPackageInfo)]
 configureDependencies verbosity use_external_internal_deps
   installedPackageSet requiredDepsMap pkg_descr enableSpec = do
-    let selectDependencies :: [LibDependency] ->
-                              ([FailedDependency], [ResolvedDependency])
-        selectDependencies =
-            partitionEithers
-          . map (selectDependency (package pkg_descr)
-                                  installedPackageSet
-                                  requiredDepsMap use_external_internal_deps)
-
-        (failedDeps, allPkgDeps) =
-          selectDependencies (targetBuildDepends =<< enabledBuildInfos pkg_descr enableSpec)
-
-        internalPkgDeps =
-          [ dep | InternalDependency dep _ <- allPkgDeps ]
-        -- NB: we have to SAVE the package name, because this is the only
-        -- way we can be able to resolve package names in the package
-        -- description.
-        externalPkgDeps =
-          [ (pn, cn, pkgid)
-          | ExternalDependency (LibDependency pn mcn _) pkgid <- allPkgDeps
-          , let cn = case mcn of
-                  Nothing -> CLibName
-                  Just n  -> CSubLibName n
-          ]
+    let
+      failedDeps :: [FailedDependency]
+      allPkgDeps :: [ResolvedDependency]
+      (failedDeps, allPkgDeps) = partitionEithers
+        [ (\s -> (dep, s)) <$> status
+        | dep <- targetBuildDepends =<< enabledBuildInfos pkg_descr enableSpec
+        , let status = selectDependency
+                (package pkg_descr) installedPackageSet
+                requiredDepsMap use_external_internal_deps dep ]
+
+      internalPkgDeps =
+        [ dep | (dep, InternalDependency _) <- allPkgDeps ]
+
+      -- NB: we have to SAVE the package name, because this is the only way we
+      -- can be able to resolve package names in the package description.
+      externalPkgDeps =
+        [ (pn, cn, pkgid)
+        | (LibDependency pn mcn _, ExternalDependency pkgid) <- allPkgDeps
+        , let cn = case mcn of
+                Nothing -> CLibName
+                Just n  -> CSubLibName n ]
 
     when (not (null internalPkgDeps)
           && not (newPackageDepsBehaviour pkg_descr)) $
@@ -1134,16 +1132,18 @@ reportProgram verbosity prog (Just configuredProg)
 hackageUrl :: String
 hackageUrl = "http://hackage.haskell.org/package/"
 
-data ResolvedDependency
+type ResolvedDependency = (LibDependency, DependencyResolution)
+
+data DependencyResolution
     -- | An external dependency from the package database, OR an
     -- internal dependency which we are getting from the package
     -- database.
-    = ExternalDependency LibDependency InstalledPackageInfo
+    = ExternalDependency InstalledPackageInfo
     -- | An internal dependency ('PackageId' should be a library name)
     -- which we are going to have to build.  (The
     -- 'PackageId' here is a hack to get a modest amount of
     -- polymorphism out of the 'Package' typeclass.)
-    | InternalDependency LibDependency PackageId
+    | InternalDependency PackageId
 
 data FailedDependency = DependencyNotExists PackageName
                       | DependencyMissingInternal PackageName PackageName
@@ -1158,10 +1158,10 @@ selectDependency :: PackageId -- ^ Package id of current package
                  -> UseExternalInternalDeps -- ^ Are we configuring a
                                             -- single component?
                  -> LibDependency
-                 -> Either FailedDependency ResolvedDependency
+                 -> Either FailedDependency DependencyResolution
 selectDependency pkgid installedIndex requiredDepsMap
   use_external_internal_deps
-  dep@(LibDependency dep_pkgname dep_mb_compname vr) =
+  (LibDependency dep_pkgname dep_mb_compname vr) =
   -- If external sublibs can someday we be used, we can simplify this
   -- case. For now, we do the error as a basic sanity
   -- check. PackageDescription.Check should give the user a nicer
@@ -1172,15 +1172,15 @@ selectDependency pkgid installedIndex requiredDepsMap
       "Should have already checked that external sub-libs are not depended on"
     (_,      _,     _)     -> do_external
   where
-    do_internal = Right (InternalDependency dep
+    do_internal = Right (InternalDependency
                     (PackageIdentifier dep_pkgname (packageVersion pkgid)))
     do_external = case Map.lookup (dep_pkgname, compName) requiredDepsMap of
       -- If we know the exact pkg to use, then use it.
-      Just pkginstance -> Right (ExternalDependency dep pkginstance)
+      Just pkginstance -> Right (ExternalDependency pkginstance)
       -- Otherwise we just pick an arbitrary instance of the latest version.
       Nothing -> case PackageIndex.lookupDependency installedIndex legacyDep of
         []   -> Left errVal
-        pkgs -> Right $ ExternalDependency dep $ case last pkgs of
+        pkgs -> Right $ ExternalDependency $ case last pkgs of
                   (_ver, pkginstances) -> head pkginstances
      where
       (legacyDep, compName, errVal) = case dep_mb_compname of
@@ -1203,10 +1203,10 @@ reportSelectedDependencies verbosity deps =
   info verbosity $ unlines
     [ "Dependency " ++ display (simplifyLibDependency dep)
                     ++ ": using " ++ display pkgid
-    | resolved <- deps
-    , let (dep, pkgid) = case resolved of
-            ExternalDependency dep' pkg'   -> (dep', packageId pkg')
-            InternalDependency dep' pkgid' -> (dep', pkgid') ]
+    | (dep, res) <- deps
+    , let pkgid = case res of
+            ExternalDependency pkg'   -> packageId pkg'
+            InternalDependency pkgid' -> pkgid' ]
 
 reportFailedDependencies :: Verbosity -> [FailedDependency] -> IO ()
 reportFailedDependencies _ []     = return ()