Rearrange the Monoid instances for Library, Executable, BuildInfo

No functional change, just moving code about.
We now define the Monoid methods directly rather than in
terms of emptyLibrary, unionLibrary etc.

Distribution/PackageDescription.hs

@@ -219,11 +219,18 @@ data Library = Library {
     deriving (Show, Eq, Read)
 
 instance Monoid Library where
-    mempty = emptyLibrary
-    mappend = unionLibrary
+  mempty = Library {
+    exposedModules = mempty,
+    libBuildInfo   = mempty
+  }
+  mappend a b = Library {
+    exposedModules = combine exposedModules,
+    libBuildInfo   = combine libBuildInfo
+  }
+    where combine field = field a `mappend` field b
 
 emptyLibrary :: Library
-emptyLibrary = Library [] emptyBuildInfo
+emptyLibrary = mempty
 
 -- |does this package have any libraries?
 hasLibs :: PackageDescription -> Bool
@@ -248,13 +255,6 @@ libModules PackageDescription{library=lib}
     = maybe [] exposedModules lib
        ++ maybe [] (otherModules . libBuildInfo) lib
 
-unionLibrary :: Library -> Library -> Library
-unionLibrary l1 l2 =
-    l1 { exposedModules = combine exposedModules
-       , libBuildInfo = unionBuildInfo (libBuildInfo l1) (libBuildInfo l2)
-       }
-  where combine f = f l1 ++ f l2
-
 -- ---------------------------------------------------------------------------
 -- The Executable type
 
@@ -266,15 +266,26 @@ data Executable = Executable {
     deriving (Show, Read, Eq)
 
 instance Monoid Executable where
-    mempty = emptyExecutable
-    mappend = unionExecutable
+  mempty = Executable {
+    exeName    = mempty,
+    modulePath = mempty,
+    buildInfo  = mempty
+  }
+  mappend a b = Executable{
+    exeName    = combine' exeName,
+    modulePath = combine modulePath,
+    buildInfo  = combine buildInfo
+  }
+    where combine field = field a `mappend` field b
+          combine' field = case (field a, field b) of
+                      ("","") -> ""
+                      ("", x) -> x
+                      (x, "") -> x
+                      (x, y) -> error $ "Ambiguous values for executable field: '"
+                                  ++ x ++ "' and '" ++ y ++ "'"
 
 emptyExecutable :: Executable
-emptyExecutable = Executable {
-                      exeName = "",
-                      modulePath = "",
-                      buildInfo = emptyBuildInfo
-                     }
+emptyExecutable = mempty
 
 -- |does this package have any executables?
 hasExes :: PackageDescription -> Bool
@@ -291,19 +302,6 @@ exeModules :: PackageDescription -> [ModuleName]
 exeModules PackageDescription{executables=execs}
     = concatMap (otherModules . buildInfo) execs
 
-unionExecutable :: Executable -> Executable -> Executable
-unionExecutable e1 e2 =
-    e1 { exeName = combine exeName
-       , modulePath = combine modulePath
-       , buildInfo = unionBuildInfo (buildInfo e1) (buildInfo e2)
-       }
-  where combine f = case (f e1, f e2) of
-                      ("","") -> ""
-                      ("", x) -> x
-                      (x, "") -> x
-                      (x, y) -> error $ "Ambiguous values for executable field: '"
-                                  ++ x ++ "' and '" ++ y ++ "'"
-
 -- ---------------------------------------------------------------------------
 -- The BuildInfo type
 
@@ -335,32 +333,56 @@ data BuildInfo = BuildInfo {
     deriving (Show,Read,Eq)
 
 instance Monoid BuildInfo where
-    mempty = emptyBuildInfo
-    mappend = unionBuildInfo
+  mempty = BuildInfo {
+    buildable         = True,
+    buildTools        = [],
+    cppOptions        = [],
+    ccOptions         = [],
+    ldOptions         = [],
+    pkgconfigDepends  = [],
+    frameworks        = [],
+    cSources          = [],
+    hsSourceDirs      = [],
+    otherModules      = [],
+    extensions        = [],
+    extraLibs         = [],
+    extraLibDirs      = [],
+    includeDirs       = [],
+    includes          = [],
+    installIncludes   = [],
+    options           = [],
+    ghcProfOptions    = [],
+    ghcSharedOptions  = [],
+    customFieldsBI    = []
+  }
+  mappend a b = BuildInfo {
+    buildable         = buildable a && buildable b,
+    buildTools        = combineNub buildTools,
+    cppOptions        = combine    cppOptions,
+    ccOptions         = combine    ccOptions,
+    ldOptions         = combine    ldOptions,
+    pkgconfigDepends  = combineNub pkgconfigDepends,
+    frameworks        = combineNub frameworks,
+    cSources          = combineNub cSources,
+    hsSourceDirs      = combineNub hsSourceDirs,
+    otherModules      = combineNub otherModules,
+    extensions        = combineNub extensions,
+    extraLibs         = combine    extraLibs,
+    extraLibDirs      = combineNub extraLibDirs,
+    includeDirs       = combineNub includeDirs,
+    includes          = combineNub includes,
+    installIncludes   = combineNub installIncludes,
+    options           = combine    options,
+    ghcProfOptions    = combine    ghcProfOptions,
+    ghcSharedOptions  = combine    ghcSharedOptions,
+    customFieldsBI    = combine    customFieldsBI
+  }
+    where
+      combine    field = field a `mappend` field b
+      combineNub field = nub (combine field)
 
 emptyBuildInfo :: BuildInfo
-emptyBuildInfo = BuildInfo {
-                      buildable         = True,
-                      buildTools        = [],
-                      cppOptions        = [],
-                      ccOptions         = [],
-                      ldOptions         = [],
-                      pkgconfigDepends  = [],
-                      frameworks        = [],
-                      cSources          = [],
-                      hsSourceDirs      = [],
-                      otherModules      = [],
-                      extensions        = [],
-                      extraLibs         = [],
-                      extraLibDirs      = [],
-                      includeDirs       = [],
-                      includes          = [],
-                      installIncludes   = [],
-                      options           = [],
-                      ghcProfOptions    = [],
-                      ghcSharedOptions  = [],
-                      customFieldsBI    = []
-                     }
+emptyBuildInfo = mempty
 
 -- | The 'BuildInfo' for the library (if there is one and it's buildable) and
 -- all the buildable executables. Useful for gathering dependencies.
@@ -394,7 +416,7 @@ updatePackageDescription (mb_lib_bi, exe_bi) p
        }
     where
       updateLibrary :: Maybe BuildInfo -> Maybe Library -> Maybe Library
-      updateLibrary (Just bi) (Just lib) = Just (lib{libBuildInfo = unionBuildInfo bi (libBuildInfo lib)})
+      updateLibrary (Just bi) (Just lib) = Just (lib{libBuildInfo = bi `mappend` libBuildInfo lib})
       updateLibrary Nothing   mb_lib     = mb_lib
 
        --the lib only exists in the buildinfo file.  FIX: Is this
@@ -412,37 +434,9 @@ updatePackageDescription (mb_lib_bi, exe_bi) p
                        -> [Executable]        -- ^libst with exeName updated
       updateExecutable _                 []         = []
       updateExecutable exe_bi'@(name,bi) (exe:exes)
-        | exeName exe == name = exe{buildInfo = unionBuildInfo bi (buildInfo exe)} : exes
+        | exeName exe == name = exe{buildInfo = bi `mappend` buildInfo exe} : exes
         | otherwise           = exe : updateExecutable exe_bi' exes
 
-unionBuildInfo :: BuildInfo -> BuildInfo -> BuildInfo
-unionBuildInfo b1 b2
-    = BuildInfo {
-         buildable         = buildable b1 && buildable b2,
-         buildTools        = combineNub buildTools,
-         cppOptions        = combine    cppOptions,
-         ccOptions         = combine    ccOptions,
-         ldOptions         = combine    ldOptions,
-         pkgconfigDepends  = combineNub pkgconfigDepends,
-         frameworks        = combineNub frameworks,
-         cSources          = combineNub cSources,
-         hsSourceDirs      = combineNub hsSourceDirs,
-         otherModules      = combineNub otherModules,
-         extensions        = combineNub extensions,
-         extraLibs         = combine    extraLibs,
-         extraLibDirs      = combineNub extraLibDirs,
-         includeDirs       = combineNub includeDirs,
-         includes          = combineNub includes,
-         installIncludes   = combineNub installIncludes,
-         options           = combine    options,
-         ghcProfOptions    = combine    ghcProfOptions,
-         ghcSharedOptions  = combine    ghcSharedOptions,
-         customFieldsBI    = combine    customFieldsBI
-        }
-  where
-    combine    f = f b1 ++ f b2
-    combineNub f = nub (combine f)
-
 -- ---------------------------------------------------------------------------
 -- The GenericPackageDescription type