Omit comments from the default cabal.sandbox.config.

Since this file is not intended to be edited by hand, don't print the default
values for empty fields in comments (just like cabal-dev).

cabal-install/Distribution/Client/PackageEnvironment.hs

@@ -10,6 +10,7 @@
 
 module Distribution.Client.PackageEnvironment (
     PackageEnvironment(..)
+  , IncludeComments(..)
   , createPackageEnvironment
   , tryLoadPackageEnvironment
   , readPackageEnvironmentFile
@@ -51,7 +52,7 @@ import Distribution.Compat.Exception   ( catchIO )
 import System.Directory                ( renameFile )
 import System.FilePath                 ( (<.>), (</>) )
 import System.IO.Error                 ( isDoesNotExistError )
-import Text.PrettyPrint                ( ($+$) )
+import Text.PrettyPrint                ( ($+$), (<+>), ($$), (<>) )
 
 import qualified Text.PrettyPrint          as Disp
 import qualified Distribution.Compat.ReadP as Parse
@@ -230,18 +231,23 @@ tryLoadPackageEnvironment verbosity pkgEnvDir = do
   base <- basePkgEnv verbosity sandboxDir (pkgEnvInherit pkgEnv)
   return (sandboxDir, base `mappend` user `mappend` pkgEnv)
 
+-- | Should the generated package environment file include comments?
+data IncludeComments = IncludeComments | NoComments
+
 -- | Create a new package environment file, replacing the existing one if it
 -- exists. Note that the path parameters should point to existing directories.
 createPackageEnvironment :: Verbosity -> FilePath -> FilePath
+                            -> IncludeComments
                             -> Compiler -> SavedConfig
                             -> IO PackageEnvironment
-createPackageEnvironment verbosity sandboxDir pkgEnvDir compiler userConfig = do
+createPackageEnvironment verbosity sandboxDir pkgEnvDir
+  incComments compiler userConfig = do
   let path = pkgEnvDir </> sandboxPackageEnvironmentFile
   notice verbosity $ "Writing default package environment to " ++ path
 
   commentPkgEnv <- commentPackageEnvironment sandboxDir
   initialPkgEnv <- initialPackageEnvironment sandboxDir compiler userConfig
-  writePackageEnvironmentFile path commentPkgEnv initialPkgEnv
+  writePackageEnvironmentFile path incComments commentPkgEnv initialPkgEnv
 
   user <- userPkgEnv verbosity pkgEnvDir
   base <- basePkgEnv verbosity sandboxDir (pkgEnvInherit initialPkgEnv)
@@ -344,14 +350,17 @@ parsePackageEnvironment initial str = do
       return accum
 
 -- | Write out the package environment file.
-writePackageEnvironmentFile :: FilePath -> PackageEnvironment
-                               -> PackageEnvironment -> IO ()
-writePackageEnvironmentFile path comments pkgEnv = do
+writePackageEnvironmentFile :: FilePath -> IncludeComments
+                               -> PackageEnvironment -> PackageEnvironment
+                               -> IO ()
+writePackageEnvironmentFile path incComments comments pkgEnv = do
   let tmpPath = (path <.> "tmp")
-  writeFile tmpPath $ explanation
-    ++ showPackageEnvironmentWithComments comments pkgEnv ++ "\n"
+  writeFile tmpPath $ explanation ++ pkgEnvStr ++ "\n"
   renameFile tmpPath path
   where
+    pkgEnvStr = case incComments of
+      IncludeComments -> showPackageEnvironmentWithComments comments pkgEnv
+      NoComments      -> showPackageEnvironment pkgEnv
     explanation = unlines
       ["-- This is a Cabal package environment file."
       ,"-- THIS FILE IS AUTO-GENERATED. DO NOT EDIT DIRECTLY."
@@ -367,10 +376,34 @@ writePackageEnvironmentFile path comments pkgEnv = do
       ,"",""
       ]
 
--- | Pretty-print the package environment data.
+-- | Pretty-print the package environment.
 showPackageEnvironment :: PackageEnvironment -> String
-showPackageEnvironment = showPackageEnvironmentWithComments mempty
+showPackageEnvironment pkgEnv = Disp.render $
+      ppFields' pkgEnvFieldDescrs pkgEnv
+  $+$ Disp.text ""
+  $+$ ppSection' "install-dirs" "" installDirsFields (field pkgEnv)
+  where
+    field = savedUserInstallDirs . pkgEnvSavedConfig
 
+    -- Customised versions of functions from D.C.ParseUtils that do not print
+    -- fields with empty arguments.
+    ppFields' :: [FieldDescr a] -> a -> Disp.Doc
+    ppFields' fields a =
+      Disp.vcat [ ppField' name (getter a)
+                | FieldDescr name getter _ <- fields]
+
+    ppField' :: String -> Disp.Doc -> Disp.Doc
+    ppField' name arg
+      | Disp.isEmpty arg = Disp.empty
+      | otherwise        = Disp.text name <> Disp.colon <+> arg
+
+    ppSection' :: String -> String -> [FieldDescr a] -> a -> Disp.Doc
+    ppSection' name arg fields cur =
+      Disp.text name <+> Disp.text arg
+      $$ Disp.nest 2 (ppFields' fields cur)
+
+-- | Pretty-print the package environment with default values for empty fields
+-- commented out (just like the default ~/.cabal/config).
 showPackageEnvironmentWithComments :: PackageEnvironment -> PackageEnvironment
                                       -> String
 showPackageEnvironmentWithComments defPkgEnv pkgEnv = Disp.render $

cabal-install/Distribution/Client/Sandbox.hs

@@ -32,7 +32,7 @@ import Distribution.Client.Install            ( makeInstallContext
                                               , pruneInstallPlan
                                               , InstallArgs )
 import Distribution.Client.PackageEnvironment
-  ( PackageEnvironment(..)
+  ( PackageEnvironment(..), IncludeComments(..)
   , createPackageEnvironment, tryLoadPackageEnvironment
   , commentPackageEnvironment
   , showPackageEnvironmentWithComments
@@ -160,7 +160,7 @@ sandboxInit verbosity sandboxFlags globalFlags = do
   -- Create the package environment file.
   pkgEnvDir <- getCurrentDirectory
   pkgEnv    <- createPackageEnvironment verbosity sandboxDir pkgEnvDir
-               comp userConfig
+               NoComments comp userConfig
 
   -- Create the index file if it doesn't exist.
   indexFile <- tryGetIndexFilePath pkgEnv