Refactor and simplify the fixup in "hc-pkg dump"

We have to set the installedPackageId if it's missing.
There's no need for this to depend on the version of ghc which
is nice since this module is not supposed to be ghc specific.

Distribution/Simple/Program/HcPkg.hs

@@ -27,7 +27,7 @@ module Distribution.Simple.Program.HcPkg (
   ) where
 
 import Distribution.Package
-         ( PackageId, packageId, InstalledPackageId(..) )
+         ( PackageId, InstalledPackageId(..) )
 import Distribution.InstalledPackageInfo
          ( InstalledPackageInfo, InstalledPackageInfo_(..)
          , showInstalledPackageInfo, parseInstalledPackageInfo )
@@ -51,6 +51,8 @@ import Distribution.Verbosity
 import Distribution.Compat.Exception
          ( catchExit )
 
+import Control.Monad
+         ( liftM )
 
 -- | Call @hc-pkg@ to register a package.
 --
@@ -124,20 +126,12 @@ dump verbosity hcPkg packagedb = do
 
   where
     parsePackages str =
-      let parsed = map parseIPI (splitPkgs str)
+      let parse  = liftM setInstalledPackageId . parseInstalledPackageInfo
+          parsed = map parse (splitPkgs str)
        in case [ msg | ParseFailed msg <- parsed ] of
             []   -> Left [ pkg | ParseOk _ pkg <- parsed ]
             msgs -> Right msgs
 
-    parseIPI s
-     | case programVersion hcPkg of
-          Nothing -> False
-          Just v  -> v < Version [6,11] [] = do
-                  ipi <- parseInstalledPackageInfo s
-                  return (fixInstalledPackageId ipi)
-     | otherwise =
-        parseInstalledPackageInfo s
-
     splitPkgs :: String -> [String]
     splitPkgs = map unlines . splitWith ("---" ==) . lines
       where
@@ -147,14 +141,21 @@ dump verbosity hcPkg packagedb = do
                            _:ws -> splitWith p ws
           where (ys,zs) = break p xs
 
--- Older GHCs did not have the installedPackageId field, so we fill it
--- as (display (packageId p)).
-fixInstalledPackageId :: InstalledPackageInfo -> InstalledPackageInfo
-fixInstalledPackageId p
-  | null ipid_str = p { installedPackageId = 
-                        InstalledPackageId (display (packageId p)) }
-  | otherwise     = p
-  where InstalledPackageId ipid_str = installedPackageId p
+
+-- Older installed package info files did not have the installedPackageId
+-- field, so if it is missing then we fill it as the source package ID.
+setInstalledPackageId :: InstalledPackageInfo -> InstalledPackageInfo
+setInstalledPackageId pkginfo@InstalledPackageInfo {
+                        installedPackageId = InstalledPackageId "",
+                        sourcePackageId    = pkgid
+                      }
+                    = pkginfo {
+                        --TODO use a proper named function for the conversion
+                        -- from source package id to installed package id
+                        installedPackageId = InstalledPackageId (display pkgid)
+                      }
+setInstalledPackageId pkginfo = pkginfo
+
 
 --------------------------
 -- The program invocations