Fix #1788

- Remove redundant space after `description:` field name.
- Eliminate trailing spaces on *some* blank lines.
- Convert all field names to lower case.
- Eliminate redundant `buildable: True` and `exposed: True` fields.
- Fix Haddock module comment.

Cabal/Distribution/PackageDescription/PrettyPrint.hs

 -----------------------------------------------------------------------------
---
+-- |
 -- Module      :  Distribution.PackageDescription.PrettyPrint
 -- Copyright   :  Jürgen Nicklisch-Franken 2010
 -- License     :  BSD3
@@ -8,7 +8,7 @@
 -- Stability   : provisional
 -- Portability : portable
 --
--- | Pretty printing for cabal files
+-- Pretty printing for cabal files
 --
 -----------------------------------------------------------------------------
 
@@ -74,6 +74,12 @@ ppSourceRepo repo                        =
   where
     sourceRepoFieldDescrs' = [fd | fd <- sourceRepoFieldDescrs, fieldName fd /= "kind"]
 
+ppFieldsFiltered :: [String] -> [FieldDescr a] -> a -> Doc
+ppFieldsFiltered removable fields x = ppFields (filter nondefault fields) x
+  where
+    nondefault (FieldDescr name getter _) =
+        name `notElem` removable || render (getter x) /= "True"
+
 ppDiffFields :: [FieldDescr a] -> a -> a -> Doc
 ppDiffFields fields x y                  =
    vcat [ ppField name (getter x)
@@ -95,16 +101,16 @@ ppFlag (MkFlag name desc dflt manual)    =
     emptyLine $ text "flag" <+> ppFlagName name $+$
             (nest indentWith ((if null desc
                                 then empty
-                                else  text "Description: " <+> showFreeText desc) $+$
-                     (if dflt then empty else text "Default: False") $+$
-                     (if manual then text "Manual: True" else empty)))
+                                else  text "description:" <+> showFreeText desc) $+$
+                     (if dflt then empty else text "default: False") $+$
+                     (if manual then text "manual: True" else empty)))
 
 ppLibrary :: (Maybe (CondTree ConfVar [Dependency] Library)) -> Doc
 ppLibrary Nothing                        = empty
 ppLibrary (Just condTree)                =
     emptyLine $ text "library" $+$ nest indentWith (ppCondTree condTree Nothing ppLib)
   where
-    ppLib lib Nothing     = ppFields libFieldDescrs lib
+    ppLib lib Nothing     = ppFieldsFiltered ["buildable", "exposed"] libFieldDescrs lib
                             $$  ppCustomFields (customFieldsBI (libBuildInfo lib))
     ppLib lib (Just plib) = ppDiffFields libFieldDescrs lib plib
                             $$  ppCustomFields (customFieldsBI (libBuildInfo lib))
@@ -116,7 +122,7 @@ ppExecutables exes                       =
   where
     ppExe (Executable _ modulePath' buildInfo') Nothing =
         (if modulePath' == "" then empty else text "main-is:" <+> text modulePath')
-            $+$ ppFields binfoFieldDescrs buildInfo'
+            $+$ ppFieldsFiltered ["buildable"] binfoFieldDescrs buildInfo'
             $+$  ppCustomFields (customFieldsBI buildInfo')
     ppExe (Executable _ modulePath' buildInfo')
             (Just (Executable _ modulePath2 buildInfo2)) =
@@ -138,7 +144,7 @@ ppTestSuites suites =
                             (testSuiteMainIs testsuite)
             $+$ maybe empty (\m -> text "test-module:" <+> disp m)
                             (testSuiteModule testsuite)
-            $+$ ppFields binfoFieldDescrs (testBuildInfo testsuite)
+            $+$ ppFieldsFiltered ["buildable"] binfoFieldDescrs (testBuildInfo testsuite)
             $+$ ppCustomFields (customFieldsBI (testBuildInfo testsuite))
       where
         maybeTestType | testInterface testsuite == mempty = Nothing
@@ -168,7 +174,7 @@ ppBenchmarks suites =
                             maybeBenchmarkType
             $+$ maybe empty (\f -> text "main-is:"     <+> text f)
                             (benchmarkMainIs benchmark)
-            $+$ ppFields binfoFieldDescrs (benchmarkBuildInfo benchmark)
+            $+$ ppFieldsFiltered ["buildable"] binfoFieldDescrs (benchmarkBuildInfo benchmark)
             $+$ ppCustomFields (customFieldsBI (benchmarkBuildInfo benchmark))
       where
         maybeBenchmarkType | benchmarkInterface benchmark == mempty = Nothing
@@ -209,6 +215,7 @@ ppCondTree ct@(CondNode it deps ifs) mbIt ppIt =
         then ppCondTree ct Nothing ppIt
         else res
   where
+    -- TODO: this ends up printing trailing spaces when combined with nest.
     ppIf (c,thenTree,mElseTree)          =
         ((emptyLine $ text "if" <+> ppCondition c) $$
           nest indentWith (ppCondTree thenTree
@@ -225,7 +232,7 @@ ppDeps deps                              =
     text "build-depends:" $+$ nest indentWith (vcat (punctuate comma (map disp deps)))
 
 emptyLine :: Doc -> Doc
-emptyLine d                              = text " " $+$ d
+emptyLine d                              = text "" $+$ d