adds rudimentary isolate capability to ghcup install ghc command

app/ghcup/Main.hs

@@ -1631,22 +1631,29 @@ Report bugs at <https://gitlab.haskell.org/haskell/ghcup-hs/issues>|]
           -----------------------
 
           let installGHC InstallOptions{..} =
-                  (case instBindist of
-                     Nothing -> runInstTool instPlatform $ do
-                       (v, vi) <- liftE $ fromVersion instVer GHC
-                       liftE $ installGHCBin (_tvVersion v)
-                       when instSet $ void $ liftE $ setGHC v SetGHCOnly
-                       pure vi
-                     Just uri -> do
-                       s' <- liftIO appState
-                       runInstTool' s'{ settings = settings {noVerify = True}} instPlatform $ do
+                (case isolateDir of
+                   Just isoDir -> runInstTool instPlatform $ do
+                    (v, vi) <- liftE $ fromVersion instVer GHC
+                    let ghcVersion = _tvVersion v
+                    liftE $ installGHCBinIsolated isoDir ghcVersion
+                    pure vi
+                   Nothing ->
+                     case instBindist of
+                       Nothing -> runInstTool instPlatform $ do
                          (v, vi) <- liftE $ fromVersion instVer GHC
-                         liftE $ installGHCBindist
-                           (DownloadInfo uri (Just $ RegexDir "ghc-.*") "")
-                           (_tvVersion v)
+                         liftE $ installGHCBin (_tvVersion v)
                          when instSet $ void $ liftE $ setGHC v SetGHCOnly
                          pure vi
-                    )
+                       Just uri -> do
+                         s' <- liftIO appState
+                         runInstTool' s'{ settings = settings {noVerify = True}} instPlatform $ do
+                           (v, vi) <- liftE $ fromVersion instVer GHC
+                           liftE $ installGHCBindist
+                             (DownloadInfo uri (Just $ RegexDir "ghc-.*") "")
+                             (_tvVersion v)
+                           when instSet $ void $ liftE $ setGHC v SetGHCOnly
+                           pure vi
+                  )
                     >>= \case
                           VRight vi -> do
                             runLogger $ $(logInfo) "GHC installation successful"

lib/GHCup.hs

@@ -229,6 +229,49 @@ installGHCBindist dlinfo ver = do
         lift $ $(logWarn) "environments). If you encounter problems, unset CC and LD and reinstall."
 
 
+-- | Installs GHC to a specified location, doesn't make any symlinks.
+installGHCBinIsolated :: ( MonadFail m
+                 , MonadMask m
+                 , MonadCatch m
+                 , MonadReader env m
+                 , HasPlatformReq env
+                 , HasGHCupInfo env
+                 , HasDirs env
+                 , HasSettings env
+                 , MonadLogger m
+                 , MonadResource m
+                 , MonadIO m
+                 , MonadUnliftIO m
+                 )
+              => FilePath
+              -> Version         -- ^ the version to install
+              -> Excepts
+                   '[ AlreadyInstalled
+                    , BuildFailed
+                    , DigestError
+                    , DownloadFailed
+                    , NoDownload
+                    , NotInstalled
+                    , UnknownArchive
+                    , TarDirDoesNotExist
+#if !defined(TAR)
+                    , ArchiveResult
+#endif
+                    ]
+                   m
+                   ()
+
+installGHCBinIsolated isoDir ver = do
+  dlinfo <- liftE $ getDownloadInfo GHC ver
+
+  lift $ $(logDebug) [i|Requested to install GHC with #{ver}|]
+
+  -- download (or use cached version)
+  dl <- liftE $ downloadCached dlinfo Nothing
+
+  liftE $ installPackedGHC dl (view dlSubdir dlinfo) isoDir ver
+
+
 -- | Install a packed GHC distribution. This only deals with unpacking and the GHC
 -- build system and nothing else.
 installPackedGHC :: ( MonadMask m