update `installHLSBindist` to take a "Maybe FilePath" argument for isolated installs

lib/GHCup.hs

@@ -530,6 +530,7 @@ installHLSBindist :: ( MonadMask m
                      )
                   => DownloadInfo
                   -> Version
+                  -> Maybe FilePath -- ^ isolated install path, if user passed any
                   -> Excepts
                        '[ AlreadyInstalled
                         , CopyError
@@ -545,12 +546,16 @@ installHLSBindist :: ( MonadMask m
                         ]
                        m
                        ()
-installHLSBindist dlinfo ver = do
+installHLSBindist dlinfo ver isoFilepath = do
   lift $ $(logDebug) [i|Requested to install hls version #{ver}|]
 
   PlatformRequest {..} <- lift getPlatformReq
   Dirs {..} <- lift getDirs
 
+  let isIsolatedInstall = isJust isoFilepath
+
+  -- we only check for already installed in regular (non-isolated) installs
+  when (not isIsolatedInstall) $
     whenM (lift (hlsInstalled ver))
     (throwE $ AlreadyInstalled HLS ver)
 
@@ -564,10 +569,17 @@ installHLSBindist dlinfo ver = do
 
   -- the subdir of the archive where we do the work
   workdir <- maybe (pure tmpUnpack) (liftE . intoSubdir tmpUnpack) (view dlSubdir dlinfo)
+  let isoDir = fromJust isoFilepath
 
+  if isIsolatedInstall
+  then do
+    lift $ $(logInfo) [i|isolated installing HLS to #{isoDir}|]
+    liftE $ installHLS' workdir isoDir ver
+  else do
     liftE $ installHLS' workdir binDir ver
 
-  -- create symlink if this is the latest version
+  -- create symlink if this is the latest version in a regular install
+  whenM (pure $ not isIsolatedInstall) $ do
     hlsVers <- lift $ fmap rights getInstalledHLSs
     let lInstHLS = headMay . reverse . sort $ hlsVers
     when (maybe True (ver >=) lInstHLS) $ liftE $ setHLS ver