Implement a dummy 'uninstall' command.

As suggested by @sclv on reddit. See
haskell#648 (comment)

cabal-install/Distribution/Client/Setup.hs

@@ -22,6 +22,7 @@ module Distribution.Client.Setup
     , listCommand, ListFlags(..)
     , updateCommand
     , upgradeCommand
+    , uninstallCommand
     , infoCommand, InfoFlags(..)
     , fetchCommand, FetchFlags(..)
     , freezeCommand, FreezeFlags(..)
@@ -837,6 +838,17 @@ formatCommand = CommandUI {
     commandOptions      = \_ -> []
   }
 
+uninstallCommand  :: CommandUI (Flag Verbosity)
+uninstallCommand = CommandUI {
+    commandName         = "uninstall",
+    commandSynopsis     = "Warn about 'uninstall' not being implemented.",
+    commandDescription  = Nothing,
+    commandNotes        = Nothing,
+    commandUsage        = usageAlternatives "uninstall" ["PACKAGES"],
+    commandDefaultFlags = toFlag normal,
+    commandOptions      = \_ -> []
+  }
+
 runCommand :: CommandUI (BuildFlags, BuildExFlags)
 runCommand = CommandUI {
     commandName         = "run",

cabal-install/Main.hs

@@ -22,7 +22,7 @@ import Distribution.Client.Setup
          , BuildFlags(..), BuildExFlags(..), SkipAddSourceDepsCheck(..)
          , buildCommand, replCommand, testCommand, benchmarkCommand
          , InstallFlags(..), defaultInstallFlags
-         , installCommand, upgradeCommand
+         , installCommand, upgradeCommand, uninstallCommand
          , FetchFlags(..), fetchCommand
          , FreezeFlags(..), freezeCommand
          , GetFlags(..), getCommand, unpackCommand
@@ -254,6 +254,8 @@ mainWorker args = topHandler $
                      regVerbosity      regDistPref
       ,testCommand            `commandAddAction` testAction
       ,benchmarkCommand       `commandAddAction` benchmarkAction
+      ,hiddenCommand $
+       uninstallCommand       `commandAddAction` uninstallAction
       ,hiddenCommand $
        formatCommand          `commandAddAction` formatAction
       ,hiddenCommand $
@@ -1004,6 +1006,17 @@ formatAction verbosityFlag extraArgs _globalFlags = do
   -- Uses 'writeFileAtomic' under the hood.
   writeGenericPackageDescription path pkgDesc
 
+uninstallAction :: Flag Verbosity -> [String] -> GlobalFlags -> IO ()
+uninstallAction _verbosityFlag extraArgs _globalFlags = do
+  let package = case extraArgs of
+        p:_ -> p
+        _   -> "PACKAGE_NAME"
+  die $ "This version of 'cabal-install' does not support the 'uninstall' operation. "
+        ++ "It will likely be implemented at some point in the future; in the meantime "
+        ++ "you're advised to use either 'ghc-pkg unregister " ++ package ++ "' or "
+        ++ "'cabal sandbox hc-pkg -- unregister " ++ package ++ "'."
+
+
 sdistAction :: (SDistFlags, SDistExFlags) -> [String] -> GlobalFlags -> IO ()
 sdistAction (sdistFlags, sdistExFlags) extraArgs _globalFlags = do
   unless (null extraArgs) $