Add Described OS and Arch

Cabal/Distribution/System.hs

@@ -50,6 +50,7 @@ import Distribution.Utils.Generic (lowercase)
 
 import Distribution.Parsec
 import Distribution.Pretty
+import Distribution.FieldGrammar.Described
 
 import qualified Distribution.Compat.CharParsing as P
 import qualified Text.PrettyPrint as Disp
@@ -132,6 +133,13 @@ instance Pretty OS where
 instance Parsec OS where
   parsec = classifyOS Compat <$> parsecIdent
 
+instance Described OS where
+  describe _ = REUnion
+    [ fromString al
+    | os <- knownOSs
+    , al <- prettyShow os : osAliases Compat os
+    ]
+
 classifyOS :: ClassificationStrictness -> String -> OS
 classifyOS strictness s =
   fromMaybe (OtherOS s) $ lookup (lowercase s) osMap
@@ -198,6 +206,12 @@ instance Pretty Arch where
 instance Parsec Arch where
   parsec = classifyArch Strict <$> parsecIdent
 
+instance Described Arch where
+  describe _ = REUnion
+    [ fromString (prettyShow arch)
+    | arch <- knownArches
+    ]
+
 classifyArch :: ClassificationStrictness -> String -> Arch
 classifyArch strictness s =
   fromMaybe (OtherArch s) $ lookup (lowercase s) archMap

Cabal/tests/UnitTests/Distribution/Described.hs

@@ -25,6 +25,7 @@ import Distribution.Types.PackageId                (PackageIdentifier)
 import Distribution.Types.PackageName              (PackageName)
 import Distribution.Types.PackageVersionConstraint (PackageVersionConstraint)
 import Distribution.Types.Version                  (Version)
+import Distribution.System (OS, Arch)
 import Distribution.Types.VersionRange             (VersionRange)
 
 import qualified RERE         as RE
@@ -43,6 +44,8 @@ tests = testGroup "Described"
     , testDescribed (Proxy :: Proxy VersionRange)
     , testDescribed (Proxy :: Proxy FlagName)
     , testDescribed (Proxy :: Proxy ModuleName)
+    , testDescribed (Proxy :: Proxy OS)
+    , testDescribed (Proxy :: Proxy Arch)
     ]
 
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