Don't update ghcup if already latest version

Fixes #2

lib/GHCup.hs

@@ -675,6 +675,8 @@ upgradeGHCup :: ( MonadMask m
                 )
              => GHCupDownloads
              -> Maybe (Path Abs)  -- ^ full file destination to write ghcup into
+             -> Bool              -- ^ whether to force update regardless
+                                  --   of currently installed version
              -> Excepts
                   '[ CopyError
                    , DigestError
@@ -683,12 +685,14 @@ upgradeGHCup :: ( MonadMask m
                    , NoCompatibleArch
                    , NoCompatiblePlatform
                    , NoDownload
+                   , NoUpdate
                    ]
                   m
                   Version
-upgradeGHCup dls mtarget = do
+upgradeGHCup dls mtarget force = do
   lift $ $(logInfo) [i|Upgrading GHCup...|]
   let latestVer = fromJust $ getLatest dls GHCup
+  when (not force && (latestVer < pvpToVersion ghcUpVer)) $ throwE NoUpdate
   pfreq <- liftE platformRequest
   dli   <- lE $ getDownloadInfo GHCup latestVer pfreq dls
   tmp   <- lift withGHCupTmpDir

lib/GHCup/Errors.hs

@@ -30,6 +30,10 @@ data NoCompatiblePlatform = NoCompatiblePlatform String -- the platform we got
 data NoDownload = NoDownload
   deriving Show
 
+-- | No update available or necessary.
+data NoUpdate = NoUpdate
+  deriving Show
+
 -- | The Architecture is unknown and unsupported.
 data NoCompatibleArch = NoCompatibleArch String
   deriving Show

lib/GHCup/Utils/Prelude.hs

@@ -240,3 +240,7 @@ addToCurrentEnv :: MonadIO m
 addToCurrentEnv adds = do
   cEnv <- liftIO $ getEnvironment
   pure (adds ++ cEnv)
+
+
+pvpToVersion :: PVP -> Version
+pvpToVersion = either (\_ -> error "Couldn't convert PVP to Version") id . version . prettyPVP