Add Text instance for License

Initially it's exactly the same format as Read/Show.
We can make it more liberal later.

Distribution/InstalledPackageInfo.hs

@@ -205,7 +205,7 @@ basicFieldDescrs =
                            disp                   parseOptVersion
                            packageVersion         (\ver pkg -> pkg{package=(package pkg){pkgVersion=ver}})
  , simpleField "license"
-                           (text . show)          parseLicenseQ
+                           disp                   parseLicenseQ
                            license                (\l pkg -> pkg{license=l})
  , simpleField "copyright"
                            showFreeText           parseFreeText

Distribution/License.hs

@@ -49,6 +49,10 @@ module Distribution.License (
 	License(..)
   ) where
 
+import Distribution.Text (Text(..))
+import qualified Distribution.Compat.ReadP as Parse
+import qualified Text.PrettyPrint as Disp
+
 -- |This datatype indicates the license under which your package is
 -- released.  It is also wise to add your license to each source file
 -- using the license-file field.  The 'AllRightsReserved' constructor
@@ -90,3 +94,6 @@ data License =
   | OtherLicense
   deriving (Read, Show, Eq)
 
+instance Text License where
+  disp  = Disp.text . show
+  parse = Parse.readS_to_P reads

Distribution/PackageDescription/Parse.hs

@@ -101,7 +101,7 @@ pkgDescrFieldDescrs =
            (text . maybe "" show) (fmap Just parseReadSQ)
            buildType              (\t pkg -> pkg{buildType=t})
  , simpleField "license"
-           (text . show)          parseLicenseQ
+           disp                   parseLicenseQ
            license                (\l pkg -> pkg{license=l})
  , simpleField "license-file"
            showFilePath           parseFilePathQ

Distribution/ParseUtils.hs

@@ -555,7 +555,7 @@ parseTestedWithQ = parseQuoted tw <++ tw
 		return (compiler,version)
 
 parseLicenseQ :: ReadP r License
-parseLicenseQ = parseQuoted parseReadS <++ parseReadS
+parseLicenseQ = parseQuoted parse <++ parse
 
 -- urgh, we can't define optQuotes :: ReadP r a -> ReadP r a
 -- because the "compat" version of ReadP isn't quite powerful enough.  In